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Old 25th Jul 2010, 7:23 am   #1
Synchrodyne
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Default Quasi-Synchronous Demodulation

A question that I have puzzled over in connection with quasi-synchronous AM demodulation is why in some cases a tank circuit is used in the reference carrier feed, usually between the limiter and the multiplier, whereas in other cases it is not.

It appears that such tank circuits are nearly always used with TV vision quasi-synchronous demodulators, sometimes with TV AM sound demodulators, but generally not with HF/MF AM radio demodulators.

Insofar as demodulation errors are reduced with greater reference carrier purity, then bandwidth limiting via a high-Q tuned circuit as well as amplitude limiting would seem to be desirable. In fact that is surely the thinking behind phase locked-loop (PLL) fully synchronous demodulation, in which a very pure reference carrier, free of both AM and PM, is generated.

Searches to date have produced some answers and allowed some deductions, but I have the sense that there is more to be elicited on this question.

As far as I know, the first quasi-synchronous vision demodulation IC developed for consumer TV receiver use was the Motorola MC1330. I have read the 1969 IEEE paper (1) referring to its development. This provides some interesting insights, although by no means does it answer all of the questions. A key feature of the design is summarized thus: “In this low-level detector, the signal and reference switching channels are separate so that the frequency response of the switching path may be operated on without affecting the main signal channel. By inserting a tuned circuit into the into the switching path tuned to video carrier frequency, the amount of unwanted chroma-sound beat is reduced, but the wanted detection of the chroma and sound sub-carrier by the video carrier is unaffected.” The goal previously stated was to avoid sideband intermodulation over the single-sideband part of the video spectrum, where it was not self-cancelling. Increased reference carrier purity helped achieve this. It seems that the Q of the tuned circuit in the reference carrier path was limited only by ease of tuning considerations. Higher Q resulted in greater reduction of the chroma-sound beat. The prototype circuit also included a chroma carrier rejector as part of the tuned circuit to help with this aspect. Nothing was said in this paper about the effects of the vision carrier being on the Nyquist slope of the IF bandpass curve; clearly this later became an issue with multichannel sound systems, hence the advent of quasi-split sound techniques. But presumably it would have resulted in vision demodulation errors as w ell.

The pattern set by the MC1330, namely the incorporation of a tank circuit in the reference carrier pathway between the limiter and the multiplier seems to have been followed for virtually all TV vision quasi-synchronous demodulator ICs, such as the TBA440, TCA270, TDA2540/41, TDA4420/21, etc. Given the need to avoid single-sideband distortion, the use of a tank circuit would seem to be virtually unavoidable. The only possible exception is an obscure – and I think short-lived – Plessey IC, about which more later. Although the MC1330 was probably designed mainly with negative modulation TV systems in mind, the same technique was applied to positive modulation systems in ICs such as the TDA2542 (positive modulation only) and TDA2549 (dual-standard).

With TV AM sound, practice seemed to vary. The TDA2543 IC combined an AM sound IF strip with a quasi-synchronous demodulator that did have a tank circuit. It appears to have been the sound channel counterpart to TDA2542. I imagine that both of these ICs would date from the mid-1970s. Whether quasi-synchronous TV AM sound demodulation was used prior to this I don’t know, but conceivably the MC1330 could have been used for this purpose, or perhaps the TBA440, which could have been operated without AGC gating and I think which did not have any video processing circuitry on the output side that might prejudice its use for audio purposes.

On the other hand, the TDA3845, which probably was released a few years later, and which was designed to handle quasi-split FM sound as well as AM sound, had an AM quasi-synchronous demodulator without a tank circuit. The FM sound pathway did have the customary quadrature tank circuit at vision carrier frequency. Perhaps keeping the AM pathway free of any tuned circuits easily allowed it to operate at more than one frequency, if required. The U4468B had a similar AM pathway to the TDA3845, even though on the FM side it had PLL fully synchronous demodulation.

I suppose that with TV AM sound, where the signal is double-sideband with very low likelihood of asymmetry, the tank circuit could be considered as not being strictly necessary, although perhaps offering incremental improvements in demodulation accuracy.

The picture changes with MF/HF AM demodulation applications, though. Here avoidance of the tank circuit seems to be the norm, although not universal. Yet even though the signals are nominally double-sideband, particularly at HF there is likely to be asymmetry due to selective fading, etc., so that the improved purity of the reference carrier that a tank circuit would provided would seem to be desirable.

An early solid-state quasi-synchronous AM demodulator design by Macario (2) did not include a tank circuit. (Interestingly, this was referenced in the Motorola IEEE paper.) On the other hand, Herbert (3) used an MC1330 with tunable tank circuit in his homodyne design. The Plessey SL624 IC, which dates from the mid-1970s, if not earlier, was intended to be multimode demodulator for communications receivers. In the AM mode, at least according to the Plessey notes, it operated as a quasi-synchronous demodulator without a tank circuit (although it might have been possible to include one if required.) In the FM mode it operated as quadrature demodulator with a tank circuit, and it could also be configured as a self-oscillating SSB demodulator.

The Herschberger design (4) for a PLL fully synchronous AM demodulator also included a quasi-synchronous demodulator for “envelope” detection. This did not have a tank circuit; nevertheless Herschberger enumerated its advantages over a simple diode demodulator.

The JRC NRD525 HF receiver used an SN16913P IC for AM demodulation. As far as I know this IC was very similar to the MC1496. The reference carrier path had additional amplification and limiting, but no tuned circuits.

The Eddystone 1570/1590 receivers used a TDA1071 IC for the AM IF amplification and quasi-synchronous demodulation function, the latter without a tank circuit. I can find absolutely no information on the TDA1071 anywhere on the web – it is almost as if it never existed. Working back from the Eddystone schematics, it appears to be very much like the Plessey SL624C at the demodulation end, with the addition of a full IF strip broken into two accessible portions, thus allowing an intermediate selectivity block if desired. A separate TDA1071 is used for AM AFC in these receivers, and as an FM IF strip and demodulator (at 10.7 MHz) in the 1570.

Perhaps for the AM case, it was thought that demodulation accuracy was adequate without a tank circuit and that anyway, where real precision was required in the presence of sideband asymmetry, the PLL approach was better. Not having a tank circuit would also have the advantage that receiver tuning would not have to place the desired carrier right in the middle of the tank circuit curve. Rather the IF selectivity could be placed pretty much anywhere over the signal bandwidth within reason, and although there would be single sideband errors, the quasi-synchronous demodulator would still be much better than a diode demodulator in such circumstances. I found that this was true for the JRC NRD525 receiver, where the carrier could be well down either slope of the IF selectivity curve without there being the gross distortion that a diode demodulator would typically show under such conditions. On the other hand, even with the carrier centred, there was not much apparent reduction of the effects of carrier nulls caused by selective fading, and I wonder if the use of a tank circuit might have provided benefits in this direction.

Now back to the TV vision case and that obscure Plessey IC that did not have a tank circuit. This was mentioned in a “Television” magazine article that I now cannot find. Probably it would have been in the late 1970s, possibly in the early 1980s. It was after the general introduction of SAW filters, but I think before the general introduction of quasi-split sound (QSS) techniques, or at least that terminology. The same article also referred to the introduction of the now well-known SL1431/32 IF preamplifiers as part of an overall IF system. One of the key innovations was the use of wideband RF AGC, derived from the preamplifier and so ahead of the SAW filter that provided the main IF selectivity. The second innovation was that the SAW filter had separate outputs for vision and sound. The main IF IC (it might have been something like SL1440, but my memory is hazy on that point) had separate channels for vision and sound, and on the sound side provided an intercarrier output. It was described as having wideband, switching demodulators. No tanks circuits were present in the circuitry shown in the article.

At the time that I read the article, I had not encountered any mention of QSS. So I had assumed that the sound channel simply processed the sound carrier alone (33.5 MHz for UK system I) and produced the intercarrier by multiplying it with the vision reference carrier derived from the vision processing pathway. And that on the vision side, a quasi-synchronous demodulator without tank circuit was used. Now in the light of much better information, it seems to me that the latter arrangement might not have been wholly satisfactory due to single sideband distortion, with the result that vision demodulation accuracy would not have been as good as say with a TDA2540/41, nor would the intercarrier have been much cleaner. As I recall the said IC did not have an AFC system, and I cannot recall whether or not the IF AGC was externally gated. Possibly though, it was not configured as I thought at the time, but was an early attempt at QSS, albeit in the same IC as the vision IF strip. Be that as it may, that IC seems to have disappeared fairly quickly. Later Plessey literature shows the SL1432/32 in IF strips that use Plessey SAW filters and the TDA2540/41 (third-party sourced) as the main IF IC.

Cheers,



(1) Lunn, G.; A Monolithic Wideband Synchronous Video Detector for Color TV; IEEE Transactions on Broadcast and Television Receivers; Volume BTR-15, Number 2, July, 1969, p.159ff.
(2) Macario, R.C.V.; How Important Is Detection?; Wireless World; April, 1968, p.52ff
(3) Herbert, J.W.; A Homodyne Receiver; Wireless World, September, 1973; p.16ff.
(4) Herschberger, D.; Build a Synchronous Detector for AM Radio; Popular Electronics; April, 1982; p.61ff.
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Old 25th Jul 2010, 8:05 am   #2
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Default Re: Quasi-Synchronous Demodulation

The first Quasi synchronous detector I came across was in 1970 in the professional UHF TV tuner used by British Relay in their line system, it was a Murphy product type MR756, it was a discreet type with a complex detector incorporating about 20 transistors, I now have only vague memories of the circuit but would like to obtain a manual again to remind me of this excellent tuner.
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Old 25th Jul 2010, 11:33 am   #3
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Default Re: Quasi-Synchronous Demodulation

Talking of professional tuners, the attached block schematic for the BBC RC1/511 (from Electronics & Wireless World, July 1984) shows some interesting facets in respect of quasi-synchronous demodulation.

I would guess that it uses semi-discrete circuitry, with small-scale, essentially monofunctional ICs. The vision reference carrier channel appears to combine amplitude limiting, careful bandwidth limiting (0.5 MHz) as well as centring of the passband over the vision carrier, thus removing any adverse effects of the Nyquist slope in the earlier SAW filter stage. There is a separate multiplier for the sound carrier, which is separated from the vision carrier early on, with the 6.0 MHz output described as “true intercarrier”.

Cheers,
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Old 26th Jul 2010, 6:58 pm   #4
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Default Re: Quasi-Synchronous Demodulation

I think you have largely answered your own question: vestigial sideband TV needs a carrier tank circuit, double-sideband audio can manage without (and may be better without - see below).

In addition, it is likely that a TV will be accurately tuned via a synthesiser or AFC whereas broadcast AM may not be. When off-tune a separate carrier tank circuit creates phase problems, but a good AM filter will simply add a time delay to the whole signal provided it is not too far off tune.
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Old 31st Jul 2010, 5:57 am   #5
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Default Re: Quasi-Synchronous Demodulation

Dave, thanks for that, which has certainly helped my understanding of the topic.

In the TV vision case I think that the use of AFC was pretty much assumed by Motorola when it designed the MC1330. The IC has a buffered, limited carrier output for feeding an AFC system. For American TV receivers, I think that AFC had been standard for many years, and in fact was one of the first functions for which ICS were developed. Later quasi-synchronous demodulator ICs, such as the TCA270 and TDA 2540/41, included the AFC function.

In HF receivers, I imagine that the ability to operate away from the correct tuning point may have been considered advantageous in terms of partially rejecting interference in one sideband. And a quasi-synchronous demodulator should have been better than a diode in such situations. So that could have been another reason for not using a tank circuit. AFC would have been rare in HF receivers (except for the point-to-point ISB type). I think that the Eddystone 1570/1590 was quite unusual in having AFC.

Incidentally, I had previously been under the impression that the term “quasi-synchronous”, if not actually coined by Motorola, came into widespread use because Motorola had associated it with the MC1330 IC. However, the term is not used in the IEE paper (by Lunn of Motorola). And in a later paper co-authored by Lunn (on the “Monomax” TV IC), the by-then conventional TV demodulator was referred to as being of the “pseudo-synchronous” type.

Cheers,
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Old 5th Aug 2010, 8:40 pm   #6
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Default Re: Quasi-Synchronous Demodulation

If you are interested in the difference between the synchrodyne and the homodyne there was an article in Nov 1998 Electronics (& Wireless) World with some practical receiver circuits including a switch-able synchrodine/homdyne circuit.
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Old 6th Aug 2010, 10:47 am   #7
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Default Re: Quasi-Synchronous Demodulation

Thanks, Nick. That might be the Slifkin & Dori article. If so, the homodyne section did not use a tank circuit between the limiter and the multiplier, unlike say the earlier Herbert design.

Homodyne and synchrodyne receivers per se seem to have been little used in commercial practice. On the other hand, their respective methods of demodulation, quasi-synchronous and [fully] synchronous have been much applied to intermediate (and subcarrier) frequencies in diverse situations. I have a partial list that I’ll post shortly.

Cheers,
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Old 14th Nov 2010, 6:11 am   #8
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Default Re: Quasi-Synchronous Demodulation

Some more thoughts on this topic:

The widespread use of quasi-synchronous vision demodulators in domestic TV receivers seems to have been facilitated by the availability of ad hoc ICs, starting with the Motorola MC1330 circa 1970. (I think Decca was an early user in the UK.) It is typically said that the technique would have been prohibitively complex to execute with discrete components, particularly on component count grounds, but I wonder whether that was really the case. Discrete component designs were used for TV colour decoders (fully synchronous demodulators) and FM stereo decoders (both quasi-synchronous and fully synchronous) before suitable ad hoc ICs were developed. So would quasi-synchronous vision demodulation with discrete components have been say an order of magnitude more difficult?

To illustrate, let’s assume a TV receiver with a 3-stage IF strip (bipolar or FET, or combination) with distributed selectivity. Normally this would feed a diode demodulator, but for the quasi-synchronous case, let’s have it feed a four diode bridge. Now to obtain the reference carrier feed for the same bridge, let’s invoke an IF side chain, taking its feed from the 2nd IF stage collector (or drain). The side chain would start with a gain stage, with input and output tank tuning arranged to cancel the Nyquist slope effect of the main IF strip. Thus the overall bandpass at the output of the gain stage would be symmetrical about the vision carrier, with appropriate bandwidth. Next would come a hard limiter, perhaps a long-tailed pair. The limiter output would then feed the previously mentioned four diode bridge demodulator, maybe via a phase adjustment network. The limiter output could also feed an AFC discriminator. And if one wanted cleaner intercarrier sound, sans Nyquist slope effects, the limiter output could also be used to generate the intercarrier in a mixer (say a dual gate mosfet) to whose other input port was sent the sound carrier extracted between the tuner and 1st IF stage.

Whilst this sounds a bit complex, IF side chains were not unknown in late 1960s discrete TV receivers. The BRC 2000 had a side chain with gain stage and limiter feeding the AFC discriminator, so incorporating many of the elements envisaged for the quasi-synchronous demodulation case. The B&O 3000 also had a side-chain, in this case a single gain stage with vision carrier central in bandpass and feeding the chroma demodulator diode, which also provided the intercarrier signal.

Thus the number of additional stages and components required for quasi-synchronous demodulation was not at all high as compared with late 1960s discrete solid state practice, and did not involve any techniques not already used in domestic equipment. Perhaps setting up adjustments would have been too finicky for run-of-the-mill receivers on the production line, although maybe not for upmarket models?

Or what am I missing here that makes my analysis overly simple?

Cheers,
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Old 22nd Jan 2011, 5:02 am   #9
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Default Re: Quasi-Synchronous Demodulation

Looking back to the original question, G8HQP Dave’s comments have prompted a close re-read of the Lunn/Motorola article plus a search for other references.

In the Lunn article, after presenting the basic layout of the quasi-synchronous demodulator, as previously proposed by both Biolotti and Macario, the comment is made that the characteristics of the switching signal can be changed by selective circuits. This established the fact that selective circuits may be used, but are not an inherent feature of the scheme.

This is reinforced by the application note AN189 for the MC1496 multipurpose double-balanced multiplier, available at: http://www.spelektroniikka.fi/kuvat/mc1496appl.pdf. Therein the basic AM demodulator case is presented as using limited, but not filtered signal carrier.

In the Lunn article, later on the basic premise is repeated as “..the signal and reference switching channels are separate so that the frequency response of the switching path may be operated on without affecting the main channel.” A key objective was to minimize the chroma-sound beat, and the original proposal included not only a tuned circuit peaked at vision carrier frequency but a notch at the chroma sub-carrier frequency. There was concern that too high of a Q in the tuned circuit might make receiver tuning difficult (a Q of 30 is mentioned) hence the anticipated need for the notch. In practice, for the MC1330 (and all that followed) a simple tank circuit seems to have sufficed. The datasheet recommends 20 to 50, with the comment that the higher the Q, the better the chroma-sound beat rejection.

Anyway, that indicates that the for the vision demodulator case, tank circuit is not there out of basic necessity, but primarily to reduce the chroma-sound beat.

Nothing is said in the article about the errors and adverse effects introduced by the Nyquist slope, but then that did not become an issue until higher quality sound was required and stereo sound was introduced, which saw the development of the quasi-split sound technique and in one or two cases reintroduction of split sound IF systems.

Some interesting comments may found in application note AN391 for the National LM1823 vision IF IC with PLL fully synchronous demodulation, available at: http://www.national.com/an/AN/AN-391.pdf. Clearly this is “selling” the idea that the PLL technique is better than quasi-synchronous, but it does reinforce G8HQP Dave’s comment that unless tank circuit tuning corresponds exactly to carrier frequency, there will be phase errors that vary with the original amplitude modulation fed to the limiter stage.

Getting back to the Lunn/Motorola concept of operating on the carrier channel frequency response, it would appear that if one wanted to correct for the Nyquist slope and so reduce sound buzz on conventional intercarrier systems, this could not be done by slightly detuning the post-limiter tank circuit, but would need to be done separately ahead of the limiter.

Cheers,
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Old 31st Mar 2012, 9:34 am   #10
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Default Re: Quasi-Synchronous Demodulation

I have since found a 1976 Motorola (Quasar) paper (1) that addresses the issue of distortion in the vision demodulation process attributable to phase modulation of the reference carrier caused by the Nyquist slope. Avoiding this problem would require either a very narrow bandwidth reference channel (just a small number of fh sidebands wide) or re-establishing double sideband integrity before limiting. The first option could be realized with PLL fully synchronous demodulation. At the time the paper was written it was thought that use of a suitable SAW filter in the reference channel would allow the second option.

Evidently though eliminating the Nyquist slope error in quasi-synchronous vision demodulator ICs was not a major concern until the arrival of multichannel sound. For example, it is not really mentioned as an issue in the papers dealing with the MC1330, the TBA440 (2). With the later MC1331 (4), Motorola seemed to be concentrating on detail improvements to the multiplier operation, along with the use of a separate intercarrier multiplier as a way of reducing the colour subcarrier-sound carrier beat. One might reasonably infer that the benefits obtainable with the standard designs, as compared with diode demodulators, were viewed as sufficient for most domestic receiver purposes, and anyway there was always the option of using fully-synchronous demodulation.

The arrival of multichannel sound forced the issue, and then for the sound demodulation process only, in order to avoid intercarrier buzz. This is discussed in detail in a 1981 Zenith paper (5). Suggested solutions were quasi-split sound (which by 1981 might already have been in use in Europe and/or Japan) and split sound using a narrow sound IF filter followed by a 2nd independent conversion to 4.5 MHz. (In practice a 10.7 rather than 4.5 MHz 2nd sound IF seems to have been used by Sony and others where this scheme was adopted.) But the split sound system was vulnerable to incidental phase modulation (IPM) on the signal as received so receivers using it typically had a conventional intercarrier sound channel for use in such situations.

Generally it seems that quasi-split sound ICs were used alongside conventional vision IF and quasi-synchronous demodulator ICs, inclusive of their Nyquist errors. I am not aware of any vision quasi-synchronous demodulator ICs that had a separate pathway from a SAW filter carrier output for the vision reference channel.

With professional equipment it may have been different, and for example the BBC RC1/511 receiver mentioned in post #3 “reconditioned” the reference channel to eliminate the Nyquist slope. The RC5M/503 UHF rebroadcast receiver (information available at http://www.bbceng.info/Designs/desig...EDI_sheets.htm) seems to have been similarly equipped, judging by the commentary:

“Selectivity and Nyquist shaping are obtained by the use of a specially developed surface acoustic wave (SAW) filter, and no IF alignment is required. A second SAW filter extracts the vision IF carrier, and amplitude modulation is removed by low-phase shift limiters; the resulting carrier is used to demodulate the vision signal. This “exalted-carrier”, or “pseudo-synchronous”, demodulation means that the effect of any incidental phase modulation (IPM) present on the input is greatly reduced.”

The RC5M/503 was intended to replace the RC5M/501 and RC5M/502, and the latter apparently had fully synchronous demodulation, it being stated:

“Synchronous detection is used, thereby removing quadrature distortion, giving improved pulse response, and a considerable reduction of chrominance-luminance crosstalk. Differential gain and phase distortions are also much reduced”.

My take is that the RC5M/502 used a PLL fully synchronous demodulator, and that it was being compared with the preceding RC5M/501, for which synchronous demodulation is not mentioned, so one assumes that it had envelope demodulation.

The LM1823 application note mentions the need for a wider (500 Hz instead of 73 Hz) PLL bandwidth when the incoming vision signal suffers from incidental phase modulation. But 500 Hz maybe enough for Nyquist slope errors to be non-negligible. So it seems that the approach used in the BBC RC5M/503 accommodates incidental phase modulation whilst avoiding the Nyquist error.

(1) Rzeszewski, “A System Approach to Synchronous Detection”, IEEE Transactions, 1976
(2) Schatter, “Monolithic TV IF System TBA440”, IEEE Transactions, 1972
(3) Wilcox, “A new TV Video/Sound Detector IC”, IEEE Transactions, 1974
(4) Fockens & Eiler, “Intercarrier Buzz Phenomena Analysis and Cures”, IEEE Transactions, 1981

Cheers,
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Old 31st Mar 2012, 6:27 pm   #11
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Default Re: Quasi-Synchronous Demodulation

Hi, have you seen any of the papers by Prof Tucker et al from both Birmingham University, and The GPO (Martelsham I think).
His group did a lot of the early work on this type of receiver in the valve era. He also published in Electronic Engineering.

Ed
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Old 6th Apr 2012, 5:57 am   #12
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Hi Ed:

Thanks. No, I don’t have the Tucker articles. The closest I have to those is the August 1948 Wireless World article by “Cathode Ray”, which seems to have been based upon Tucker’s work. The synchrodyne receiver shown therein used a locked oscillator (not a phase locked oscillator) and a diode ring demodulator. I think that was around the same time that the single valve locked oscillator FM demodulator first appeared, which thus might be considered the FM counterpart to the AM synchrodyne demodulator. As a corollary, the nonode FM demodulator from about the same time would be the FM counterpart to the AM homodyne demodulator, and also the antecedent of the solid state quadrature FM demodulator that came into favour, in IC form, in the late 1960s.

The TV vision demodulation case is an interesting one. References cited in the Rzeszewski paper suggest that the problems with envelope demodulation of vestigial sideband signals was know from the earliest days. But realization of improved methods was very difficult in the valve era. I have a 1957 (?) RCA paper entitled “Synchronous and Exalted-Carrier Detection in Television Receivers” that suggests that there is a continuum of improvements available between simple envelope demodulation through exalted carrier techniques to fully synchronous.

One thus supposes that when the IC era arrived and allowed economic realization of improved demodulation techniques, the early efforts, such as the Motorola MC1330, were aimed at the same increment that would be available from exalted carrier technique, and were not reaching for perfection. A rough qualitative assessment of what this could bring is provided by those HF receivers that have quasi-synchronous AM demodulators, for example the JRC NRD525. Here the receiver may be detuned to the point where the carrier is slipping down the edge of the IF bandpass without the intrusion of the gross distortion that typically would be evident with a conventional diode demodulator.

Talking of HF receivers, whilst there was a whole plethora of ad hoc ICs for developed television vision quasi-synchronous demodulation, few seem to have been developed for HF AM work. Maybe it was assumed that most designers would build their demodulators from single-function ICs, such as the MC1496. Two ad hoc ICs that I have identified are the Plessey SL624C, for which I have limited information, and the TDA1071 (used by Eddystone in the 1570/1590 series), about which there is otherwise absolutely nothing on the web. (It seems to have sunk without trace.) Working from the Eddystone schematics, the TDA1071 appears to have combined a two-part IF amplifier, with accessible interstage, followed by a multiplying demodulator that may be configured for quasi-synchronous AM or quadrature FM, and was used by Eddystone in both roles in the 1570, and also for AM AFC. I have a vague notion that the TDA1071 was used by a portable receiver maker (Roberts?) but then I might be confusing it with the Philips/Mullard TDA1072 AM radio subsystem.

Coincidentally, some of the issues that PLL synchronous demodulation resolves as compared with the quasi-synchronous approach, essentially related to purity of the reference carrier, come out in the Wireless World September 1970 Portus and Haywood article on their phase-locked stereo decoder that was recently posted in the thread: “Wireless World MC1310 pll decoder article” at: https://www.vintage-radio.net/forum/...t=81838&page=2.

Cheers,
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Old 6th Apr 2012, 6:54 pm   #13
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Default Re: Quasi-Synchronous Demodulation

Hi, I've just had a look at my ancient Plessy handbooks on ICs (Nov 1974). This lists the SL624C as a new product in about 2 pages with a max operating freq of 30Mhz.
There are 3 app circuits given; Synchronous AM detector, FM quad detector and Self oscillating product detector.
PM me with your e-mail if you would like a scan of the pages.

Ed
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Old 15th Apr 2012, 2:04 am   #14
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Default Re: Quasi-Synchronous Demodulation

Thanks very much, Ed. The SL624C was certainly a versatile IC.

I also had a vague recollection of the LM373 from the same era. For this, some data is available on the web. It was somewhat different, though, being a communications IF amplifier, which could be operated with agc (for AM and SSB) or in limiting mode (for FM) with AM, FM and SSB demodulators. There was a separate AM demodulator (of the rectifying type), and a multiplier that could be used as either an SSB demodulator (fed by an external BFO) or as an FM quadrature demodulator. But it is noted that it could be used as a synchronous demodulator. In this case I imagine that an external limiting amplifier would be required to provide a reference feed to the multiplier.

I have since found some additional useful references on the web, namely:

Prof. Tucker’s 1954 article at:

http://www.thevalvepage.com/radtech/synchro/synchro.htm

Pat Hawker’s 1972 Wireless World articles at:

http://www.epanorama.net/sff/Radio/C...0Part%201.pdf; and:

http://www.epanorama.net/sff/Radio/C...20Part%202.pdf

And the 1957 Costas paper:

http://www.costasarrays.org/costasre...ynchronous.pdf

Cheers,
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Old 17th Jun 2012, 8:41 am   #15
Synchrodyne
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Default Re: Quasi-Synchronous Demodulation

Referring back to my original question, about the use or not of a tank circuit in the reference carrier channel of quasi-synchronous demodulators, I have recently come across the data for the Siemens TDA2460-2 TV sound IF IC, which seems to be a successor to the original TBA460, and handles both AM sound and intercarrier FM sound channels. In the AM case, the quasi-synchronous demodulator may be used with or without a tank circuit, the latter allowing deletion of an LC circuit and the need to tune it. It was claimed that the LC circuit was unnecessary because of the excellent capture ratio features of the limiter. Working back from that, one might infer that with earlier ICs such as the TBA460, the need for a tank circuit with symmetrical AM signals such as TV AM sound was to correct for less-than-ideal limiter performance. And good limiter performance is harder to obtain at higher frequencies.

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Old 6th Jan 2014, 11:07 pm   #16
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Default Re: Quasi-Synchronous Demodulation

Revisiting this topic in the context of HF receiver quasi-synchronous demodulation applications, where, unlike the TV case, there seem to have been fewer ad hoc ICs developed, and as a corollary, there was a tendency to use relatively standard components.

The MC1496 double-balanced multiplier and like components seem to have been central, and the AM demodulation case is included in the application not, excerpt attached. The 8-pin IC limiter in the reference channel is unidentified, although Motorola’s own IC FM limiter was the 16-pin MC1355.

But possibly predating the MC1496 is an application found in the mid-1968 Bilotti (Sprague) IEEE paper “Applications of a Monolithic Analog Multiplier”. In essence this considered alternative applications for the Sprague ULN2111A IC, which was an FM IF subsystem including amplifier/limiter and quadrature demodulator. If not the first, then it was likely one of the very earliest FM IF subsystems to include a quadrature demodulator. Amongst other applications, it could be deployed as an AM quasi-synchronous demodulator by using the limiter for the reference channel and feeding unlimited AM into the other multiplier port. An excerpt from the paper is attached. Whether directly or indirectly, this use of an FM subsystem IC for AM quasi-synchronous demodulation appeared to have influenced future practice.

An example HF receiver using the MC1496 as an AM quasi-synchronous demodulator was the Racal RA1772, demodulator schematic attached. Here the reference channel limiting is provided by an MC1357, which is the Motorola clone of the Sprague ULN2111A. The MC1357 also served as the NBFM quadrature demodulator. The reference channel signal for the MC1496 was tapped off between the limiter and the multiplier parts of the MC1357. The MC1496 was also used for SSB demodulation, and in this case the BFO signal was routed via the MC1357 limiter, with switching between BFO and AM inputs then done ahead of the MC1357 and one imagines at fairly low levels.

A different approach was used in the Marconi Oceanic marine HF receiver. Here, a TBA120A IC was used for quasi-synchronous AM demodulation. The TBA120A (from Siemens, I think) was an early TV FM sound subsystem including an amplifier/limiter and quadrature demodulator. In the Oceanic it was deployed in exactly the same way as the ULN2111A in the Sprague paper. An MC1496 was used separately for SSB demodulation. Interestingly, the final IF stage in the Oceanic was an MC1349, basically a TV vision IF IC, and an improved version of the MC1350, which was one of the earliest TV vision IF ICs.

Turing to dedicated ICs, the previously mentioned Plessey SL624C may be seen as following the precedent set by the ULN2111A. It could be used as an FM quadrature demodulator or an AM quasi-synchronous demodulator. It was also designated for use as an SSB demodulator, which is probably something that could also have been done by the ULN2111A and its ilk. The inclusion of an AF volume control in the SL624C also followed TV FM sound practice in that some of the ICs for the latter, such as the TBA120S, included similar facilities. Plessey recommended that individual SL624C ICs be used for each demodulation function rather than switching a single IC to do two or three jobs.

Possibly predating the Plessey SL624C was the National LM373, which was described as a self-contained AM/FM/SSB IF strip with built-in agc. FM demodulation was quadrature, and the multiplier was also used for SSB demodulation. AM demodulation was normally by a peak rectifier, but the LM373 was stated to be suitable for use as an AM synchronous demodulator.

And that leads to the TDA1071, about which zero information is to be found on the web. Three of these were used in the late 1970s Eddystone 1570 series receiver. One was used as an FM IF subsystem at 10.7 MHz, another as an AM IF amplifier and quasi-synchronous demodulator at 455 kHz, and a third as an AM AFC discriminator at 455 kHz. However, it was not used for SSB, for which amplified IF was tapped off from the TDA1071 and then fed to a TCA240 multiplier IC, which was similar to an MC1496.

Working back from the Eddystone 1570 schematic, the TDA 1071 looks as if it has a two stage IF amplifier ahead of the demodulator, with internally generated agc applied to at least the first stage. That agc could be disabled and replaced by mgc. Coupling between the two IF amplifier stages was external, allowing the use of an interstage filter. The IF amplifier section was followed by a limiter and multiplier, which fairly obviously could be used for FM or AM demodulation. There did not appear to be any AF processing.

That makes the TDA1570 look like a latter-day LM373, albeit perhaps with synchronous AM demodulation capability as a primary feature rather than a secondary possibility. Or maybe not, and Eddystone just choose to use it that way. One would need to find a maker’s datasheet to know one way or the other.

Anyway, that takes us to today’s question, which is has anyone here come across any information the TDA1071 IC, such as a datasheet. I certainly cannot find any such on the web, either now or from earlier searches. I also wonder who developed it. The Eddystone 1570 receiver dates to around 1978, which would seem to be about the right time for the “1071” series number. It was around then that the Philips group TDA1028/9 (audio switches), TDA1072 (AM radio subsystem) and TDA1074 (audio volume and tone control) appeared. Apart from the Eddystone applications (1590 as well as 1570), I have a notion that the TDA1071 was used in a Roberts model or iteration thereof (RMF33?).

Maybe some ICs are just web-shy. Information on two others pertinent to this thread, namely the TDA2543 and the SL1440 (https://www.vintage-radio.net/forum/...ad.php?t=96530), was also difficult to track down, although eventually found.

The return to this topic was prompted by the recent thread on the Marconi HR22 HF receiver (https://www.vintage-radio.net/forum/...d.php?t=101833) which used a different approach to synchronous AM demodulation, namely filtering out the carrier using a very narrowband filter, a technique that dates back to early HF SSB/ISB receivers in the 1930s. More on this for another day, as also for further thoughts on the tank circuit issue (the original question in this thread) prompted by the Bilotti paper.

Cheers,
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Old 3rd Feb 2014, 12:10 am   #17
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Default Re: Quasi-Synchronous Demodulation

I have since been able to deduce some information on the TDA1071 IC after finding a couple of the schematics for some Roberts models that used it, namely the RM30 and RM33. I still have not found a datasheet for it, but I have discovered that it was referred to in Mullard Technical Communications 14, No. 134, 1977. Mullard Technical Communications appear to be unobtainium, but this reference does suggest that it was a Philips/Mullard device. Thus it is surprising that it was not mentioned in any of the Ambit catalogues 1 through 3, which otherwise covered the Philips/Mullard RF and AF ICs of the era, including the TDA1005A, TDA1028/9, TDA1072 and TDA1074. Given Ambit’s apparent enthusiasm for the Plessey SL624C, one imagines that the TDA1071 would have appealed, and that interesting uses would have been found for it beyond those delineated in the application notes.

Essentially the TDA1071 appears to have been a consumer IC, intended to handle many of the FM-AM radio receiver functions. In that sense it was not unusual. On the FM side, it provided IF amplification, limiting and quadrature demodulation. On the AM side it included frequency changing (with separate mixer and oscillator), IF amplification with agc, and quasi-synchronous demodulation. The last-mentioned was its point of departure, and it was probably rare amongst consumer application ICs in specifically catering for AM quasi-synchronous demodulation. The Plessey SL624C for example was primarily a professional applications IC. And other FM-AM radio ICs, such as the TDA1220, tended to use more conventional forms of AM demodulator, even when they incorporated quadrature-type FM demodulators.

In the TDA1071 the same multiplier was used for both FM and AM demodulation. The feed from the limiter to the multiplier was internal. The second feed to the multiplier was external, and by external switching could either from the limiter output via a π/2 phase shift and tank circuit for FM, or from the IF amplifier output direct for AM.

In the Eddystone 1570/1590 case, the AM front end was discrete, using dual-gate mosfet RF and mixer stages, with a dedicated agc loop for the RF amplifier. So the frequency changing capability of the TDA1071 was not needed. Instead, the mixer was used as an additional gain controlled amplifier stage, ahead of the main selectivity, which was obtained by switching amongst various bandwidth filters between the mixer and the IF amplifier sections of the IC. The IF from the front end went into the mixer RF port, and it looks as if the two local oscillator ports were used for gain control, one for agc and the other for mgc. The agc voltage was obtained from the IC itself, possibly from the IF amplifier agc decoupling pin. I should guess that the TDA1071 mixer was of the “transistor-tree” multiplier form, as in the MC1496, and that reminded me that way back when I had seen the schematic (attached) for a 10.7 MHz IF strip that used a couple of MC1496s, with variable DC on the “carrier” inputs, to provide agc. Also, the first stage of the Motorola MC1350 TV IF amplifier IC was a transistor-tree with agc DC applied to the upper section, thus acting as a variable gain differential cascode amplifier with input impedance essentially unaffected by the gain setting.

Eddystone used a second TDA1071 for AM AFC on the 1570/1590, configured with a quadrature demodulator. This was fed from the IF amplifier input of the main TDA1071, and as in that case, the “mixer” section was used to provide gain, although in this case without agc. The FM IF section of the 1570 used yet another TDA1071, with the “mixer” section used as an IF pre-stage ahead of the main selectivity filter, and fed directly from the UM1181 tuner module. (I think it was a Mullard module; anyway, it was described in Ambit Catalogue #3.)

One wonders whether, given the apparently unusual nature of the TDA1071 in respect of AM demodulation, there was some reluctance to use the quasi-synchronous technique for AM (LF, MF and HF) broadcast reception, whereas once executable in IC form, it had adopted quickly for TV vision demodulation. I seem to recall some criticism as compared with diode demodulation voiced in a Practical Wireless magazine of the late 1990s. The Australian manufacturer Allen Wright included both precision rectifier and quasi-synchronous demodulators in Wideband AM Tuner 2 (reviewed in Electronics Australia August, 1980 edition.) The rationale for the quasi-synchronous demodulator (which was based upon an MC1330 IC) was given as: "While this detector gives a little more distortion, it can be useful at night to reduce 'monkey chatter' from adjacent signals."

On the other hand, parts of the NAB Engineering Handbook are available on line at Google Books and the section on C-QUAM decoding (page 709) includes the comment: “The envelope detector demodulates the monophonic, L+R information. It may be a simple diode detector; however, most stereo demodulator integrated circuits utilize a limiter/multiplier approach that offers superior performance. Distortion measurements in the 0.1% to 0.3% region are commonly found at 99% negative modulation when this technique is used”. Thus it would seem that any distortion problems, if in fact they exist, relate to execution and not the concept itself.

If the TDA1071 demodulator section was like that in the SL624C, then the limited reference would have been fed into the bottom part of the transistor-tree, with the AM signal into the top half. This was the reverse of what might be considered normal, as found in TV vision quasi-synchronous demodulators and for the MC1496 when configured for AM demodulation. But this “reverse” arrangement does seem to have been normal for FM quadrature demodulator ICs, and so was also used when such ICs were employed for AM demodulation.

Cheers,
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Old 6th Dec 2014, 9:05 pm   #18
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Default Re: Quasi-Synchronous Demodulation

Although data on the TDA1071 IC remains elusive, I have established that there were a couple of Mullard articles about it, as shown below. These appear to be unavailable on the web.

The TDA1071 radio IC in communications receivers
J.A. Garters and J.S. Malcolm
Mullard Technical Communications 132, October 1976


The TDA1071 IC in a.m./f.m. radio receivers
J.S. Malcolm
Mullard Technical Communications 134, 1977


I’ll place an enquiry for these in the “Wanted” section.

Cheers,
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Old 6th Dec 2014, 9:31 pm   #19
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Default Re: Quasi-Synchronous Demodulation

My late-1970s/early-1980s synchronous-demodulation experiments [aimed at getting better reception of medium-wave broadcast stations after dark] focussed on a selective-sideband approach: feed the IF signal through three filters - a USB/LSB pair to separate the two sidebands, and a really-tight [200Hz bandwidth] filter to extract the unimpeded-by-modulation carrier.

Then mercilessly amplify/ clip the carrier to the point where it's a square-wave - and use it to feed a pair of diode-quad mixers [product-detectors] driven by the outputs of the upper- and lower-sideband filters.

And then combine the two resulting audio-signals in appropriate phase.

The theory was fine; the reality was that the group-delay characteristics of the three filters was inconsistent so when combnining the two outputs their phase-differences varied horribly with frequency.

I got far better results by pushing the IF signal through a tighter filter - essentially throwing-away one sideband - and using the 'clipped-carrier' to lock a PLL based around the NE561 chip to provide the carrier-coherent component for the demodulator.

If doing this today I'd feed the 450KHz signal into an analog-to-digital converter and after that the rest becomes a few hundred lines of non-challenging software....
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Old 6th Dec 2014, 11:22 pm   #20
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Default Re: Quasi-Synchronous Demodulation

I think that your results were more-or-less as would be expected. That is why, I imagine, that when broadcasters used valve-era professional three-channel ISB receivers for reception of HF relays for rebroadcast, they mostly used one or other of the sidebands but as far as I know did not attempt to combine the two after demodulation. The results seemed to have been good enough given that the technique was apparently quite widely used. (See the ISB Receiver thread; https://www.vintage-radio.net/forum/...d.php?p=660609.) When demodulating a single sideband, the reinserted carrier phase less important. But synchronous demodulation of a double sideband signal requires that the reinserted carrier phase be closely aligned with that of the original carrier. If the sidebands are demodulated separately with a non-phase aligned reinserted carrier and then recombined, then I suspect that the effects of the carrier phase error would reappear, compounded by whatever other errors were occasioned by the separate processing of the sidebands.

But the fully synchronous PLL approach, when done well, is definitely better for fading MF and HF signals. With full carrier AM, the carrier filter appears not to be needed, as the PLL itself acts as a very narrow band filter. And separation of the sidebands after demodulation could be done by the phasing method rather than using separate sideband filters. Thus switching between DSB, USB and LSB was done fully in the audio domain. With later solid state three-channel ISB receivers using PLL carrier recovery, the carrier filter was still used, I imagine for noise reasons given that the pilot carrier was 16 or 26 dB down.

Way back when I had the opportunity to discuss this topic with the designer of the Phase Track Liniplex HF receivers, which probably had one of the best implementations of PLL synchronous demodulation. At the time I did not have the knowledge to ask as many questions as I would today, and nor did I take any notes. The difficulties of attempting carrier and sideband separation using IF filters were mentioned though, as well as the need for a tracking PLL. Receiver synthesizer phase noise was an issue that showed up with USB and LSB operation, which is why the original Liniplex design used a crystal-controlled oscillator. An interesting comment was that with DSB AM, even better results could be obtained by using as Costas loop, because this regenerated the carrier based upon the sideband information, and so had it phase-aligned with the whole signal. The obverse of that, though was that sideband separation was reduced in the USB and LSB modes.

The group delay problems introduced by separate sideband filters could have been why Crosby did not attempt to separate the sidebands in his exalted carrier receiver proposal. I had written the following a couple of days back, intending to edit it and post it before long, but as it fits here I’ll include it as is, including any redundancies.

Related to this topic, I recently came across an article by M.G. Crosby about exalted carrier AM reception, in “Communications” magazine for 1945 February (available on the excellent AmericanRadioHistory.com site).

Here the primary objective was to minimize the effects of carrier fading distortion on MF and HF reception. In essence this was achieved by extracting the carrier (at 2nd IF in a double-conversion receiver) from an AM signal using a very narrow bandwidth crystal filter, amplifying and limiting it, and then after appropriate phase adjustment either recombining it (at exalted level) with the full bandwidth IF signal for conventional diode demodulation or using it as the reference input to a product demodulator whose other input was the full bandwidth IF signal. Also included was an AFC system driven from the narrow bandwidth carrier channel.

Insofar as reconditioned incoming carrier was used as the reference, I think that this would qualify as quasi-synchronous demodulation, at least when a product demodulator was used.

Actually though the Crosby receiver looks much like the point-to-point SSB/ISB receivers that were originally developed in the 1930s. These also separated and reconditioned the carrier for use as a reference after extraction with a very narrow band width filter. The sidebands were also filtered out separately and then individually demodulated in product demodulators that used the reconditioned carrier as reference. Usually provision was also made for using a locally generated carrier for use with those SSB transmissions that had fully suppressed carriers. Carrier-driven AFC was the norm for these receivers. They could be and were used for the reception of AM signals in order to minimize the effects of carrier fading distortion. In that case one or the other sideband was used according to which had the least interference.

So one may ask what did the Crosby circuit bring to the table beyond what existing SSB/ISB receivers could do. Use of both sidebands simultaneously – assuming neither was suffering from undue interference – would have reduced the amplitude variations due to selective fading in the recovered audio, as typically only one sideband at a time is affected as the fade combs through. My first-hand experience with the Liniplex F2 confirmed that this was a non-trivial gain, at last for SW programme listening (it showed up in my ad hoc BBC WS Play-of-the-Week test; DSB, where one could use it, was definitely better than LSB and USB). Also, there would have been no loss of lower audio frequencies. The sideband filters in SSB/ISB receivers cutoff points varied with type, say 300 Hz for voice filters (300 to 3400 Hz) and 100 Hz for broadcast relay filters (100 to 6000 Hz), so there was always some loss, and the 300 Hz number was very inappropriate for broadcast listening.

Crosby also proposed that his receiver could be used for reception of phase-modulated transmissions in fading conditions, for which purpose both sidebands would have been required. This would have required a π/2 phase difference between the reference and signal feeds to the product demodulator, rather than the zero difference required for AM demodulation.

Whether the Crosby receiver was ever realized in practice I do not know. One candidate for consideration was the Press Wireless exalted carrier diversity receiver, mentioned in the BBC description of its Tatsfield monitoring station, see: http://www.bbceng.info/Operations/Re...rch%201961.pdf. Although I have not found any information specific to the Press Wireless receiver, the associated patent provides some clues. It appears that the Press Wireless approach was to use a locked oscillator to generate the local reference carrier, this normally being locked to the incoming IF. The oscillator output was mixed with the full bandwidth IF before demodulation. When the incoming carrier level was too low to allow locking of the oscillator, the latter was suppressed, and normal AM demodulation took place. That suggests that the demodulator was of the rectifying type, not the product type. Be that as it may, it looks as if the Press Wireless receiver had fully synchronous rather than quasi-synchronous demodulation, and it did not have a narrow bandwidth carrier filter.

Cheers,
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