UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 10th Dec 2013, 2:59 am   #61
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,541
Default Re: FET Questions

Very interesting thread - which I can only sit and read with eyes wide open. Much of what is being talked about relates to that period when the commercial equipment for Amateurs was still 'understandable'. A little later, the circuitry, components and construction took things to a level where some professional expertise was all but essential. What goes on inside my more recent Trios and Kenwoods is something I have rarely (if ever) delved in to .
Bazz4CQJ is offline  
Old 10th Dec 2013, 3:01 am   #62
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

James Bryant’s writings on the SL600 series were certainly very readable, even to the layman. And it was that 9 MHz IF strip with three SL612Cs in the signal path that I had in mind when I was thinking that post-main filter gain in consumer-level HF receivers could have been done by bipolar ICs as an alternative to discrete FETs.

Maybe I am drawing the wrong inference here, but I had the impression that Plessey specialized in particularly linear bipolar RF ICs (well,, aside, of course, from the logarithmic radar devices). As an example, the late-1970s SL1430 and SL1431/2 TV IF pre-amplifiers seem to fall into this category. They were designed to go between the tuner and the then new-fangled SAW filters (in which field Plessey was an early mover) to compensate in advance for the filter gain loss. Thus they operated in a relatively wideband part of the circuit. And the rationale (cross-modulation reduction) for using dual-gate mosfets in VHF TV tuners (at least in North American and Japanese reception conditions) would seem to apply equally to any following wideband IF stage. Plessey evidently addressed this by appropriate design, and did publish cross-modulation and intermodulation data for this family of ICs. I’d guess that one IC advantage here would have been the easy provision of a differential output for good matching to SAWFs. And the SL1431/2 added a wideband RF AGC loop, and idea that had also turned up in FM front ends and elsewhere in that time period.

Although discussion of RF/IF ICs may seem somewhat off-centre here, as previously observed, FETs and ICs appeared, in consumer equipment at least, at about the same time and to some extent competed with each other. Thus any broad discussion of FET applications reasonably includes those where they could have been used but were not because designers chose the IC pathway.

Cheers,
Synchrodyne is offline  
Old 23rd Dec 2013, 7:17 pm   #63
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

Mentioned previously in this thread was the fact that whereas use of dual-gate mosfets in VHF-TV and FM receiver front ends commenced circa 1968, it was not until the mid-1970s that they appeared in UHF TV front ends. Apparently the latter application awaited the development of (economic) devices that had enough gain towards 1000 MHz.

But an apparent oddity was that whereas in the VHF-TV and FM cases, dual-gate mosfets were adopted for both the RF amplifier and mixer positions, in the UHF-TV case they were used only in the RF amplifier position.

The resultant question is: was this because dual-gate mosfets were simply just not effective mixers at UHF, or were there other reasons.

An early example (circa 1976) of a UHF-TV tuner with a mosfet RF amplifier is the RCA KRK-226, schematic attached. This used a diode mixer. Now it seems possible that this choice could simply reflect the inertia of established US practice where UHF-TV tuners were concerned. The hitherto standard layout was a bandpass tuned RF input feeding a diode mixer, with a valve (such as the 6AF4A) or bipolar local oscillator. The UHF-TV tuner output was then fed into the VHF-TV tuner, passing through both the RF stage and then the mixer stage acting as an amplifier.

So it could be that the RCA UHF-TV tuner at interest was conceived as being more-or-less the standard format with the addition of a tuned RF amplifier stage, rather than a complete rethink. Against that, an interesting aspect of the RCA tuner was that it included what might be called an “IF preamplifier” in the form of a grounded base bipolar stage. Also, although it then fed into the corresponding VHF tuner (KRK-228), it bypassed the RF stage, being connected directly to the mixer stage acting as an amplifier. I suspect that this may have been done to simplify the switching; whereas with a turret tuner it was easy enough to have an extra position for switching the UHF tuner output through the whole tuner, doing that may have been more complex with a varactor tuner. Anyway, a possible take from RCA’s design choice is that the combination of a diode mixer with an IF preamplifier was better (more gain perhaps?) than a dual-gate mosfet mixer.

Some of the RCA TV receivers in which the KRK-226 and KRK-228 tuners were used had distributed selectivity IF strips based upon three dual-gate mosfets, probably unusual by the second half of the 1970s when ICs were dominant for this function. In the VHF case, the signal path through to the vision demodulator thus consisted of five consecutive dual-gate mosfets. In the UHF case, the active device sequence was something of a “sandwich”, namely one dual-gate mosfet, one grounded base bipolar and then four consecutive dual-gate mosfets. This departed from the somewhat conventional wisdom that the mosfets be used “up front” with bipolar stages reserved for later stages after the main IF selectivity.

There is an interesting parallel in that around the same time (1977?), Mullard made a step-change in its UHF-TV tuner range with the introduction of the U321. Like the RCA KRK-226, this used a diode mixer followed by a grounded base bipolar IF pre-amplifier. Previous Mullard UHF TV tuners had used a self-oscillating bipolar mixer, which was I think fairly standard and established UK practice, going back to valve days when self-oscillating triode (PC86) mixers were used. Where Mullard differed from RCA was in using a grounded base bipolar RF amplifier preceded by a PIN diode attenuator for AGC. I wonder if Philips/Mullard had an aversion to using mosfets at the time, even though it did adopt them for RF amplifiers a few years later, perhaps when they were better and/or cheaper than the early UHF-capable devices. Anyway, the Mullard U321 supports the notion that there was some merit in using a diode mixer followed by a gain stage at UHF rather than an active mixer with inherent gain.

Cheers,
Attached Thumbnails
Click image for larger version

Name:	RCA KRK-226 UHF TV Tuner.jpg
Views:	2288
Size:	85.6 KB
ID:	87105  
Synchrodyne is offline  
Old 23rd Dec 2013, 7:41 pm   #64
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,869
Default Re: FET Questions

Active mixers are much noiser than an amplifier using the same technology. Passive (e.g. switching) mixers can be very quiet. This means that when getting near the limit of what your technology can do the first thing that goes is active mixers, then amplifiers, then passive mixers.
G8HQP Dave is offline  
Old 23rd Dec 2013, 8:11 pm   #65
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 7,888
Default Re: FET Questions

Pondering this, I wonder - in the US UHF TV took a long time to get off the ground - to the point where it was considered very much an afterthought to TV designers until well into the 1980s. Remember, unlike the UK they had 525-line NTSC colo(u)r on VHF from the 1950s... so no real pressure to put much effort into UHF... and UHF doesn't cover US-style distances well either.

I also ponder whether the US FET TV- and FM-radio tuner designers were more focussed on coping with lots of strong competing stations on VHF [particularly on the VHF-TV and 'Band II' FM broadcast radio-band] whereas in more-regulated State-controlled-media Europe we tended to have many fewer stations. Cross-modulation resistance (hence early use of MOSFETs and diode-mixers) could have been more important to the US designers? When I visited Dallas/Fort Worth in the early-1980s tuning a FM radio from one end of the band to the other revealed precious few unoccupied channels.

Then in the context of base/mobile two-way radio, I know that US designers from the 1960s onwards had very rigorous targets to meet for handling high power adjacent-channel signals without desensitization: a typical US Police low-band [30-50MHz] mobile-radio installation in the 1960s had a 100-Watt transmitter and the base-stations were often Kilowatts.
Minimal pre-mixer gain [often a grounded-grid FET, or a high-current bipolar] and diode-mixers were the order of the day. The front-end of a similar-era Pye Westminster [with a humble 2N3819 or two up to a low-oscillator-injection 2N3819 first-mixer] just wouldn't handle it.

Last edited by G6Tanuki; 23rd Dec 2013 at 8:18 pm.
G6Tanuki is offline  
Old 27th Dec 2013, 2:58 am   #66
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

Quote:
Originally Posted by G6Tanuki View Post
Pondering this, I wonder - in the US UHF TV took a long time to get off the ground - to the point where it was considered very much an afterthought to TV designers until well into the 1980s. Remember, unlike the UK they had 525-line NTSC colo(u)r on VHF from the 1950s... so no real pressure to put much effort into UHF... and UHF doesn't cover US-style distances well either.
Definitely UHF TV was of secondary importance in the USA. The three main networks, ABC, CBS and NBC, were on the VHF channels in the great majority of locations, and PBS was also on VHF in most areas. Thus the UHF channels were occupied mostly by local and independent broadcasters. Dallas-Fort Worth (DFW) was a typical example, with VHF TV stations on channels A2 (PBS), A4 (CBS), A5 (NBC), A8 (ABC), A11 (Independent) and A13 (PBS), and with 3 or 4 independents on UHF.

Thus UHF tuners were very simple devices until the 1970s. At some stage, it might have been early in the 1970s, FCC mandated that TV receivers must tune the UHF channels as well as they did VHF channels, and this resulted in detented tuning mechanisms, although not necessarily with any electronic changes. The addition of RF amplifiers, as in the RCA KRK-226 case, might have coincided more-or-less with the advent of varactor tuning. Possibly the use of varactors resulted in greater loss and/or higher noise, such that low-noise gain ahead of the diode mixer became desirable?

Quote:
Originally Posted by G6Tanuki View Post
I also ponder whether the US FET TV- and FM-radio tuner designers were more focussed on coping with lots of strong competing stations on VHF [particularly on the VHF-TV and 'Band II' FM broadcast radio-band] whereas in more-regulated State-controlled-media Europe we tended to have many fewer stations. Cross-modulation resistance (hence early use of MOSFETs and diode-mixers) could have been more important to the US designers? When I visited Dallas/Fort Worth in the early-1980s tuning a FM radio from one end of the band to the other revealed precious few unoccupied channels.
With US VHF TV tuners, cross-modulation was evidently the key performance parameter as an indication of front end non-linearity, and one that had gone backwards significantly with the transition from valves to bipolar transistors. The use of mosfets solved the problem, and allowed solid state tuners to equal or match the best valved models. TI at least had also proposed the use of jfets, but mosfets were better for the application. At the time the cross-modulation concern would have been more about the adverse effects of alternate – or even further away - channels than about the effects of adjacent channels, but the latter became important once TV receivers were fitted with “cable ready” front ends.

On the face of it, the European VHF TV situation might have been less demanding in that fewer stations were available in any given area, and that in turn might have justified the continued use of bipolar VHF tuners. But it was not necessarily so. My own experience with two Philips receivers, a K9 in Auckland during the 1970s and a later model (I do not recall which chassis) in Wellington during the early 1980s was that their respective VHF tuners were quite miserable in terms of cross-modulation performance. At each site, there was just one Band I and one Band III channel available. At both, reasonable outdoor aerials were needed to obtain interference- and ghost-free reception. And at both, something like 20 dB attenuation (at the set input) was needed to get rid of what was very visible cross-modulation. In the Auckland case, using an aerial array that I had installed, an earlier valved HMV monochrome receiver had no problem with the same signal. By playing around with the internal RF agc offset control, one could get the front end into overload, but even just before onset of same, there was no evidence of cross-modulation. The Wellington aerial was a professional installation, and the installer had to add the attenuator to get rid of the cross modulation. My conclusion was that it was poor or careless design on the part of Philips; it probably should have used a mosfet-based tuner in these receivers. But at the time Philips seemed to be bipolar-oriented.

With US FM there were similar concerns as with VHF TV. As you say, the FM Band was often pretty much full, and in DFW it was end-to-end occupied with what were mostly highly compressed transmissions “shouting” at the potential audience, with KERA (PBS) and WRR (Dallas City-owned classical) being in a very small minority with minimal compression. Both from time-to-time received questions from poorly-informed listeners as to why they were relatively “quiet”. I can’t recall their exact responses, but KERA at least implied that compression was a form of distortion and thus highly undesirable (which I don’t suppose went down too well with the purveyors of compression equipment who no doubt preferred to put a positive spin on their distortion mcahines).

Anyway, FM receivers with bipolar front ends could and did have difficulties in that environment. For example my Beolit 707 was not too happy. At least judging by the RCA and TI papers, the key parameter relating to FM front end linearity was the half-IF response, and FETs were generally better than bipolar devices at minimizing this. Here though, the European and UK FM tuner makers adopted FET-based front ends in the same time period, if not earlier than their Japanese and US counterparts, I imagine because they were weighing heavily the needs of the export markets including the USA. Some delayed the change from valves to solid state until FETs were available and economic, and so avoided bipolar woes. On timing, I don’t have the exact dates but I think that B&O and Sony both updated their respective “5000” models from bipolar to FET front ends at around the same time, circa 1969. Radford offered an “export” FET front end option for its otherwise bipolar FM tuner by 1967.

For FM, jfets as well as mosfets were used in the early days. The VHF-TV objections, such as limited agc range for an RF jfet cascode, and minimal gain for a jfet mixer, were of much less consequence in FM applications.

Returning to UHF TV, the UK might have had the most severe reception conditions, with its transmitter network carefully designed to provide national coverage. Whilst the service area maps showed the boundaries for good reception, I suspect that had they also showed the boundaries at which transmitters could cause material interference, these would have been much further out, an inevitable consequence of designing for contiguous service areas. On the other hand, for many US cities, except perhaps in the northeast, it would be unlikely that any UHF transmissions from other cities would be strong enough to cause interference problems. In DFW, one could not find any out-of-area UHF TV transmissions using the ICOM R7000 VHF-UHF communications receiver, and scarcely any out-of-area VHF TV transmissions, either. Anyway, fairly exacting standards were set by BREMA for UK UHF TV tuners from the start, and they more-or-less had to be four-gang. Cross-modulation of some of the early bipolar designs was evidently not brilliant, but then signal strengths as delivered to the receivers might not have been that high. Still, the previously-mentioned Mullard U321 seems to have included a higher front end overload threshold as part of its design brief, albeit in this case by using a grounded base bipolar stage, I imagine with reasonable current. Maybe a mosfet would have been better still, though. It did not occur to me before, but I wonder if a diode mixer was chosen because a realizable bipolar mixer similar to that used in earlier designs, might have overloaded before the new, higher-headroom RF amplifier did.

Quote:
Originally Posted by G8HQP Dave View Post
Active mixers are much noiser than an amplifier using the same technology. Passive (e.g. switching) mixers can be very quiet. This means that when getting near the limit of what your technology can do the first thing that goes is active mixers, then amplifiers, then passive mixers.
So the take on this is that (appropriate) mosfets were quiet enough for use as RF amplifiers for both the VHF TV and UHF TV cases.

And the gain available at VHF was sufficient to allow a mosfet mixer to follow a single mosfet RF stage without prejudicing noise performance.

But at UHF, the gain available from a single mosfet RF stage was insufficient to offset the noise contribution of a following mosfet mixer, so it was preferable to use a passive mixer followed by a low noise amplifier.

Cheers,
Synchrodyne is offline  
Old 4th Jan 2014, 4:12 am   #67
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

I have since found some more information on the previously-mentioned RCA UHF TV tuner that perhaps throws some light on the very recent discussion.

It was said to have used a new mosfet device for its RF amplifier. So that gives us an approximate date (1976) for the advent of UHF-capable mosfet devices that were economic for TV receiver uses, so answering one of the early questions in this thread. That also appears to be about the time that mosfets were first used at UHF in commercial VHF-UHF communications receivers, but I’ll cover that in a separate post.

The use of the mosfet RF stage gave a cross-modulation performance improvement of around 12 dB as compared with the previous bipolar type tuner. Here I need to correct my earlier assertion that US UHF TV tuner practice jumped direct from the simple type with no RF amplifier and a diode mixer to the more complex type with a mosfet RF amplifier and diode mixer. Not so, evidently there was an intermediate stage in which a grounded base bipolar RF amplifier preceded the diode mixer. Reading between the lines of the available RCA information, the need for an RF amplifier arose with the introduction of varactor tuning, which was lossy as compared with variable capacitor tuning. Thus the RF amplifier was required to offset varactor losses.

Looking at the timeline, it would appear that the use of mosfets in VHF TV tuners (1968-69) just predated the arrival of varactor tuning (1970 or so), confirmed by the fact that the references to early such tuners showed them as having turret-type tuning. Then varactor tuning required the addition of RF amplifiers, initially bipolar, to UHF TV tuners. As this happened after the advent of mosfets in VHF TV tuners, one assumes that bipolar RF amplifiers were more-or-less a forced choice, and perhaps regarded as interim until UHF-capable mosfets became available. RCA had been a leader in mosfet technology, making the case for initially single-gate then dual-gate small-signal devices (whereas TI seemed to have leaned towards jfets before moving on to dual-gate mosfets), so I’d take its 1976 introduction of a mosfet-based UHF TV tuner as being a very early example of the art, although perhaps the Japanese makers got there at around the same time.

RCA also noted that the use of a mosfet RF amplifier in the UHF tuner made it compatible with the VHF tuner for agc purposes.

I imagine that varactor losses would vary by type, but RCA quoted 4 dB per circuit at the bottom end of the UHF band for those used in its UHF tuner. So that is 12 dB total loss for the three pre-mixer tuned circuits. Presumably the loss was maximum at the low end of the band. The RF amplifier mosfet had a gain of 15 dB at the top end of the UHF band, and “considerably better” (whatever that meant) at the lower end. So the RF amplifier covered the varactor loss with not very much to spare, and I should think there was not enough net gain to allow the use of (overcome the noise of) an active mixer such as a dual-gate mosfet, hence the retention of the diode type per established American practice. The choice of a grounded base bipolar pre-IF amplifier following the diode mixer was chosen to provide the diode mixer with a relatively constant impedance load, and the gain was adjusted to allow the same RF agc delay for the UHF and VHF cases. Working back from this, presumably agc considerations made it preferable for the IF signal not to be routed through the VHF RF amplifier, but rather to the VHF mixer via an IF pre-amplifier.

The corresponding RCA VHF TV tuner had its mixer (dual-gate mosfet) performance adjusted to minimize the so-called channel 6 color beat, which seemed to be a general bellwether of VHF TV mixer linearity performance, somewhat like the half-IF response for FM tuners. The mechanism was that a three-way mix (f1 + f2 – f3) of the channel 6 vision (83.25 MHz) and sound (87.75 MHz) carriers and the local oscillator (129 MHz) produced an in-IF band 42 MHz spur, which then demodulated against the colour subcarrier as a 170 kHz beat. I suppose that it was one of those circumstantial artefacts. The American TV channel frequencies were established before the advent of colour, and the migration to the 45.75 MHz standard vision IF frequency (upon which UHF channel allocations were based) from the previously used numbers around 25 MHz happened in the very early 1950s, just before NTSC-II defined the colour standards. And it seemed not to have been a major problem with valved VHF tuners, rearing its head with the arrival of bipolar transistor tuners.

All of this speaks to G6Tanuki’s comment about cross-modulation, intermodulation, etc., performance being very important in typical US reception conditions.

Once established as both VHF TV and UHF TV RF amplifiers, dual-gate mosfets seemed to stay prominent in that role throughout most of the remaining analogue era. But the mixer function in both cases was usurped by bipolar IC double-balanced multiplier devices, although I do not have timeline information for their early use. I suppose that such devices could be very linear (their primary mixing function excepted) if kept below overload level, and that could be done with an agc’d mosfet amplifier coupled with adequate selectivity. In US VHF TV tuner practice, at least 50 dB RF gain reduction by agc was the desideratum.

Actually, there were HF and MF precedents for having a mosfet RF amplifier feeding into a bipolar IC mixer. The Marconi Apollo marine receiver of the late 1960s had a 40673 mosfet RF amplifier with bandpass input, coupled to an SL640C IC mixer by a single-tuned interstage and an emitter follower. The high-quality (including PLL demodulation) AM side (MF-only) of the Sansui TU-X1 “supertuner” of the later 1970s had a 3SK41 mosfet RF amplifier followed by an LM1496N IC mixer. The earlier TU-S77AMX and TU-D99AMX tuners with multisystem AM stereo capability used a 2SK192 jfet AM RF amplifier with tuned input, more I think as a buffer to feed the LA1245 AM IC, which likely included a double-balanced bipolar mixer.

Cheers,
Synchrodyne is offline  
Old 10th Jan 2014, 3:27 am   #68
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

To return to the previously reviewed issue of local oscillator injection – gate 1 or gate 2 – for the VHF TV tuner dual-gate mosfet mixer case, it has just occurred to me there is another, perhaps somewhat “sideways” way of looking at this that does eventually point to the gate 1 option.

To get to this destination it is necessary to go back to the early days of valve TV receivers. By around 1950, the typical American 13-channel turret tuner used a 6AG5 high-slope VHF pentode RF stage followed by a 6J6 double triode mixer-oscillator (Standard Coil example attached). The RF pentode was rather noisy at Band III (high band) frequencies, but was a lot easier to use and more robust for mass-production than any triode option, although some makers did use, for example, push-pull triode RF amplifiers based around the 6J6. IFs were then around 25.75 MHz, well below the bottom edge of Band I (low band), so triode mixers were relatively easy to use.

But IFs were on the cusp of moving upwards, both for spurii reduction and in anticipation of the advent of UHF transmissions, the FCC channel allocation plan for which was to assume a 45.75 MHz receiver vision IF. However, a vision IF that was nudging the bottom end of Band I created problems for triode mixers because of Miller feedback. Neutralization was one solution, but that was not liked by the makers, as evidenced by their preference for pentode RF amplifiers. A more robust solution for a high-volume mass-produced product that was nevertheless a precision product was to use a pentode mixer. But such were extremely noisy at Band III frequencies, and to be made usable thereat, needed to be preceded by a high gain, low noise RF amplifier stage. That I think is why the series cascode RF amplifier and triode-pentode mixer-oscillator appeared at essentially the same time, in 1951. RCA developed the former and its associated 6BQ7 valve from the original Wallman shunt cascode circuit, which had been intended for IF strip use and evidently was not well-suited to tunable RF stage applications. In part the series cascode would have been an answer to the independent quest to find a robust way of using triodes in VHF TV tuner RF amplifiers, but it was also an essential element in the implementation of the higher IF, as it was low noise and provided enough gain to allow the use of a pentode mixer even at Band III frequencies. Of course, a triode mixer would have been fine at Band III; it was the lower Band I requirement and its proximity to the IF that pointed to the pentode as being a more robust solution than a neutralized triode. RCA developed its 6X8 triode-pentode at the same time as it developed the 6BQ7, and someone who was not RCA (Tung-Sol, I think) also developed the 6U8 at around the same time.

Thus did VHF TV tuner practice “flip” from pentode RF amplifier plus triode mixer to (cascode) triode RF amplifier plus pentode mixer. I guess that a cascode double triode would also have been effective as a mixer, with both signal and oscillator on gt1, but that would have required a separate oscillator valve given that accommodating three triodes on a B9A base presented some difficulties, although later on it was done for FM front end applications (6EZ8).

The European situation was similar, although on a slightly later timescale. A 1953 Philips 10-channel VHF TV tuner (AT7501) had an EF80 (pentode) RF amplifier followed by an ECC81 (double triode) mixer-oscillator, and a 23.75 MHz IF output. A 1954 Philips VHF TV tuner (AT7530) had a PCC84 (cascode) RF amplifier followed by a PCF80 (triode-pentode) mixer-oscillator, and 38.9 MHz IF output. The PCC84 and PCF80 had been released mid-1953.

Anyway, the key point arising from this preamble, is that there were cogent reasons for using a pentode mixer rather than a triode mixer, and these were still valid in the solid state era. That is, any device used as a VHF TV mixer should desirably have pentode-like characteristics.

As previously noted, American VHF TV tuner performance was at a premium, and even though early solid state bipolar types were not as good as their valve predecessors, there was some use of cascode bipolar mixers with both signal and oscillator fed into the base of the first stage. I hadn’t pondered this much before; it was easy enough to see it as a desire to obtain a bit more mixer gain, although such was probably just as easily obtained in a post-mixer stage. But taking in the rationale for established valve practice, one can see that the more likely reason for the use of cascode bipolar mixers was because these had some pentode-like characteristics, in particular tending to block Miller feedback that could be particularly deleterious on the lower Band I channels. Single bipolar mixers stages either had lowish gain (common base) or required neutralization (common emitter).

The arrival of the dual-gate mosfet may thus be seen as having been close to a drop-in solution for the VHF TV mixer case. It was inherently a cascode device, thus had the pentode-like characteristics that had become a desideratum from the early 1950s. Also it was triode-like from a noise viewpoint, and overall delivered performance equal to or better than triode-pentode valve mixers. The use of g1 for both signal and oscillator injection would have been the “natural” thing to do, based upon the prior art with the pentode and cascode bipolar mixers, and of course it was necessary to preserve the cascode characteristics in respect of both inputs. So the g2 oscillator injection possibility might not have been much considered for the VHF TV application, although it became the norm for FM front ends, where the IF was an order-of-magnitude separated from the signal and local oscillator frequencies. The Zenith paper is not completely clear on this item, simply stating: “The dual-gate MOS FET mixer in the cascode configuration (the gate 2 RF bypassed) provided the best overall mixer performance among the solid-stage mixers investigated. It has a conversion gain of approximately 13dB at channel 13, and has less cross-modulation distortion than the 6GJ7 tube mixer.” However, the level of detail supplied in respect the various RF and mixer possibilities suggest that had g2 oscillator injection option been considered for the mosfet, it would have been mentioned.

One might say that in the TV case, the dual-gate mosfet was used as a mixer because it was a cascode device, whereas in the FM case it used because it was a two-port input device. In the latter, the valve analogy does not hold up in full. In a cascode double triode, only gt1 has control over cathode (k1) current (excepting the extreme case where gt2 is taken negative enough that it cuts off), whereas in a dual-gate mosfet, both g1 and g2 control source current. One might liken a dual-gate mosfet with inputs on both g1 and g2 to a dual-control pentode or a heptode, but in the valve cases, g1 alone controls cathode current whereas g3 swings that current between screen and anode rather than modulating the cathode current, so there is a difference. Of the two valve analogies, the dual-control pentode might be the better as in that case g3 is not screened from the anode, just as in a dual-gate mosfet g2 is not screened from the drain. A wild supposition - if one wanted to make the mosfet look more like a heptode in that respect, then maybe a single-gate mosfet with (AC) grounded gate could be used in series with the dual-gate device that used both g1 and g2 as input ports.

The foregoing basically just stitches together in a different way previous contributions on this item, particularly those from Radio Wrangler and G0HZU_JMR. As with any post facto “why did they do it that way” question, it may be unwise to assume exhaustive analysis on the part of the original designers and allow for the possibilities of inertia, “conventional wisdom”, and “others were doing it that way”.

Having written the foregoing, I remembered a possible counterargument in that that the RCA databook showed its own-design (40nnn series) VHF TV mixer mosfets with g2 oscillator injection, although industry standard 3N2nn varieties were shown with g1 injection. But the circuits shown were test circuits, not application circuits, and in the mixer case for 200 MHz-to-44/45 MHz conversion gain. So this was not necessarily how the devices would be used, and anyway at 200 MHz, g2 injection should be fine, as g1 injection was driven by requirements at the low end of the range, around 55 MHz.

Cheers,
Attached Thumbnails
Click image for larger version

Name:	RCA 3N212 Test Circuit.jpg
Views:	3779
Size:	77.3 KB
ID:	87660   Click image for larger version

Name:	RCA 40821 Test Circuit.jpg
Views:	1485
Size:	53.9 KB
ID:	87661   Click image for larger version

Name:	Standard Coil Turret Tuner.jpg
Views:	752
Size:	74.4 KB
ID:	87662   Click image for larger version

Name:	Philips AT7501.jpg
Views:	1083
Size:	88.6 KB
ID:	87663  
Synchrodyne is offline  
Old 27th Jan 2014, 3:23 am   #69
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

Returning to the issue of FM tuners and front end performance in difficult reception conditions, this comment made in the Radford entry in the 1967 London Audio Fair catalogue is I think illustrative:

“The standard FMT.2 tuner uses low noise silicon planar transistors in the signal frequency unit. This tuner will now be available to special order fitted with field effect transistors. The F.E.T. model is being made available primarily for foreign markets where conditions demand the highest possible performance in respect of cross modulation, image rejection and adjacent channel selectivity”.

What variety of FET was used by Radford was not stated, but given the early 1967 timing, jfets seem more likely. The FMT.2 had been shown at the 1966 Audio Fair, and in the catalogue it was described as having a four-gang front end with a double tuned circuit ahead of the input transistor.

One may surmise that Radford found FMT.2 front end performance not good enough in some export markets, hence the introduction of the FET option. Radford was somewhat unusual in that it introduced a solid state FM tuner before it introduced solid state amplifiers, the opposite of what several others, such as Leak, Quad and Rogers did, their hesitation in some part due to the knowledge that valve performance could not be matched with bipolar devices.

Radford’s use of a double tuned circuit ahead of the RF amplifier was interesting in that it went against the conventional wisdom that at VHF, it was better – for noise performance - to have a double tuned interstage and single tuning at the input. Presumably Radford was concerned to get as much selectivity as reasonably possible ahead of the first bipolar transistor. Whether the FET version was similarly configured is unknown. But Rogers chose the same configuration for its Ravensbourne FM tuner in 1968, that is a bandpass input circuit ahead of the dual-gate mosfet RF amplifier followed by a single-tuned interstage to the dual-gate mosfet mixer. But placing the bandpass between the RF amplifier and the mixer appears to have been more usual with following designs such as the Revox A76, although some makers stayed with three-hang front ends, as in the Leak Stereofetic and the Quad FM3. The next step, adopted for “supertuners”, was the use of two dual-gate mosfet RF stages, usually with bandpass interstages and so six gangs in all. The EF5804 FM tuner head shown and described in Ambit catalogue #3 was an example. Actually, the Ambit catalogues #1 and #3 between them illustrate a hierarchy of FM front end designs and their performance characteristics, with commentary (I think it was done by Bill Poel) that is quite accessible from the layperson’s viewpoint. It is evident that the use of two RF stages and associated additional tuned circuits was primarily driven by a desire to reduce spurious responses, including but not limited to half-IF. On the other hand, the extra gain ahead of the mixer could compromise strong signal handling, and the EF5804 was fitted with a pin diode attenuator ahead of the 1st RF amplifier and operated by a wideband agc loop driven from the immediate post-mixer signal. This was additional to the “normal” agc on gate 2 of both RF mosfets and derived from the IF strip. (As an aside, somewhat later – early 1980s I think – the National LM1865 FM IF IC – was designed to provide both narrow and wideband RF agc outputs in addition to the other functions that had previously been delivered by the CA3089 and its ilk.) The EF5804 gain was quoted as 46 dB, whereas the Larsholt 8319, a representative four-gang/one RF amplifier design, was quoted at 32 dB. So the second RF amplifier was adding around 12 dB gain, and commensurately reducing mixer overload headroom, the later being compensated for by the input attenuator. But apparently such was required to address the challenge of “sorting out” multiple strong stations in the typical US urban environment whilst retaining good fringe reception capability.

The use of two RF stages probably had no precedent in valve FM front end practice. Even the use of four gangs, with say a bandpass interstage, was probably rare in valve days. Nevertheless, it could be said that in the solid state era, domestic FM tuner front ends followed the pattern set in the valve era and then improved upon it with additional pre-mixer selectivity and gain. The dual-gate mosfet was the active device of choice for the RF amplifier and mixer stages in order to achieve maximum spurious response suppression. There was no general move to an altogether different approach, such as the use of a bullet-proof mixer with high level oscillator injection preceded by a minimum amount of RF gain and selectivity. Nor I think was there any significant move to the use IC bipolar mixers in place of dual-gate mosfets, even though such devices were developed for TV tuner use. Single bipolar ICs that did the whole front end job, such as the TDA1062, seem not to have made much impression at the higher performance end. There was also the SD6000, which was an IC containing two dual-gate mosfets, one for the RF amplifier and one for the mixer. It needed an external oscillator though.

ICs suitable for use in consumer equipment became available at about the same time as FETs, and these dominated in FM tuner IF strips from that time, initially with one simple “building block” IC, such as a µA703 or CA3028 per stage, followed by increasing aggregation of functions. The arrival of the multifunctional CA3089 in 1970 was a major nodal point in the transition to ICs. Nevertheless, sometimes additional IF gain ahead of the CA3089 was desired, as well as buffering between cascaded ceramic block selectivity filters. This was sometimes done with FETs rather than bipolar transistors or ICs. The Meridian 104 was an example, with a single agc’d BF961 dual-gate mosfet between the front end and the block filter (which fed into an HA12411 IC, one of the CA3089 derivatives). As the gain stage was ahead of the block selectivity, one assumes that the designers used a mosfet in part for best linearity and overload performance. Another, perhaps unusual example was the Eddystone 1002 FM-HF combination receiver. Here two dual-gate mosfet stages, without agc, preceded the single ceramic filter and a CA3089. But the unusual part was that those mosfet IF stages followed a Mullard module bipolar front end. I imagine that the Mullard module was quite low gain, perhaps lower than that provided by the kinds of front ends that might normally be used with a CA3089-based IF strip. So the use of two mosfets might have been to make up the gain deficit to enable the CA3089 to limit on suitably low signal levels. The previously mentioned National LM1865 FM IC included a pre-IF gain/buffer stage that obviated the need for discrete pre-IF stages.

Cheers,
Attached Thumbnails
Click image for larger version

Name:	Rogers Ravensbourne Front End.jpg
Views:	1273
Size:	85.0 KB
ID:	88296   Click image for larger version

Name:	Revox A76 Front End.jpg
Views:	2092
Size:	86.4 KB
ID:	88297   Click image for larger version

Name:	Larsholt 8319.jpg
Views:	1749
Size:	95.6 KB
ID:	88298   Click image for larger version

Name:	Ambit EF5804.jpg
Views:	1270
Size:	99.8 KB
ID:	88299  
Synchrodyne is offline  
Old 27th Jan 2014, 3:27 am   #70
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

Additional attachments pertaining to the most recent posting.
Attached Thumbnails
Click image for larger version

Name:	Meridian 104 IF.jpg
Views:	3420
Size:	61.9 KB
ID:	88300   Click image for larger version

Name:	Eddystone 1002 FM IF.jpg
Views:	1676
Size:	54.0 KB
ID:	88301   Click image for larger version

Name:	Ambit FM IF.jpg
Views:	1956
Size:	90.6 KB
ID:	88302  
Synchrodyne is offline  
Old 27th Jan 2014, 9:21 am   #71
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 12,704
Default Re: FET Questions

In any historic study of what was done in some branch of technology, it is equally interesting to consider what was *not* done.

In the early 70's there were low noise high dynamic range bipolar amplifiers using transformer (noiseless) feedback - see the patents of David Norton. These had many dB more dynamic range than dual gate mosfets. There were the beginnings of VMOS... I experimented with a couple of VMP4s some years later. The SD6000 has been mentioned, but no-one at that time knew what the SD5000 was going to do for receiver dynamic range in the 90's. There were bipolar mixers, but not high dynamic range ones... I'mnot sure when the SL6440 was released, but it was a lower frequency part really.

For dynamic range, I was using bipolar amplifiers running appreciable power and diode ring mixers. Together they offered a lot better signal handling than any dual gate mosfet, but at the cost of power and price. The mosfets were very easy to use, were cheap, offered convenient impedances for embedding and worked well enough for most purposes in the UK.

Diode rings were locally made toroidal transformers and matched quads of HP schottky diodes, MD108, SBL1 weren't known.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 27th Jan 2014, 9:57 am   #72
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

Quote:
Originally Posted by Radio Wrangler View Post
For dynamic range, I was using bipolar amplifiers running appreciable power and diode ring mixers. Together they offered a lot better signal handling than any dual gate mosfet, but at the cost of power and price. The mosfets were very easy to use, were cheap, offered convenient impedances for embedding and worked well enough for most purposes in the UK.
Which is why I think dual-gate mosfets were used in FM tuners and in those Japanese HF receivers aimed more at the upper end of the consumer market, whereas the the higher cost, higher power consumption and better performance alternatives were used in seriously professional equipment.

Cheers,
Synchrodyne is offline  
Old 27th Jan 2014, 10:46 am   #73
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

Quote:
Originally Posted by Radio Wrangler View Post
There were bipolar mixers, but not high dynamic range ones... I'mnot sure when the SL6440 was released, but it was a lower frequency part really.
I think one of the first well-known bipolar integrated doubly balanced mixers based upon the six-transistor Gilbert cell was the Motorola MC1496. I cannot pinpoint when it was introduced, but I’d guess around 1968-69. Plessey had a broadly similar device, the SL640C, which probably arrived soon after the MC1496.

The MC1496 seemed to be usable both at the back end of signal-chain, for example as an SSB demodulator, and towards the front end, as a mixer, at least in situations where it was appropriate for the signal levels and bandwidths encountered. The previously mentioned Sansui TU-X1 “supertuner” used a 1496 as mixer on the AM side, and the Marconi Apollo marine HF receiver used an SL640C as first mixer. Even earlier in using a Gilbert cell for demodulation was the Sprague ULN2111 FM IF sub-system of 1967, which combined a limiting amplifier with a quadrature demodulator. It was quickly followed by a whole host of similar devices, many aimed mostly at TV FM sound. Motorola had previously used what was effectively a Gilbert cell for the 1st, agc’d stage of its MC1350 TV IF amplifier block, and then again in 1969 for its MC1330 TV quasi-synchronous demodulator, which spawned a whole raft of functionally similar and progressively more complex ICs. In fact it might be said that the bipolar mixer form was pivotal in establishing the use of ICs for consumer equipment, and this was happening concurrently with the introduction of fets for the very front end work. With ICs, whilst the FM tuner makers were comfortable in the early days with using building block ICs such as the µA703 and CA3028, each IC typically replacing one transistor, apparently this was resisted by the mass-producers, particularly in respect of TV receivers. Motorola’s MC1350, which replaced the first two regular IF stages (with the MC1330 replacing the third and the diode demodulator) followed the MC1550, which functioned as a single IF stage, but which evidently was not accepted by the industry. And the MC1350 was very quickly segued into the MC1352, which added a gated agc circuit to increase functionality. Motorola was also early with an IC colour TV subcarrier demodulator based upon bipolar multipliers, whereas RCA had been proposing dual-gate mosfets for this role. Here I think the IC approach won decisively and quickly.

Getting back to the theme of what was not done or seldom done, finding an example of an essentially consumer-level item that used a high current bipolar feedback RF amplifier might be difficult, whereas DBMs can be found here and there.

Cheers,
Synchrodyne is offline  
Old 27th Jan 2014, 11:35 am   #74
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 7,888
Default Re: FET Questions

OK it's not FET but one of the late-1960s RSGB books I've got [Amateur Radio Techniques?] mentions a US "Fisher" FM-tuner where the antenna is fed through a single tuned-circuit then into a 4-hot-carrier-diode ring-mixer - the article mentions that this was done to give better strong-signal handling than could easily be achieved with current-generation bipolar devices.
G6Tanuki is offline  
Old 27th Jan 2014, 12:05 pm   #75
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 12,704
Default Re: FET Questions

Yes, that was reported by Pat Hawker in tech topics. It's the only mention I've come across of an FM tuner with a diode ring mixer.

I'm not sure whether the MC1496 or the RCA CA3027/3028 family came first.

One thing is definite, they aren't 'Gilbert Cells'. The transistor tree mixer predates the Gilbert cell, and the Gilbert cell is a logarithmic current-based 2 or 4 quadrant linear mixer.

What Barrie Gilbert invented was the idea of adding diode connected transistors on the substrate so that currents could be converted bach to voltages with a logarithmic law which compensated for the antilog law of current partitioning in a long-tailed pair.

His cell is in 4-quadrant linear multipliers and a lot of interesting stuff from ADI, but it seems that everyone sticks his name on the thing that he improved, and don't know of the final thing. It's a bit like everyone attaching Baird's name to every theatre and not knowing about television.

Cheers
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 27th Jan 2014, 2:56 pm   #76
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,869
Default Re: FET Questions

I think putting two dual-gate MOSFETs in front of a mixer on Band 2 is asking for trouble, unless you can be sure that stations will be sufficiently widely spaced in frequency that the excessive gain is more than compensated for by the improvement in RF selectivity.

Some years ago I designed and built an FM tuner, partly based on what I gleaned from the Ambit catalogues. I had a dual-gate RF stage, followed by a packaged diode mixer. The RF stage had a single tuned circuit before it, but a double tuned circuit between it and the mixer. The mixer was followed by a simple diplexer and common-gate J310 IF preamp. An LTP IF stage then fed into a Hitachi CA3089-like IF chip. My aim was something approaching communications receiver dynamic range, yet with hi-fi sound quality. Sadly, it is now compromised by a very high local noise floor during sunny weather!

Last edited by G8HQP Dave; 27th Jan 2014 at 2:57 pm. Reason: typo
G8HQP Dave is offline  
Old 28th Jan 2014, 1:50 am   #77
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,454
Default Re: FET Questions

Quote:
Originally Posted by G6Tanuki View Post
OK it's not FET but one of the late-1960s RSGB books I've got [Amateur Radio Techniques?] mentions a US "Fisher" FM-tuner where the antenna is fed through a single tuned-circuit then into a 4-hot-carrier-diode ring-mixer - the article mentions that this was done to give better strong-signal handling than could easily be achieved with current-generation bipolar devices.
Thanks. I was unaware of that one. I suspect that it was quite rare in the FM tuner world.

Quote:
Originally Posted by Radio Wrangler View Post
I'm not sure whether the MC1496 or the RCA CA3027/3028 family came first.

One thing is definite, they aren't 'Gilbert Cells'. The transistor tree mixer predates the Gilbert cell, and the Gilbert cell is a logarithmic current-based 2 or 4 quadrant linear mixer.

What Barrie Gilbert invented was the idea of adding diode connected transistors on the substrate so that currents could be converted bach to voltages with a logarithmic law which compensated for the antilog law of current partitioning in a long-tailed pair.

His cell is in 4-quadrant linear multipliers and a lot of interesting stuff from ADI, but it seems that everyone sticks his name on the thing that he improved, and don't know of the final thing. It's a bit like everyone attaching Baird's name to every theatre and not knowing about television.
Ah, I wasn’t aware of that distinction, but is does explain why Gilbert’s papers on the topic seem to have arrived at about the same time as ICs incorporating the transistor-tree mixer, and not before.

Many sources seem to miss this. This site is probably typical: http://www.radio-electronics.com/inf...multiplier.php. The diode addition is mentioned, but it is not shown as the key difference that makes a Gilbert cell out of a transistor tree. Traps for the unwary (such as me).

I think that the RCA CA3028 predated the MC1496, and was aimed mostly at military and commercial applications. I am not sure what was the target market for the MC1496. Not consumer, as then it would have had a MC13xx number. But MC15xx was used for industrial/military components, and there was an MC1596 that I think was a MIL-spec MC1496.

The CA3028 was quite versatile, and could be used as a cascode or differential amplifier with various agc options, as a differential limiter and as a singly-balanced mixer. As far as I know the Fairchild µA703 was differential only, and the Motorola MC1550 cascode only. RCA also had the CA3026 that contained a pair of “half-trees” that could be used together as a full-tree mixer, and was so deployed by Leak in the stereo decoder part of its Stereofetic tuner. I don’t know if RCA or others proposed using dual-gate mosfets for the mixing/switching part of stereo decoding, but Motorola released its first IC-base decoder (non-PLL type) around 1968, and that was the direction in which the industry went, moving to PLL in 1971.

Given that FETs and ICs both competed with each other and complemented each other in the transitional era, some more than passing discussion of ICs in this thread seems unavoidable. Leak well summed up the situation in the name that it chose for its first solid-state FM tuner.

Quote:
Originally Posted by G8HQP Dave View Post
I think putting two dual-gate MOSFETs in front of a mixer on Band 2 is asking for trouble, unless you can be sure that stations will be sufficiently widely spaced in frequency that the excessive gain is more than compensated for by the improvement in RF selectivity.
One gets that impression from the Ambit commentary on the EF5804, and its inclusion of PIN agc as well as regular agc. Yet two RF stages became the norm for “supertuners” by the later 1970s. To some extent that might have been marketing-driven, perceived as necessary to justify its positioning even if of dubious or even negative technical merit. That seemed to happen in the audio field, although as observed earlier in this thread, the upper end consumer HF receivers were not immune, and so they had features that were not found on professional equipment.

Whereas the use of two FET RF stages in FM tuners had no significant precedent in the valve era, conversely, the use of two RF stages and four gangs was common practice in valved HF receivers, more often in single conversion types with IFs around 450 kHz, but also in some double conversion types (e.g. Marconi) with 1st IFs around 1.6 MHz or even higher. On the other hand it would appear that the use of two RF stages in those HF receivers that used FETs in the early signal stages was very rare. Admittedly, the early FET era corresponded with major changes in HF receiver topology including upconversion to high 1st IFs, which eliminated the image problem, and the use of very linear mixers that could handle high signal levels and needed less “protection” from out-of-band signals, these doing away with the need for exceptional front end selectivity. But even where FET-based HF receiver signal pathways followed valve practice, single RF amplifiers were the order of the day, and the fourth gang was obtained by using bandpass inputs. This suggests that the FET mixers used in such receivers were less tolerant of large signals than their valve predecessors, and so would not be happy with too much RF gain. One example was the Eddystone EC958, which had a jfet/mosfet cascode single-stage RF amplifier feeding into a dual-gate mosfet mixer, with a 1st IF of 1335 kHz. On the HF bands, the input was double-tuned bandpass, with a single-tuned interstage between the RF and mixer stages. Given that Eddystone had used two RF stages (and 4 gangs) on its top valve receiver, the 880, which had a higher 1st IF, one might reasonably infer that the single RF stage choice for the EC958 had something to do with not overwhelming the mixer.

Marconi, notwithstanding its frequent use of two RF stages in the valve era, also used a single RF stage, dual-gate mosfet, in its Apollo marine HF receiver. It had a bandpass input and single-tuned interstage. In this case the mixer was an SL640 bipolar unit which might have needed a bit more protection than say a dual-gate mosfet. The Marconi Hydrus HF ISB receiver might be a useful case to study here. It seems to have been the solid-state successor to valve receivers such as the HR22, and according to the advertisements, it was FET-based. However, circuit information for the Hydrus seems to be unobtainium.

Anyway, perhaps the professional HF receiver examples with one mosfet RF stage ahead of the mixer are more indicative of the best trade-off for overall performance (within the boundaries of what was possible with mosfets), whilst the FM examples with two mosfet RF stages reflect the influence of perceived marketing needs overriding sound technical judgment.

Cheers,
Synchrodyne is offline  
Old 28th Jan 2014, 9:57 am   #78
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 12,704
Default Re: FET Questions

Lots of RF gain in a supertuner is OK just so long as you have a supermixer to handle it.

Very few of these firms were into careful design at the system level. A standard failing was that there were hundreds of articles and books written on planning the frequency conversions of a receiver and combatting VFO drift, and getting noise figure of a stage down, BUT there was a general lack of info on how to design a system for good dynamic range.

Consequently there are tuners, receivers etc where the gain/loss distribution and the distribution of selectivity aren't poorly thought out, they weren't thought out at all. A lot of choices were done by copying someone else's and maybe tweaking something a bit.

For planning such systems, there was a lot of tedious arithmetic needed, so it was fertile ground for what to do with that new computer the department had bought.
Later on, spreadsheets were great for the task. Here's the one I built and used.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 28th Jan 2014, 10:01 am   #79
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 12,704
Default Re: FET Questions

Ah, the screenshot didn't make it. Looking at the size, I think I see why. Here's a smaller JPEG

Nope, it's not going to work. Not enough resolution for it to be readable, and I don't want to set it loose on the world on an open forum in its .xls form.

Sorry

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

Last edited by AC/HL; 28th Jan 2014 at 1:11 pm. Reason: PS merged
Radio Wrangler is offline  
Old 28th Jan 2014, 12:12 pm   #80
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 7,888
Default Re: FET Questions

From the traditional valved HF communications-receiver perspective the "two RF-amps" thing had several purposes:

[1] To allow three loosely-coupled RF tuned-circuits ahead of the mixer. This helped reduce image response [which despite 3 tuned-circuits was still a problem at 25+ MHz if your IF was only 465KHz]

[2] To overcome mixer-noise [old multi-electrode mixers like the ECH35 and 6SA7 had equivalent-noise-resistances of 250,000 Ohms-plus and you needed significant RF amplification to get to a point where first-RF-tuned-circuit noise overcame mixer-noise]

[3] An extension of [1] - the additional pre-mixer selectivity could give reduced cross-modulation effects in the mixer [but paradoxically the extra pre-mixer amplification could increase the risk of it!]

[4] Pre-mixer selectivity provided greater isolation of the LO signal from the antenna - potentially important in some military/commercial applications where multiple receivers were fed from the same antenna [or if you were at risk of the enemy DFing your LO signal].

In the context of FET or bipolar VHF/UHF TV tuners, image interference [where extra tuned-circuits ahead of the mixer would be a Good Thing] never seemed to be a big issue - not in the UK anyway. Bands IV and V had national/regional station-planning so that you rarely had a significantly-powerful local transmitter on the image-frequency of another station serving your locality: though I must admit to having experienced image-reception of aircraft on a Band-II receiver.

I can see the logic in having two MOSFET RF amps in a broadcast-band tuner: the first one essentially being noise-figure-optimised [with the AGC delayed quite a bit], the second one being deliberately designed to provide relatively low-gain (tap the gate-connectors well down the tuned-circuits so you get the highest possible tuned-circuit 'Q') 'selective-matching device with a bit of gain' then into the mixer.

I must dig out some of the old diagrams and suchlike that I've got which relate to 1960s-vintage American PMR gear: the likes of GE and Motorola knew their stuff when it came to providing utterly bomb-proof-but sensitive VHF receiver front-ends and used circuitry which looks quite alien [4-chamber helical filters for example] when compared to something like a Pye Westminster [which used ordinary slug-tuned coils and a humble 2N3819 in the first RF-amp].
G6Tanuki is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:19 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.