UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV 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 6th Aug 2013, 6:21 am   #1
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default FET Questions

Before the actual questions, a preamble to set the background: dual-gate mosfets started to be used in consumer equipment around 1968, following by a year or a couple of years the initial use of junction fets. An early application was in FM hi-fi tuner front ends, as RF amplifiers and/or mixers. Whilst junction fets had also been used for the same applications, mosfets seemed to be preferred once they were available. There were also some applications in AM LF/MF and HF receivers, but these were more sporadic, I think.

In the USA, mosfets were also adopted early on for use in TV VHF tuners, as RF amplifiers and mixers, in particular to improve cross-modulation performance in an environment where most localities had multiple channels available. Reversion to valve tuner performance levels seems to have been the objective; bipolar transistor tuners falling well short of this. As far as I know, it was well into the 1970s before European TV makers adopted mosfet VHF tuners; I have a vague recollection that Tandberg might have been an early mover.

With regard to mosfets used as RF mixers, the conventional wisdom as I understand it is that the signal is fed to gate 1 and the local oscillator injected at gate 2, so that multiplicative mixing obtains. This seems to hold true generally for FM front ends, example attached, and MF/HF applications.

Thus I was surprised when looking closely at an American TV VHF tuner schematic (attached) to find that the mixer had the local oscillator injected at gate 1 along with the signal, and not at gate 2. I think that implies additive mixing, with the mosfet functioning in the cascode mode with a set bias on gate 2. Further checking across other sources suggested that this practice, if not universal, was quite widespread.

So the first question is, why would additive mixing, with both signal and local oscillator going into gate 1, be used for VHF TV applications, whereas multiplicative mixing, using both gates was normal for FM tuners and other applications?

To the second question, approximately when did dual-gate fets become available that were suitable (and economic) for use as RF amplifiers in domestic TV receiver UHF tuners? My own recollection is that it was in the late 1970s. The earliest example that I can find, attached, is from a 1979 publication, which tends to confirm this. And as a corollary question, what were the limitations that precluded the use of the early generation dual-gate fets at UHF? Was it that they were too noisy at such frequencies?

Thanks in advance.

Cheers,
Attached Thumbnails
Click image for larger version

Name:	Mosfet VHF TV Tuner.jpg
Views:	2892
Size:	85.2 KB
ID:	82594   Click image for larger version

Name:	Mosfet FM Front End.jpg
Views:	2220
Size:	101.7 KB
ID:	82595   Click image for larger version

Name:	Mosfet UHF TV Tuner.jpg
Views:	2094
Size:	84.6 KB
ID:	82596  
Synchrodyne is offline  
Old 6th Aug 2013, 8:42 am   #2
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

Frequency mixing is always a multiplicative process. The only real additive mixers are the things found in recording studios. Even if both signal and LO are applied to the same port of an active device, there is reliance on the non-linearity of that device forming product terms, and it is those product terms which are on the wanted sum-and-differece frequencies. If such a device had no non-linearities whatsoever, its output would simply be amplified versions of all its inputs, all on their original frequencies, all strictly proportional to the input amplitudes, and nothing else would come out.

Databooks have traditionally divided active analogue parts into 'linear' and non-linear' groups. Opamps and RF amps go into the linear book, True RMS detectors, logarithmic converters etc go into the 'non-linear' book. Mixer chips are put in non-linear, but is this right? We spend a lot of money in the pursuit of highly linear mixers, but linear things can't boogie. So that term can't be right either.

Huh?

In a radio system, we stuff a lot of signals together into one mixer port, and an LO into the other. We want perfect linear behaviour between the applied group of signals. We don't want them mixing with each other. But we do want mixing between the LO and each of the signal components in turn. It looks so far like an ideal mixer would be an ideal multiplier. X*Y=Z. This would let the amplitude of Z be proportional to the amplitude of X and also proportional to the amplitude of Y.

The dual-gate mosfet does this. It has imperfections and signals do intermodulate somewhat, but the mixer 'gain' is strongly influenced by the LO amplitude applied to gate 2. This means that if the paths to gate 1 and gate 2 are swapped, normal mixing service is continued, and that TV tuner works.

I suspect that LO-in-gate-1 circuit may have been inspired by bipolar practice where LO modulation of the quiescent current of a bipolar device modulates its Gm, and hence modulates its gain... and that is multiplication, and that is mixing. Because of its characteristic curves, both ports of the mosfet modulate the Gm of the other. Signal to gate 1 was found to allow a larger LO voltage to gate 2 and you got a bit higher gain for most common devices, so this architecture got set in stone and is used without thought.

But mixers don't have to be this way.

The LO port carries only one frequency at a time, and it does only non-linear things with the signals in the other port. The LO port doesn't have to be linear. THe LO port could have a large input, could have large gain and could be driven into limiting so the LO function looks like a square wave when it meets the signals. Voila! we have the switching mixer.

At first glance, this doesn't look good, the signals are going to mix with all those LO harmonics as well as the LO fundamentals. There is going to be hell. In reality out normal mixer devices do quite well at making LO harmonics anyway and squarewave LO, or switching mixing, however you prefer to think of it only makes the unwanted stuff several dB bigger. We want to filter unwanted products and want to get them maybe 100dB or more down in a really good receiver, so the squarewave LO only makes things a little worse in this respect. What it does in other respects is more interesting, though less obvious. It's possible to use circuitry which gets hammered into on/off switching by the LO, but is usefully more linear in the matter of interaction between different signal components. These mixers have a good linear signal flow almost without intermodulation from the signal path input and out the IF port. The LO however goes straight into a high-gain clipping amplifier.

Swap signal and LO on one of these mixers and there will be devastation.

If you have a play with a diode ring mixer you can interchange the three ports and use any for LO, IF, RF. Put in a big LO drive to any port and it will start driving diodes into switching, the other two ports work fairly linearly. One port works down to DC but the other two are transformer coupled, so this may force the choice of which port to use, and then you choose based on isolation of leaked LO. This must be the ultimate example of swapping signal ports in a mixer.

For a cheap mixer for mass-produced stuff where every ha'penny breaks a director's heart-string, the dual gate mosfet offers a bit better performance than a bipolar. I remember an article in one of the Mullard transistor radio and audio circuits books on how bipolar mixers could be run to lower currents than mosfets, and on lower voltages, offering better battery life. It was a bit of a rear-guard action in some respects. Battery portables stayed with bipolars, but mosfets took over everywhere.

The first dual gate mosfets I came across were all RCA, with RCA all-numerical type numbers.

It took a while before mosfet gate capacitances came down, and cases got low-enough inductance to make them useful at UHF. Bipolars operate at higher current densities in the silicon, so for a given current capability, their structures are smaller so capacitances can be made lower. Bipolars also have secondary carrier lifetime limitations, giving the Ft sort of behaviour. Mosfets just start looking like RLC networks rolling-off.

Quite early on Jfets and Mosfets could beat the noise figures that valves could offer. That was relatively easy. Amateurs stopped making 2 metre converters with 6CW4s and whatever exotica they could find, and switched to the 2N3819. It was a while before the loss of big-signal performance and overload effects were noticed.

In the US, the available tv channels were much more heavily populated. In the UK with restricted numbers of programmes, so UK channel assignments were more carefully managed from the beginning to protect the image frequencies of standard tuners and to dodge some of the intermod problems.

In the UK, mosfets went into TV tuners when their gain got good at those frequencies. Noise figure wasn't a limitation at all.

For ultra high performance receivers, the dual gate mosfet never got a look-in. The diode ring was king for a long time. Hugh Walker and Guy Douglas got them up to the +40dBm third order intercept point level of performance in test equipment, but since then, switching with DMOS devices as passive switches have come to the fore. Rafuse published a paper on DMOS in place of diodes in rings back in 1968. Racal used his ideas in the wonderful RA1772. In the 1990s several people independently invented a better structure for those DMOS parts, and Colin Horrabin was the guy who published it. He called it 'H-mode' If you stump up 9 grand for a posh ICOM transceiver, or Yaesu, or Trio/Kenwood. Guess what's inside to give it that claimed +40dBm TOI point?

Both typing fingers now sore!

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 6th Aug 2013, 10:02 am   #3
ukcol
Rest in Peace
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
Default Re: FET Questions

Quote:
Originally Posted by Radio Wrangler View Post

Both typing fingers now sore!

David
Both my brain cells are now sore just reading post 2.

What a fascinating subject and hats off to the level of knowledge demonstrated. My knowledge in this area is not at this level and so sadly I am not able to contribute anything useful.

Colin M
ukcol is offline  
Old 6th Aug 2013, 12:41 pm   #4
lesmw0sec
Octode
 
Join Date: Jul 2009
Location: Carmel, Llannerchymedd, Anglesey, UK.
Posts: 1,498
Default Re: FET Questions

I recall using dual-gate fets for radio-mic receivers in the 70's, so they must have been well established then.

Les.
lesmw0sec is offline  
Old 6th Aug 2013, 1:04 pm   #5
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: FET Questions

Quote:
Originally Posted by ukcol View Post
What a fascinating subject and hats off to the level of knowledge demonstrated.
Seconded!

Quote:
Originally Posted by Radio Wrangler View Post
Frequency mixing is always a multiplicative process. The only real additive mixers are the things found in recording studios.
I'll have to mildly disagree here - mixers found in a recording studio are linear, additive (the output is proportional to the sum of the inputs). An RF frequency-changer mixer can also be additive - the output is a function of the sum of the inputs, it's just not a linear function.

Frequently, one of the signals (the oscillator) is much larger than the other, and it turns out that the amplitude of the wanted output signal (ie the IF component) is approximately proportional to the amplitude of the small input signal (the RF input).

Probably the simplest additive mixer is got by just putting the two input signals (oscillator and RF) in series - clearly adding them - and then putting the sum to an output circuit via a diode. The diode provides the non-linearity. The instantaneous diode current is a function of the sum of the instantaneous input voltages, so it's additive. A big oscillator amplitude - you can even make it square wave - creates a infinite spectrum of output frequencies, but a bit of Fourier analysis shows that the amplitude of the wanted component is, as I said, proportional to the RF input, as long as it is much smaller than the oscillator amplitude. That's what we mean by a 'linear' RF mixer.
kalee20 is offline  
Old 6th Aug 2013, 5:35 pm   #6
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

It's a matter of how you look at it, but if two signals are applied voltage-additively to a non-linear function, then one of the terms created will be simple product of the two voltages, and that resolves by the the usual trig identities to two components at the sum and difference frequencies. Of course there are other components but the sum and difference creation is mathematicalli identical to a multiplication process. I suspect that every frequency mixing process involves either a multiplication process or else something with a term mathematically identical with one. I think I'm getting close to the business of 'were William Shakespeare's plays written by him, or were they by someone with the same name?' If sum and difference terms are formed, the trig identities prove it was identical to multiplication if we logick backwards.

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 7th Aug 2013, 6:06 am   #7
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FET Questions

David:

Thanks very much for taking the trouble to pen your comprehensive and in-depth response. I’ll freely admit that I am stretched to take it all in; well out of my comfort zone.

Here is more detail on what I have found on the chronology of fet applications; it was too much to put in the initial posting. As mentioned, FM tuners seemed to be early beneficiaries. An early UK example was the Rogers Ravensbourne, from 1968 (or perhaps even late 1967), which used dual-gate mosfets for both the RF and mixer stages. The following and lower-price-point Ravensbrook model, from circa 1969, had a dual-gate mosfet RF stage but a bipolar mixer. In that case the determination seemed to be that where only one mosfet could be afforded, it was best used for the RF stage. It also suggested that at the time, mosfets were significantly more costly than bipolar transistors, such that there use needed to be rationed, as it were. Before Rogers though, in early 1967, Radford had offered a special export market version of its silicon bipolar FM tuner that had a fet front end. I don’t know the details, but it seems more likely that it was based upon jfets. The Leak Stereofetic of 1969 had a dual-gate mosfet mixer, but unusually a single-gate mosfet RF amplifier. Then Sugden, with the R21/R51 in 1970, and Quad, with the FM3 in 1971, both used dual-gate mosfets for RF and mixer stages, by which time that configuration seemed to have become the norm. The RCA 40822 as RF amplifier and 40823 as mixer seemed to be a common pairing, as well. As an aside, this group of tuners shows interesting progress in integration of the non-front end parts of the circuit, the 2½th iteration of the Quad FM3 reaching what was something of a plateau in terms of basic form. Leak and Quad at least had held back from producing solid state FM tuners until valve performance could be matched. The inference is that this could not be done with bipolar front ends; the mosfet was the enabling feature. Outside of the UK, in 1968, Sony updated its ST-5000 FM tuner to the ST-5000F with a fet front end, using jfets I think but I am not sure. I have the impression that some of the Japanese equipment makers entered the fet FM front end era with jfets, including cascode RF amplifiers, before migrating to mosfets. Bang & Olufsen changes to a fet front end (jfets I think) around 1968-69. Apart from the fact that jfets probably became economic for (sparing) use in consumer products ahead of mosfets, I wonder if the perceived fragility of the latter – at least the early versions – steered some makers towards using jfets even when mosfets were available.

In MF/HF applications at the consumer level, the Drake SPR-4 of 1969 seemed to be an early example. It used dual-gate mosfets throughout the RF and IF chain except for the mixer, which was of the push-pull jfet type. In the late 1960s and into the 1970s, few makers of hi-fi equipment appeared to pay more than cursory attention to the AM side of tuners, where fitted, so fet applications there were fewer. But the initial (1975?) version of the Tandberg TR-2075 tuner-amplifier had a dual-gate mosfet MF RF amplifier, as recorded in one of the Gordon J. King books. The Sansui TU-X1 “supertuner” of circa 1979 had a dual-gate mosfet AM RF stage feeding an IC mixer. One curiosity though is that hi quality wideband AM-only tuners of the 1970s (a rare species), such as the McKay-Dymek AM5 and Wright LDT-3A used jfets not mosfets in their RF and IF stages.

Professional HF receivers from 1968 or thereabouts that included fets were the Eddystone EC958, Marconi Apollo and Marconi Hydrus. Eddystone used dual-gate mosfets for all three mixers and a jfet + single-gate mosfet cascode rf amplifier. Apparently the latter was chosen so that it could handle very big rf signals. The same combination was used in the following 1830 and 1000 series receivers. The Marconi Apollo had a dual-gate mosfet RF amplifier, followed by an IC mixer. It would seem that the EC958 and Apollo represented a final stage of traditional professional HF receiver design, with much attention paid to RF selectivity ahead of the mixer, an approach that was soon overtaken by the Racal RA1772, which as David has said, went well beyond what could be done with fets. (The related article at: http://www.premium-rx.org/marconimishap.pdf makes for interesting reading.) The Marconi Hydrus I know little about other than it was a point-to-point ISB receiver that was advertised as using fets, and had upconversion to a 40 MHz 1st IF (synthesized or Wadley loop?).

Use of mosfets in American VHF TV tuners appears to have started in 1968-69. As well as RCA, TI was an advocate of this approach. TI was also a proponent of the use of mosfets in TV IF strips, which also started at around the same time, but had to compete with ICs, in particular Motorola’s MC1350 and MC1352. I think that the IC approach won after a few years. TI’s initial TV mosfet set was 3N201 (RF amplifier), 3N202 (mixer) and 3N203 (1st and 2nd IF stages. The 3rd IF stage was bipolar. The second set, all with higher gain, added a 3N313 for the 3rd IF stage, along with the 3N211 (RF, 1st & 2nd IF) and 3N212 (mixer).

The advent of mosfet RF amplifiers in American UHF TV tuners seems to have been associated with a step change in their configuration. Whether this was simply a result of suitable mosfet availability or whether it drove mosfet development is unclear, but I suspect the latter given the FCC’s efforts to make UHF transmissions more accessible. Through the valve era through the bipolar solid sate era, typical American UHF TV tuners lacked an RF amplifier, and the signal was passed via bandpass input to a crystal mixer fed by a valve or bipolar local oscillator. The IF then went via the VHF tuner, including the RF stage to provide the necessary gain. So the addition of a mosfet RF amplifier and another tuned circuit would, one should think, have improved performance noticeably.

Referring to David’s comment about adequate gain being the main determining factor that in the application of mosfets at UHF, it could be that in the American case, the need for better tuner performance without introducing cross-modulation problems means that mosfets were accepted when they reached an earlier point on the development curve than was the case in Europe. With a history of including RF amplifiers in UHF TV tuners in the valve and bipolar eras, the European makers may have deferred somewhat the change to mosfets until the latter could match or better established bipolar transistor gain.

Cheers,
Synchrodyne is offline  
Old 7th Aug 2013, 9:05 am   #8
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

Interesting!

The dual-gate mosfet came along and replaced the bipolar device in a number of consumer receivers around that time. What we've overlooked so far is that it also offered easy and effective AGC. Compared to the bipolar designs that came before, the mosfet receivers were a step up in performance, and once production quantities were rolling, the price fell. I remember the RCA parts being first. The interesting thing is that used properly, bipolars were capable of out-performing the dual-gate mosfets. But this meant using greater numbers of parts and larger amounts of power.

The mosfets also operated directly in high-impedance RF networks, so they connected directly into high-Q tuned circuits. They were components that offered a wide package of advantages and only one real problem.

The problem was that the quiescent current was not well controlled. The gate threshold voltage was widely variant. Most circuit designers didn't bother much and allowed quiescent current to vary quite a bit from unit to unit, and it was typically much more than the predecessor bipolars. Mosfets went into domestic hifi tuners and then TVs, portables stuck with bipolars.

The latest dual gate mosfets include low voltage parts for 5v supplies, using a miniature version of themselves as a current mirror on gate 1, so that gate 1 is biased with a small current and the main device biases for a multiple of it as quiescent. I used some of these in a recent transceiver design. They do the job well.

Jfets also have the problem with large variation in pinch-off voltage and Idss.

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 7th Aug 2013, 1:26 pm   #9
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: FET Questions

Quote:
Originally Posted by Synchrodyne
One curiosity though is that hi quality wideband AM-only tuners of the 1970s (a rare species), such as the McKay-Dymek AM5 and Wright LDT-3A used jfets not mosfets in their RF and IF stages.
MOSFETs have a reputation for having a high 1/f noise corner frequency. Although low noise at VHF and UHF they may be no better (or perhaps worse) at much lower RF frequencies. JFETs are good from RF down to audio.
G8HQP Dave is offline  
Old 7th Aug 2013, 1:49 pm   #10
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

Another quirk is that Gallium Arsenide devices have worse noise figures than can be achieved with silicon devices below about 300-500MHz.

The Agilent Noise Figure Analysers have one of each type as a preamp, and switch between them at 500MHz

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 7th Aug 2013, 10:23 pm   #11
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: FET Questions

Yes, there was a DIY fashion for a while to use GaAsFETs for VHF - where they are likely to be inferior to a bog-standard silicon MOSFET. Tech bragging rather than good engineering.
G8HQP Dave is offline  
Old 7th Aug 2013, 10:29 pm   #12
turretslug
Dekatron
 
turretslug's Avatar
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
Default Re: FET Questions

I also recall the big hi-fi names falling over themselves to shout "GaAs- FET!" in their FM tuner blurb. BS-FET, perhaps.
turretslug is offline  
Old 7th Aug 2013, 11:25 pm   #13
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

Their noise goes up like a ski-jump as frequency falls.

Some hifi got designed on religious grounds by 'true believers' of something or another, some got designed by marketing departments desperate for one more bullet-point in the advert.

The sane stuff, the stuff designed by people who understood what they were doing, the Baxandalls of this world, is precious.

The trouble is that the true believers are noisy because they only feel secure if they can get everyone converted. The marketing departments are noisy because they get paid to be. Somewhere in the mountains of repetitive hype, the good stuff is hard to see.

GaAs in FM tuners had me in tears when I first read of it.

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 8th Aug 2013, 6:23 am   #14
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FET Questions

I vaguely recall that the initial use of fets in UHF TV tuners was with GaAs devices.

Re mosfets vs jfets at lower frequencies, perhaps any jfet advantage manifests itself somewhere in the MF to lower HF region? That would explain the empirical evidence that jfets were used for the RF and IF stages of wideband, hi-fi AM tuners whilst mosfets found their way into HF receivers quite early on. In the latter case the focus could well have been minimizing noise at the higher end of the HF range.

At AF, I recall the Linsley Hood Wireless World articles in the 1980s about the then newer op amps that were candidates for audio applications. The TI TL071 series, with jfet input, was a bit lower in noise than the RCA CA3040, with mosfet input, I think. But Hugh Walker had something to say about jfet 1/f noise (with low impedance sources) in the WW Stereo Mixer article recently pointed to by Radio Wrangler in this thread: https://www.vintage-radio.net/forum/...066#post623066.

Fet (jfet) use in audio equipment also dates back to circa 1968, seemingly adopted where high input impedances were beneficial. An early example was the Ferrograph Series 7 tape recorder, which had a fet input stage (source follower I think) for its line input. Whilst the 2M2 impedance of this input would have allowed the direct connection of (self-equalized) ceramic cartridges, I suspect that high impedance was also desired to allow backwardly compatible interfacing with valve amplifiers and control units. Typically these had unbuffered tape record outputs (e.g. from a triode or pentode anode) and needed to look into 500k or more. Some confirmation that this was the case is provided by the contemporaneous Revox A77, whose auxiliary input had a 1M impedance, in this case achieved by bootstrapping the input of a bipolar amplifier. Ferrograph repeated the fet input in its 1969 F307 amplifier, in that case for the purposes of matching ceramic cartridges. Tripletone, whose budget amplifiers were oriented to use with ceramic cartridges, and had previously used a bootstrapped emitter follower input, was another early adopter. If I remember this correctly it used a simple Mullard IC that had a jfet input stage. Also I think Tripletone had a budget FM tuner with a mosfet RF stage quite early on.

Returning to mosfet mixers, possibly the applying of both signal and LO to gate 1 in VHF TV tuners was a carryover from bipolar practice. Evidently cascode bipolar mixers, with both signal and LO going into the common emitter stage, had been used in American practice, as indicated by the attached excerpt from Benson & Whitaker (1). Another excerpt from the same book shows a cross-modulation performance comparison of various devices in VHF TV tuners.

Re the “multiplicative” and “additive” terminology for mixers, older texts, such as Witts (2), use the term multiplicative for such as hexode/heptode mixers where LO and signal are fed to separate grids, and additive for such as triode and pentode mixers where both the LO and signal are fed to the control grid. But as already said, one cannot get to say sin (A±B) from sin A and sin B without there being a multiplying step, and multiplication does not happen unless the transfer curve is non-linear.

If one then views “mixer” and “multiplier” as being somewhat interchangeable in this context, then perhaps an adequate working definition for an “additive mixer” that reasonably respects both actuality and tradition is one in which the two inputs are added before being multiplied, this addition often happening at the grid/base/gate of the multiplier. Against that, “multiplicative mixer” looks to be a bit awkward, tautologous even, but the working definition could be a mixer in which the two signals are not added before multiplication, but are applied to separate ports of the multiplier.

Cheers,


(1) K. B. Benson, rev. J.C. Whitaker; Television Engineering Handbook, Revised Edition; McGraw-Hill, 1992; ISBN 0-07-004788-X.
(2) A.T. Witts; The Superheterodyne Receiver, Seventh Edition; Pitman, 1961.
Attached Thumbnails
Click image for larger version

Name:	Benson & Whitaker p.13.90.jpg
Views:	931
Size:	39.1 KB
ID:	82661   Click image for larger version

Name:	Benson & Whitaker p.13.88.jpg
Views:	707
Size:	53.4 KB
ID:	82662  
Synchrodyne is offline  
Old 8th Aug 2013, 7:33 am   #15
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

Rafuse's paper in 1968, in the International Journal of Solid State Circuits makes interesting reading. He uses DMOS fets in a ring circuit, simply grafting new 2-port devices into a topology that became popular for minimising issues with 1-port devices - diodes. He measured the intermodulation performance and compared it to the Collins R390, which did not come out well at all. The R390 has a rather soft front end like most equipment of that era, though criticising the R390 is like hunting pandas with blunderbusses in the eyes of some people.

There wasn't much HF gear that used dual gate mosfets in RF or mixer stages, It was fairly apparent that they had not got the dynamic range needed at the top end. It was around this time that the 'Big Three' in Japan were taking over the amateur radio market and they quickly moved from valves to diode rings. Mosfets were commonly used in IF stages after 9 (ish) MHz crystal filters because their high impedances made matching and gain easy, but most of all, they were good for AGC.

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 13th Aug 2013, 5:43 am   #16
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FET Questions

Mention of the Japanese OEMs prompted a look at the schematic for the JRC NRD525 HF receiver. This was a mid-1980s design that was generally viewed as being a bit better than the best contemporary HF receiver offerings from ICOM, Kenwood (Trio) and Yaesu Musen. On the other hand, I don’t think that it pretended to compete with the fully professional receivers such as those from Racal, Plessey, etc., and so would not match them in terms of front end performance.

In its class the NRD525 was a little unusual in having a fully tuned (varactor) bandpass input that tracked with the main tuning control, rather than say the octave filters that ICOM used. In a way I suppose it was a throwback to pre-RA1772 design practices. But this tuning circuit could be bypassed if desired. Then came an RF amplifier that was of the push-pull, grounded gate jfet type, without agc, which fed the 1st mixer, also of the push-pull jfet type with signal fed to the sources and local oscillator to the gates. There was one gain stage at the 70.455 MHz 1st IF using a dual-gate mosfet with agc, feeding the 2nd mixer which like the 1st, was also of the push-pull jfet type but with signal fed to the gates and local oscillator to the drains. Then followed three gain stages at 455 kHz, all dual-gate mosfets with agc. All four agc’d stages were fed from the same agc line; there was no grading. There was also a sidebar 70.455 kHz strip feeding the noise blanker, this having a dual-gate mosfet followed by two bipolar stages, all with agc developed within the subsidiary strip.

So the NRD525 does seem to have been illustrative of the general situation. Dual-gate mosfets were used as agc’d IF amplifiers, but jfets were used for the RF amplifier and the mixers. The use of two different mixer configurations is interesting, and I wonder if it has something to do with the fact that the 1st mixer had a potentially wideband input and so needed to be very linear, whereas the 2nd mixer had to deal only with a narrow band input (the 1st IF filter had something like 12 kHz bandwidth) and more-or-less a single local oscillator frequency, thus perhaps allowing some sacrifice of wideband linearity for say extra conversion gain?

Cheers,
Attached Thumbnails
Click image for larger version

Name:	JRC NRD525 RF & M1.JPG
Views:	1824
Size:	57.6 KB
ID:	82858   Click image for larger version

Name:	JRC NRD525 IF70.455 & M2.JPG
Views:	836
Size:	33.4 KB
ID:	82859  
Synchrodyne is offline  
Old 13th Aug 2013, 7:25 am   #17
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

The RA1772's real thing was a surprisingly good mixer, better than any of its predecessors, but in terms of the input stages it was very similar with a hand-operated preselector that wasn't ganged in any way to the main tuning. You tuned the receiver and then peaked the preselector yourself. Under remote control or scanning around, you needed to switch out the preselector and suffer anything it had been protecting you from.

I have grave doubts about the usefulness of varactor tuned preselector filters given the dynamic ranges on HF. Marconi published an interesting paper in their review studying levels found across the HF region and receiver overload causing loss of reception. The figures were fair, but it was famous for B M Sosin trying to push his wonderbox, the H2900 receiver by taking a poke at the competitor's performance. Famously Marconi backpedalled and sent a new expurgated edition out and asked recipients to destroy the old copies. Yeah, right.

In the late '80s early '90s the 3SK125 was the RF amplifier FET of choice, and also used in push-pull mixers. THe NRD525 and family as well as the IC765 by my elbow used them. Their performance was quite good, but better was about to happen. The Japanese designers had a terrible tendency to say 'Well that's the signal through a crystal filter, we can forget about linearity, now' and so the closer-in intermod performance degraded terribly. THe audio distortion was bad too.

The designers liked to give customers variable bandwidth IF filters, and did it by sliding two filters at 8 point something MHz and 455kHz against each other. So you wound up with 16 poles of crystal filters and bad ringing and group delay unflat enough to even become audible. Ugh!

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 13th Aug 2013, 10:13 am   #18
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: FET Questions

It's always interesting to compare designs: my RA217 has a manually-tuned preselector which then feeds two simple bipolar transistors as a RF amp ahead of the first balanced-diode mixer. There is a "wideband" position on the preselector but that's unusable when you've got a 100-foot longwire attached!

I wonder how well960s/1970s FET mixers compared on HF to the approach used in the Atlas 210/215 transceivers - no RF gain, preselector straight into a 4-diode double-balanced-mixer with lots of l.O. drive, then into the first IF filter. It always seemed to make sense to me to put the signal into a nice narrow filter as soon as possible.
G6Tanuki is offline  
Old 13th Aug 2013, 12:52 pm   #19
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: FET Questions

The RA217 didn't have many fans, even at Racal! Strong signal performance was considerably worse than its predecessor RA17 series kit. It was solid state, though....
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 13th Aug 2013, 2:03 pm   #20
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: FET Questions

I have an RA1217 in the collection. It's an interesting set, but certainly not one for big antennae and 40m.

I suppose I got one because one of the Racal people involved was a lecturer at uni and there was one in the lab. It looked rather impressive in 1972!

The Atlas got a lot of things right. THe G3PDM receiver in Radcom/The bull is also worth a look.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 4:08 pm.


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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.