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Old 7th Dec 2014, 10:26 am   #21
Radio Wrangler
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Default Re: Quasi-Synchronous Demodulation

Horses for courses as they say.

Plain diode:

These can be viewed in several ways. Think of it as a rectifier working on the IF waveform followed by a filter to dump the IF frequency components and to let the envelope waveform through. Or think of it as a switch being beaten up by the large carrier component and switching the smaller sidebands on and off at the carrier frequency. Or think of it as a square law device where the carrier and sideband components mix and intermodulate, Each sideband mixing with the carrier gives a baseband component, the other one of a pair adds extra and in the right phase. They also make components around twice the IF, which the lowpass filter hunts down and kills. All of these views are valid. SSB proves that to shift the information, you don't need both sidebands and you don't need a carrier. You can look on an AM transmitter as something which sends the carrier reinsertion signal needed by your product detector packaged neatly along with the signal itself.

Switching Demod:

THe diode above can be improved by making it into a switch. Extract the carrier, filter and amplify it then use the big squarewave to operate a switching mixer. This can give a very highly linear mixer and can operate with signals below what would be the threshold of a plain diode demod. Getting the phase right and keeping it right over a range of signal levels is the difficulty. The carrier path is a limiting amplifier and AM to PM conversion is a problem.

Flywheel:

In the carrier reinsertion switcher above, extraction of the carrier involves filtering. Make the filter narrow and it will ring to cover up any wobblies of the incoming signal's carrier. This can be good if the wobblies are in the amplitude dimension. It can also be bad if the sidebands have the same phase wobblies as the carrier because then a less filtered carrier would track them.

PLL:

This is the extreme case of the flywheel carrier reinsertion. Good for fading which drops the carrier and maybe one set of sidebands, poor for phase wobblies.

Pythagoras:

Most digital receivers downconvert the digitised IF (or sometimes RF) and make two channels centred on zero hertz. Usually called I (in-phase) and Q (Quadrature) to demod AM from these, any of the above schemes can be executed in digital form, but the simple rectifier approach needs to look at the amplitude of the vector sum of I and Q data, so Pythagoras' well known thing about hypotenuses is used. Square the numbers of both I and Q channels, Add the results, and take the square root of that. Unlike the diode, it doesn't have a problematic lower threshold and is quite linear if you don't take too many short cuts in the arithmetic.


So there are AM demods for all reasons. Do you want to quickly track phase variations? Do you want to steam-roller across amplitude variations? Do you want cheap? or aare you painting-by-numbers?

Which you choose depends on what you think the biggest problem is going to be.

David
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Old 10th Dec 2014, 12:54 am   #22
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Default Re: Quasi-Synchronous Demodulation

Thanks for the nice summary of AM demodulator types.

Regarding the PLL type fully synchronous demodulator and its handling (or not) of PM, that came up in the discussion I had with the Liniplex designer. The Liniplex demodulator was designed to track relatively slow carrier movements (e.g. Doppler shifts) but I was advised that it would not follow phase modulation, such as data transmissions piggybacked on LF and MF transmissions, or AM stereo transmissions. Thus any PM on the signal would be demodulated in the Q channel. As well as transmitted PM, this applied also to oscillator phase noise. That was of no consequence in the DSB mode, which used only the I-channel demodulator output, but it did show up in the LSB and USB modes, which used both the I and Q channel outputs. The Liniplex was actually very tenacious once locked on to a transmission. A good test of HF receiver PLL demodulators was how they handled the Big Ben chimes on the BBC World Service. The Liniplex F2 stayed locked, but the Sherwood SE3 often unlocked briefly. The Sony ICF2010 almost always unlocked and wandered around for a while. I suspect that it unlocked when it saw the Big Ben chimes coming...

As an example of “horses for courses”, the ICs used for C-QUAM AM stereo decoding used an interesting mix of techniques. Envelope demodulation was wideband quasi-synchronous, using limited but unfiltered signal as the reference, so that none of the (stereo) PM came through into the envelope audio channel. But a PLL was used for I and Q demodulation, presumably because precise phase and phase relativity were required.

When PLLs were used for TV vision demodulation, a wider PLL bandwidth of around 500 kHz was suggested by National Semiconductor in respect of its LM1823 IC to ensure that incidental phase modulation (ICPM) was tracked by the reference and so did not transfer to the output. Interesting here is that the BBC, for its rebroadcast TV receivers, moved from PLL fully synchronous demodulation (in the RC5M-502) to quasi-synchronous demodulation (in the RC5M-503) in which the carrier was extracted by a narrow band filter and conditioned in a low-phase shift limiter, with the objective of minimizing the effects of ICPM. Again a “horses for courses” choice, I think.

The problem of AM-to-PM conversion during limiting in the carrier path for the quasi-synchronous case prompted the thought that this problem might have been eased by the availability of integrated circuit differential pair limiters. In the late 1960s and early 1970s, RCA often made the point that such ICs provided symmetrical limiting – and lots of it - over a wide range of input signal strengths. That might explain why ICs that incorporated wideband quasi-synchronous demodulators, that is with the reference obtained by hard limiting unfiltered input signal became quite common from the late 1960s. Previously, with valve or discrete solid state technology, wideband quasi-synchronous demodulators without carrier filters seem to have been scarce.

Cheers,
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Old 11th Dec 2014, 3:39 pm   #23
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Default Re: Quasi-Synchronous Demodulation

I never worked on TV demodulators, but from what I recall, there are several factors which need to be simultaneously addressed, and one of some importance (esp. in the early days of UHF TV Rx's) was phase-noise.

If you construct an "ideal" synchronous demodulator, and the incoming IF has significant LF phase-noise (predominantly from the UHF tuner's low-cost VCO), the side chain should ideally track the phase-perturbations to avoid pm- to-am conversion. I think a quasi-synchronous demod with a not-too-high Q meets this criteria, in that LF phase-noise is effectively tracked, but HF (e.g chroma and sound carriers) are not.


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Old 11th Dec 2014, 11:55 pm   #24
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Default Re: Quasi-Synchronous Demodulation

That phase noise issue is nicely illustrated in the block schematic for the BBC RC1/511 TV receiver, copy attached.

This had a consumer-type front-end module (Mullard U321) and used quasi-synchronous vision demodulation in which the carrier IF channel was separated from the main IF channel after amplification and processed through a narrower bandwidth filter.

The highlighted part of the schematic includes the carrier filter, with the caption “B.W. ½ MHz centred on vision carrier (allows Ø noise through).”

Other important QS demodulation features shown are the carrier channel phase adjustment ahead of the filter and limiter, the filter ahead of type limiter, and the limiter being of the low-phase shift type.

Not shown, but I imagine very likely included in the carrier channel, perhaps simply by shaping/offsetting of the bandpass filter, would have been an “anti-Nyquist” filter that cancelled the Nyquist slope imparted by the main IF filter and so eliminated spurious PM from that source. The 500 kHz filter bandwidth was well within the DSB portion of the vision signal (±1.25 MHz) so there was no PM from inherent sideband asymmetry.

The carrier channel filter bandwidth chosen by the BBC, 500 kHz, was that same as that suggested by National Semiconductor for the PLL bandwidth of its LM1823 IC to enable it to better deal with ICPM.

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Old 13th Dec 2014, 1:24 am   #25
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Default Re: Quasi-Synchronous Demodulation

Continuing with this theme, slightly wider bandwidth appears to have been used over the vision carrier in consumer-type quasi-split sound (QSS) SAWF filters of the double-humped variety (camels for courses?) Eyeballing a typical SAWF curve (attached) suggests something around 1 MHz or slightly more at 6 dB down. But that was still well within the DSB portion (±0.75 MHz narrowest case) of vision signals, and well below the colour subcarrier and its sidebands. So PM caused by sideband asymmetry, the Nyquist slope and the colour subcarrier were avoided. Possibly mass production tolerances and the need to avoid too many factory-setup adjustments had something to do with the bandwidth choice.

In contrast. in the conventional wideband quasi-synchronous demodulator, represented by the MC1330 IC and the like ICs that followed it, the reference was taken from the main IF channel after the customary bandpass filtering and amplification and was then limited in “as found” condition as it were, meaning that it was phase modulated because of the sideband asymmetry, exacerbated by the Nyquist slope, as well as carrying any ICPM including phase noise. The post-limiter tank circuit was then used to minimize the nastier PM sidebands, particularly the colour subcarrier, but it did not undo the PM caused by the Nyquist slope, which adversely affected the intercarrier sound channel and was the undoing of this type when multichannel sound arrived.

One might say that with the MC1330 and its post-limiting carrier tank circuit, Motorola had adopted the “cure (partial, anyway) rather than prevention” approach. To put this in perspective, though, Motorola’s objective was not solely to produce a near-immaculate vision QS demodulator. Rather it wanted an IC that could replace the 3rd vision IF stage and the customary diode demodulator in existing circuits, with demodulation done at relatively low level and some of the overall gain provided by post-demodulation video amplification. It already had the MC1350 and MC1352 ICs that replaced the 1st and 2nd vision IF stages. These had been preceded by the MC1550, which was intended to replace a single TV vision IF stage, providing agc with virtually no input impedance change over the agc range, but apparently the setmakers showed little interest in a three-transistor IC that would replace but a single circuit stage. So Motorola was looking to develop multifunctional ICs that could replace two or more stages. The wideband QS demodulator, as outlined by Sprague and Macario, fitted the need for a low-level demodulator that was also relatively simply. The wideband carrier channel avoided the need for pre-limiting phase adjustment and a single stage of limiting minimized phase shift. The net result, although perhaps not a text-book implementation, was enough better than diode demodulation – mostly because it was effectively of the exalted carrier type - to be a significant step forward. In fact the MC1330 set a precedent for subsequent vision demodulator ICs, such as the TBA440, TCA270 et al. Interestingly, Bilotti of Sprague, when proposing the ULN2111A IC for use as a QS AM demodulator, had noted that with the chosen method of carrier recovery, that is limiting of the full bandwidth input signal, correct demodulation of the envelope would occur only in the case of a DSB signal, implying that any sideband asymmetry would cause errors.

The BBC had an earlier general-purpose TV receiver, the UN1/642, which incorporated the MC1330 vision demodulator. Information about it is available here: http://www.bbceng.info/EDI%20Sheets/10089.pdf; and here: http://www.bbceng.info/ti/eqpt/UN1_642.pdf. It was said to include several new design features, including “Low-level, exalted-carrier balanced demodulation, giving a reduction in quadrature distortion compared with earlier types of circuit.” It appears that the use of the MC1330 was justified by that benefit alone.

I have not found comparable information on the RC1/511, but the brief commentary available here: http://www.bbceng.info/Designs/RDCE/...0/ipage_50.htm shows that it was designed with Teletext in mind, one requirement for which was a low-distortion demodulator. That would explain the move from the MC1330 to a more exacting type of quasi-synchronous demodulator circuit. Horses for courses again.

Cheers,
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Old 14th Dec 2014, 3:54 am   #26
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Default Re: Quasi-Synchronous Demodulation

Upthread, post #20, I had wondered whether or not the Crosby exalted carrier receiver was ever realized in practice.

Well, apparently it was, judging by the attached item and advertisements from the early 1950s. Evidently Crosby Laboratories manufactured both SSB and exalted carrier receivers, amongst other equipment.

The Press Wireless endorsement (1953 July advertisement) is of note, given that this organization had its own manufacturing division (http://www.tmchistory.org/PressWirel...facilities.pdf) and eventually developed its own exalted carrier receiver. Perhaps there was some kind of connection between Crosby and Press Wireless, and its starting point was the Crosby receiver, which it then refined by the use of its version of the locked oscillator technique.

Another related idea of the late 1940s was that of Norgaard of GE, for the generation of ISB signals by wideband AF phase-shifting and quadrature modulation. The proposed receiver was of the locked oscillator type, with I and Q demodulation, audio phase-shifting and dematrixing, recognizable as the basic technique used in more recent HF receivers with PLL fully synchronous selectable sideband demodulation. A brief description of the Norgaard system was provided in Communications magazine for 1948 April, available here: http://www.americanradiohistory.com/...ns-1948-04.pdf, see page 12.

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Old 14th Dec 2014, 7:01 am   #27
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Default Re: Quasi-Synchronous Demodulation

There were systems using reduced carrier power as a pilot tone for PLL receivers to lock onto. DSB, SSB and ISB modulation were all used.

I used to use a Redifon GK203N synthesised exciter unit as my HF transmitter (with an RA117 receiver). The Redifon's mode switch had several reduced carrier AM modes on it which I never used.

I also remember, in an old Shortwave Magazine, an article on a self-clarifying ssb adaptor. I can't remember the year of the magazine. The adaptor was valved. I can't remember whether it tried to find a bit of residual carrier or whether it tried to align audio harmonics, though I think it was the latter. It was a magazine on loan to me, so that would have put it in the middle 1950s to middle 1960s era. I'd expect that the author must have been someone doing such things professionally.

Editors don't dare put anything like that in their magazines nowadays. They have an ethos which asks "Will enough people build one?" when back then editors asked themselves "Will anyone learn anything from it?"

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Old 17th Dec 2014, 2:03 am   #28
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Default Re: Quasi-Synchronous Demodulation

As far as I know, the SSB HF radiotelephone links developed in the 1930s generally used a pilot carrier. Fully suppressed carrier HF SSB may have been more of a military development post-WWII, although I am not sure about this. But some of the very early LF links were of the fully suppressed carrier type.

At some stage it appears that carrier levels were standardized; 16 dB down for SSB and 26 dB for ISB. Concomitantly receivers sometimes had adjustable gain in the carrier channel to allow matching to the transmission mode. For example, the Marconi HR24 had a switched attenuator; 0 dB for ISB, 10 dB for SSB and 32 dB for AM (DSB). I should guess the 32 dB number for AM derives from the fact that the carrier is 6 dB above peak maximum power, which would put it 32 dB above the ISB pilot carrier. This kind of three-channel ISB receiver – and I think others of its era – had quasi-synchronous demodulation in that reconditioned carrier for driving the demodulators was obtained by filtering, amplifying and limiting the incoming carrier. I imagine though that there would have been some valve-era SSB/ISB receivers that were fully synchronous in that they used locked oscillators or PLLs to regenerate the carrier. As mentioned upthread, the locked oscillator technique was used by Press Wireless in its exalted carrier receiver, and was proposed by Norgaard for his SSB/ISB receiver.

To what extent DSB pilot carrier and DSB suppressed carrier were used at HF were used I do not know. DSB suppressed carrier falls into the realm of “synchronous communications” as proposed by Costas of GE. In this case PLL techniques were used to recover the carrier from the sidebands alone, all the required information being contained therein. For communications traffic at least the carrier was redundant, although it would still have been needed, if only in pilot form, to establish an agc reference level for broadcast relay links. I am not quite sure – or really I can’t work out - what a Costas loop would do if presented with an SSB signal. Apparently it could deal with DSB corrupted by selective fading, in which case it effectively steered the instantaneous phase of the regenerated carrier. If the loop is endeavouring to keep the Q output at zero, then with an SSB input it would seem that it would need to recreate a symmetrical DSB signal in order to do that. But that is not a conclusion that I feel too comfortable with. Another way of looking at is that provides some crumbs of comfort is that whereas DSB is pure AM, SSB is AM modified by PM. So what the loop might be doing to SSB is applying “reverse” PM to return it to pure AM. Insofar as the loop is swinging the phase of the regenerated carrier to keep the Q output at zero, it does not look impossible, just improbable. By the way, an interesting commentary on the development of the Costas Loop may be found here: http://williamhaywoodjones.blogspot....gineer-by.html.

The Crosby exalted carrier system evidently had wider use than I had at first guessed. For example, it was used by TMC (Technical Materiel Corporation) in its TDRS triple diversity receiver, see: http://www.virhistory.com/tmc/tmc_pa...mc.ssb_143.pdf. Background information on TMC is provided here: http://www.virhistory.com/tmc/tmc_pages/, which page shows that there was a connection to Press Wireless.

RCA included a Crosby exalted carrier receiver in its work on multi-channel radio-telegraph systems, as recorded in RCA Review for 1948 December (http://www.americanradiohistory.com/...w-1948-Dec.pdf). Its performance, in terms of error rate, was comparable to SSB, both much better than conventional AM, which is confirmation that it worked well enough. RCA appeared to see the exalted carrier technique as having interim value until existing AM circuits were converted to SSB. But it also noted that an SSB receiver could also be used to receive AM with the same result of avoiding selective fading distortion. Eventually that seems to have become the preferred approach for point-to-point working, and I imagine that it happened once SSB telegraph and telephone circuits became commonplace, along with SSB receivers.

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Old 17th Dec 2014, 9:08 am   #29
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Default Re: Quasi-Synchronous Demodulation

Think of a signal entering a receiver.

What features are there in the signal to allow the receiver to be accurately tuned and for the user to tell that it's accurately tuned?

With a pilot carrier mode, there is one very obvious answer. The FDM systems I used to work on had up to 1800 SSB channels and a sprinkling of pilot tones to allow good demodulation. The pilots weren't simply the missing carriers, but just one shared for every group, supergroup and mastergroup.

With fully suppressed carrier DSB, you don't have that option. But if it's not ISB but true DSB, then you can use the knowledge that the sidebands must be mirror images around the missing carrier. Inserting an off-frequency carrier will split each tone component in the recovered audio. Tune around to minimise the splits, or to get best correlation between basebands recovered from USB and LSB sides.

With SSBSC, you lose that opportunity as well, and you're left trying to find non-sinusoidal components in the speech so you can slide the frequency with a linear offset until you get harmonic structures coming out with integer relationships.

The second harmonic of 1kHz is 2kHz but that of 1.01kHz is not 2.01kHz and the human ear hears that something sounds 'off'

Fine tuning an ssb receiver manually when you don't know the person speaking has to come down to this approach because you don't know what the speaker's natural pitch is. A lot of people do this successfully without ever knowing what they're really doing.

There was a programme to try to pack more PMR channels into limited bandspace back in the Early 70s. The Woolfson foundation was the sponsor and it involved Gosling and Macario. Their approach was to change from FM in 50kHz/25kHz channels and use SSB. One tool used was pilot-controlled companding, but the difficulty was frequency correction to get a good natural sounding voice. THe gear had to be simple enough to be affordable. A friend was doing his Phd in amongst that bunch.

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Old 17th Dec 2014, 9:59 am   #30
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Default Re: Quasi-Synchronous Demodulation

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There was a programme to try to pack more PMR channels into limited bandspace back in the Early 70s. The Woolfson foundation was the sponsor and it involved Gosling and Macario. Their approach was to change from FM in 50kHz/25kHz channels and use SSB. One tool used was pilot-controlled companding, but the difficulty was frequency correction to get a good natural sounding voice. THe gear had to be simple enough to be affordable. A friend was doing his Phd in amongst that bunch.
I recall something similar being tried in the US on their 220MHz band: it involved a curious digitally-created signal which, when presented to a discriminator produced the traditional FM-style low-DC component which varied in sign around the mid-point depending on whether the receiver was on the high or low side of correct. This was then used to control a locally regenerated carrier and fed to a more-traditional SSB detector.
It was quite effective but at low signal-strengths the discriminator-signal became jagged with noise and the 'CIO' unlocked. So overall the performance was no better than FM except that it occupied a narrower bandwidth (10KHz rather than the standard 20KHz channels which the US uses).
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Old 18th Dec 2014, 1:42 am   #31
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Default Re: Quasi-Synchronous Demodulation

On SSB, thanks for those comments RW, I can now see that the Costas Loop could not create DSB from SSB. It might work to create a mirror image sideband about a notional carrier, but it would have no way of finding the correct carrier frequency. Each sideband would end up with the same magnitude (but opposite sign) displacement error, which would probably derive from the happenstance initial relativity of the incoming sideband and the CIO.

As I recall both Macario and Gosling had an interest in demodulation. Certainly Macario wrote an article that was published in WW 1968 April that described what was a wideband quasi-synchronous AM demodulator. And was there not some interest in DSBSC and DSBDC (diminished carrier) systems for mobile radio around that time.

On the use of DSBDC at HF, the attached item from WW 1955 September about the GPO Rugby station suggests that it could have been used in point-to-point work the 1950s, along with SSB and ISB. The pertinent comment, in respect of the telephony transmitters, is: "When supplied with two separate telephony (or audio) inputs between 100 and 6,000 c/s, the drive equipment provides two independent sideband outputs or, if required, a double sideband output. It is also capable of providing an output of four telephony speech channels 300 to 3,000 c/s wide for operating the main transmitter as a four-channel independent sideband system."

Presumably the DSB output could have the pilot carrier at -26 dB, as with ISB, although it could be at any other level as well. In those days, DSB could have been received one sideband at a time using an SSB/ISB receiver, or by using an exalted carrier receiver that could handle diminished carrier levels.

The Crosby and TMC exalted carrier receivers could also demodulate PM (FM) by the exalted carrier method. In this case the reference carrier was simply phase-shifted by π/2, so that effectively it corresponded with the quadrature (Q) axis. This was thus quadrature demodulation with a narrow-band reference channel. Some other quasi-synchronous quadrature FM demodulators developed in the 1940s had wideband reference channels. These included the Zenith gated beam valve (6BN6) and the Philips nonode (EQ40, EQ80). Also appearing in this time period was the Philco locked-oscillator FM demodulator (FM1000 heptode), which belonged to the fully synchronous group. In the 1950s, RCA developed another quadrature FM demodulator based upon the 6DT6 valve. This acted as a locked-oscillator at lower signal levels, and as a quasi-synchronous demodulator at higher signal levels.

Synchronous techniques were also used – in fact required - for TV colour subcarrier demodulation. Back in the valve era, at the quasi-synchronous end of the spectrum, passive subcarrier regeneration by using the colour burst to ring a very high-Q crystal and then amplifying and limiting its output was probably an extreme example of the flywheel technique. At the fully synchronous end, both locked oscillators and PLLs were used, an example of the latter being the Quadricorrelator circuit.

Valve-era FM stereo decoders – and for that matter those from the discrete transistor era - seemed to be either quasi-synchronous, where the 19 kHz pilot tone was doubled and amplified to provide the 38 kHz reference, or of the locked oscillator type. As far as I know PLL was not used until the IC era, whence it quickly became the preferred horse for the course.

An interesting observation is that during the valve era, FM quadrature demodulation of the wideband type, and the two subcarrier examples mentioned, were widely used for consumer products. But the Crosby exalted carrier AM and PM demodulation system, and the two/three-channel SSB/ISB system, appeared to be confined to professional equipment, notwithstanding that they would have been very advantageous for shortwave broadcast listening. I guess that overall cost and complexity would have been prohibitive for consumer equipment, and I suspect that the very narrow carrier extraction filters were particularly expensive components. The simplest valve-era carrier recovery receiver that I am aware of was the Marconi HR22, which was a lot smaller and a lot less complex than its fully-fledged siblings such as the HR21 et seq. I think it was intended more for smaller-scale broadcast relay work, particularly shipboard. But it was still well beyond the sphere of domestic receivers.

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Old 2nd Dec 2015, 9:59 pm   #32
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Another related idea of the late 1940s was that of Norgaard of GE, for the generation of ISB signals by wideband AF phase-shifting and quadrature modulation. The proposed receiver was of the locked oscillator type, with I and Q demodulation, audio phase-shifting and dematrixing, recognizable as the basic technique used in more recent HF receivers with PLL fully synchronous selectable sideband demodulation. A brief description of the Norgaard system was provided in Communications magazine for 1948 April, available here: http://www.americanradiohistory.com/...ns-1948-04.pdf, see page 12.
Quite recently I happened across confirmation that the Norgaard system was actually used in its early days in a commercial product, namely the General Electric (GE) YRS-1 Single Sideband Selector. It was covered by an article in Wireless World for 1948 July, p.244ff, “Single Sideband Selector - Unit for Attachment to Communications Receivers”, which may be found here: http://www.americanradiohistory.com/...ld-1948-07.pdf.

Further searching then found a similar article in Radio News for 1948 August, p.53ff, “A Single Sideband Selector for Ham Use”: http://www.americanradiohistory.com/...-1948-08-R.pdf. Interesting is that it was aimed primarily at the amateur market rather than the commercial market, and was considered to be simpler than commercial approaches that included the use of filters. I have attached the page from that article which shows the block schematic. Both articles include the full schematic. Also attached is a GE advertisement for the YRS-1.

Its primary function appears to have been to enable the selection of either sideband of an AM transmission. It also provided exalted carrier demodulation of DSB AM signals, and demodulation of reduced carrier SSB signals. All of these were done fully synchronously using a locked oscillator. It could demodulate suppressed carrier SSB and CW signals, in which case its oscillator served as CIO and BFO respectively. Sideband selection was done by the phase-shifting and matrixing method, not by filters, and was reasonably wideband, 70 to 7000 Hz. Oscillator locking was done via an AFC loop, using the Q demodulator DC output and a reactance valve. Its input was 455 kHz (or thereabouts) IF from the associated communications receiver. All of this required 14 valves, including a power supply with voltage stabilization. Overall, it looks to have been quite an impressive product for the late 1940s, particularly as it was aimed at the consumer market.

I hasten to add that the GE YRS-1 used fully synchronous demodulation, not the quasi-synchronous type. So strictly speaking it is off-centre in this thread, topic-wise, but in practice it is difficult to discuss one without some mention of the other. And perhaps the distinction can be a bit blurred. TMC has been mentioned previously. Its TDRS triple-diversity receiving system used Hammarlund SP600X HF receivers modified inter alia to incorporate Crosby ECC exalted carrier adaptors. The Crosby exalted carrier demodulation system was quasi-synchronous; the carrier was extracted in a narrow filter and then limited and used to drive the demodulators. In the Crosby adaptor as used by TMC, the incoming receiver IF was down converted to a 200 kHz IF for exalted carrier processing. This was done by a local oscillator that had reactance-valve AFC controlled by a discriminator acting on the filtered and limited carrier signal. The primary purpose of this AFC system was to keep the 200 kHz IF closely centred even if the incoming IF was wandering somewhat. It seems to me that the presence of this “long” loop did move the Crosby demodulation system some way along a line drawn between quasi-synchronous and fully synchronous. Not only that, but sometimes both fully-synchronous and quasi-synchronous demodulation were used in the same unit, or even in the same IC. For example the Motorola MC13020 CQUAM AM stereo decoding IC – and its successors – used PLL fully synchronous demodulation to retrieve the I and Q signals, but (wideband) quasi-synchronous demodulation to retrieve the envelope signal.

The GE YRS-1 was a “surprise” find, and reminded me of the maxim that when doing casual and scattered lightweight “research”, one should not be surprised by surprises. This was purely a happenstance find, though, as I was looking at WW 1948 for other reasons and nearly didn’t even look at the GE YRS-1 article. Effectively this unit presaged the solid-state outboard synchronous demodulators for (consumer and semi-professional) HF receivers that were offered in the 1980s. Some of the latter also included quasi-synchronous as well as PLL full synchronous demodulators.

Cheers,
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Old 9th Dec 2015, 9:34 am   #33
Synchrodyne
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Join Date: Jan 2009
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Default Re: Quasi-Synchronous Demodulation

Another recently encountered older article on synchronous demodulation was “Second Detectors in TV”, in the journal RCA Engineer for 1958 October-November, p.39ff, available at: http://www.americanradiohistory.com/..._Issue_Key.htm.

It provided a concise and clear treatment of the distortions produced by envelope (diode) demodulation of vestigial sideband signals. Then the article went on to show the improvements that could be obtained by demodulating the in-phase component rather than the envelope. Methods suggested for obtaining in-phase component (IPC) demodulation were enhanced carrier and sampling using a product demodulator. The concluding statement was: “There is probably little risk in predicting that in the near future we shall see more of enhanced-carrier techniques using envelope detectors to obtain approximate IPC detection. Eventually, with greatly improved frequency memories becoming available, sampling detectors may well become important in home television receiver design.” I have attached a copy of the last page of the article, which also includes some useful diagrams.

In the event, and to the best of my knowledge, enhanced carrier techniques as described in the article were not widely used in domestic TV receivers in the valve era. But “approximate IPC detection” suitable for general use in TV receivers did arrive at the end of the 1960s, albeit in the form of the IC-based quasi-synchronous demodulator, for example the Motorola MC1330. As previously recorded, Motorola’s motivation was not solely improved demodulation. For its vision IF IC set it needed a low-level demodulator, and found this in the recent work of Macario – who was in search of better AM demodulation, and of Sprague, who had outpointed that its ULN2111A FM limiter/demodulator IC could also be used as a low-distortion AM demodulator.

The “afc-locked product detector”, to use terminology from the article, appeared in consumer IC form in the mid-1970s. The RCA CA3136 was the first PLL vision demodulator that I am aware of, although it seems to have been quite obscure. The National LM1821/2/3 of the early 1980s probably did more to put the type “on the map”. Nevertheless, discrete solid-state circuitry for PLL vision demodulation had earlier been used in professional equipment, such as the BBC RC5M/502 TV receiver. Overall the article was quite prescient, although I suspect that the timeline was longer than the author expected. The critical step, for consumer TV receiver applications where cost was so important, was the availability of suitable integrated circuits.

That aspect was emphasized in another article in RCA Engineer for 1958 October-November, namely “High Performance Television Receiver Experiment”, p.80ff. The article may be summarized by this quote from the introductory section:

“This paper describes an experiment that has been performed indicating an approach to an improved, high performance, color-TV receiver which is completely compatible with broadcast standards. The original work was conducted some years ago and served at that time to prove the validity of the principles involved: however, the cost/performance tradeoffs at this early period discouraged development in the consumer TV receiver product environment. The interim developments of integrated circuit technology and the growing availability of lower cost signal processing components may make the development of such a receiver both desirable and practical. This background, coupled with the growing interest in cable TV and high performance color television systems, make a review of the fundamentals worthwhile.”

Quite when the original work was done was not stated in the article, but it was evidently back in the valve era. One of the improvements was the use of synchronous demodulation, amongst others to eliminate cross-modulation in the demodulation process. It was said: “Any properly operating synchronous detector, such as the integrated circuit units including bridge-circuit balanced diodes with an associated AFC loop, will meet the requirements. However. the particular unit available for test consisted of 6J6, injection locked, product detector operating at the RF level on Channel 4 frequency. Fig. 3 is a diagram of the RF and detector portion of the complete receiver.”

I have attached a copy of the ‘Fig. 3’ referred to. The 6J6-based circuit looks surprisingly simple. The valve choice – the erstwhile 6J6 from the summer of 1942, and one of the very early heater-cathode miniatures - may have been quite specific, as elsewhere in the circuit the newer 12AT7 was used. The experimental receiver was a channel A4 TRF. Two other key features of the experimental receiver were the use of a delay line-based comb filter and sound carrier cancellation rather than notching out. In respect of the latter, it somewhat presaged the principle of the pilot-cancelling technique used in mid-1970s FM stereo decoder ICs from Hitachi and Toko.

Cheers,
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