Homodyne/Direct Conversion
 

Having read Al's ( Skywave) general question about short superhet definition it made me think about Direct conversion receivers. Without interfering with his thread I set this thread off to answer the simple question of why direct conversion is not widely used as opposed to a superhet design.

Ideally the Homodyne is simpler and uses less stages and components, more efficient detection and does not suffer the effects of image frequencies etc.

Why has it not been more widely used in not just Domestic but also in Commercial radio.

Mike

Re: Homodyne/Direct Conversion
 

Could it be because of the audio image that this design produces?

Al. / Skywave

Re: Homodyne/Direct Conversion
 

Theoretically there is more selectivity why would the audio image be such a problem.

Mike

Re: Homodyne/Direct Conversion
 

My initial thought did not take into account the necessary filtering, post-mixer, to remove the audio image. However, this link goes some way to providing an answer to your Q. (And does a better job than I could :-) )

http://en.wikipedia.org/wiki/Direct_conversion_receiver

Al.

Re: Homodyne/Direct Conversion
 

I'm not sure what people mean by the audio image, in this context. An AM signal has two sidebands each carrying exactly the same information. The issue with the homodyne is getting the reinserted carrier exactly at the right frequency and phase. This needs either a very narrow filter and precise tuning, or a phase locked loop. Either would be very expensive with 1940's-60's valve technology. Modern ICs can do it cheaply, but still not as cheaply as a simple diode envelope detector.

The image is a problem with SSB reception, but then you don't need a PLL as the frequency doesn't have to be exact and the phase doesn't matter at all. So you save money by losing the PLL, but instead need a good filter or an image-cancelling mixer (otherwise known as phasing SSB).

Re: Homodyne/Direct Conversion
 

G8HQP Dave has confused me! Until now, I had considered a homodyne to be a direct conversion receiver, with the locally generated carrier derived from the input itself with suitable limiting and filtering. so the frequency is always right. In other words, it's a special case of a synchrodyne (which does have a local, critically tuned oscillator, with PLL or some other sync system).

One thing which interests me - does a direct-conversion receiver work these days with Radio 4 on long wave? I ask this because the transmitter is now phase modulated with data to control electricity meters. It's transparent to a conventional envelope detector - but to a synchrodyne - ?

Re: Homodyne/Direct Conversion
 

This may help:

http://dictionary.sensagent.com/dire...eceiver/en-en/

Al.

Re: Homodyne/Direct Conversion
 

Hi Gents, the champion of direct conversion was Prof D G Tucker of Birmingham uni, he published several articles in Electronic Engineering in the 50's and I think a "re-visited" article in WW in the 70's. From memory neither of these sets were particularly "twitchy".

Ed

Re: Homodyne/Direct Conversion
 

I recall, John Linsley-Hood published a design in Wireless World, in the 1980's, also. This was for a synchrodyne.

Thanks for the link, Al Skywave.

Re: Homodyne/Direct Conversion
 

The Linsley-Hood synchrodyne receiver was described in Wireless World (or by then Electronics and Wireless World) issues January, February and March, 1986. As well as the final design, the article discusses some of the practical issues involved in making the synchrodyne concept workable.

A combined synchrodyne/homodyne receiver was described by Slifkin and Dori in Electronics World of November, 1998. And a simple homodyne receiver, based upon the MC1330 television vision demodulator IC, was described by Herbert in Wireless World of September, 1973.

Although true direct conversion synchrodyne and homodyne receivers are quite rare, the use of synchrodyne (fully synchronous) and homodyne (quasi-synchronous) demodulators in conjunction with conventional superhet receivers seems to have been reasonably widespread. The need for amplification ahead of demodulation, and the non-linearities inherent in such indicate that reasonable pre-demodulation selectivity is desirable, and for the homodyne case, suppression of adjacent channel signals is also desirable as these will demodulate directly (although I think to a lesser extent than with envelope demodulators) as well as translate to out-of-AF band signals.

Macario described a homodyne demodulator, using hard-limited carrier as the reference frequency, in Wireless World of April, 1968, and commented upon its performance when used in conjunction with an Eddystone 940 receiver. Brook described an add-on phase-locked loop synchronous demodulator in Electronics and Wireless World of September, 1989, and Hershberger described another in Popular Electronics for April, 1982. The latter also included a homodyne demodulator for “envelope” demodulation, as well as post-demodulator sideband selection by AF phase-shifting.

Use of homodyne demodulators based upon hard-limiting of the IF signal to obtain the reference carrier seems to have become more common from the later 1970s onwards in commercial and consumer receivers. One example was the Eddystone 1570/1590, which used the apparently multipurpose (but mysterious – no information available on the web) TDA1071 IC. As well as IF and AGC functions, it seems that this IC could also be configured for both homodyne AM demodulation and quadrature FM demodulation. The Plessey SL625 IC was similarly configurable in respect of demodulation functions. Homodyne demodulators as used in AM receivers seemed mostly to use hard-limited carrier as the reference without a tank circuit between the limiter and the demodulator itself. However, in the hi-fi world, the second version of the Australian Allen Wright Wideband AM tuner (MF only) included an MC1330-based homodyne demodulator as well as a precision rectifier envelope demodulator. (The original version was designed to match or better, in solid-state form, Quad AMII/AM3 tuner performance.)

However, the use of homodyne demodulation in production receivers predates the solid-state era. The point-to-point professional ISB receivers of the 1950s, such as the Mullard GFR 552 and a similar Marconi model whose designation I do not know. These extracted the pilot carrier in a 60 Hz bandwidth filter (at 100 kHz second IF) prior to its limiting and use as the reference in the sideband demodulators. They also had selectable bandwidths of 3.5 and 6 kHz, the latter allowing their use in SSB and ISB broadcast quality program links. I imagine that these or similar receivers were also used for receiving regular shortwave broadcasts for rebroadcasting purposes. The late 1960s solid-state Marconi Hydrus ISB receiver might have offered similar facilities, but there seems to be little information about it.

A professional HF/ISB receiver with an apparently early synchrodyne demodulator application was the Eddystone EC958/12, circa 1974. This retained separate USB, LSB and carrier filters with their separate IF amplifier strips, but used the recovered carrier to drive a phase-locked loop. I think that other commercial receivers of the period, such as some of the Plessey models, may have been similarly configured, but I am not sure.

For consumer and semi-professional receivers, the use of synchrodyne demodulators, based upon phase-locked loop techniques, and usually with a selectable sideband facility based on AF phase-shifting, seems to date from the later 1970s. The Sansui TU-X1 hi-fi tuner was an early example. Perhaps one of the better known was the Sony ICF2001D/2010 shortwave portable , which used ICs originally developed for AM stereo decoding purposes (and used, for example, in the SRF-A100 AM stereo portable receiver) to provide selectable-sideband synchronous demodulation. An outstanding example was the Liniplex F2 (and preceding F1) with its tracking phase-lock loop demodulator. Having used a Liniplex F2 (feeding a hi-fi system) for extended shortwave program listening, I would say that it was an excellent tool for that purpose. Sherwood in the USA offered (and I think still does) an outboard phase-locked lop synchrodyne demodulator unit, and I have also used one of these in conjunction with a JRC NRD525 receiver. It was not quite as good as the Liniplex F2, though.

One of the most common uses of the homodyne demodulator in superhet circuits was in consumer television receivers, as a vision IF demodulator. The Motorola MC1330 IC, circa 1969, seems to have set the pattern, using hard-limited IF signal to generate the reference carrier, which was then fed via a tank circuit to the demodulator multiplier. That the vision carrier being limited was on the Nyquist slope of the IF curve does not seem to have been much of an issue until the advent of stereo sound systems. Still, that said, the BBC RC1/511 TV receiver, described briefly in Electronics & Wireless World of July, 1984, used a more elegant approach with the carrier IF separated after amplification and then conditioned by filtering and phase-shifting before being used for vision demodulation and intercarrier generation. There was also some use of synchrodyne demodulators for vision IF, as in the RCA CA3136E and national LM1823 ICs.

Clearly the advent of integrated circuitry made it much easier to include homodyne and synchrodyne demodulators in consumer receivers. Still, some early American colour TV receivers included phase-locked loop techniques in their chroma demodulator sections, based totally on valve technology. (Carnt and Townsend Volume I describes some interesting examples.) And if one allows that quadrature-type FM demodulators are within the synchronous family, then these go back well into the valve era. The gated beam valve (6BN6) and nonode (EQ80) would appear to lean more to being of the homodyne type, whereas the later locked-oscillator (EH90) type looks more like a synchrodyne. Insofar as single valves did the limiting and demodulation jobs, they were early examples of “integration”. And FM quadrature demodulation was an early function to be integrated. Back in the 1970s, I had a Bang & Olufsen 24 inch monochrome TV receiver whose circuitry was all discrete solid-state except for the TAA570 FM quadrature demodulator. Returning to the valve circuits, one assumes that for FM demodulation, close phase-locking of the reference carrier was not required, as it seems that such simple circuits would not have worked well for AM demodulation. Perhaps an interesting experiment could be done with one of the Eddystone receivers, such as a 940, equipped with a self-oscillating SSB demodulator (EK90), but tuned to an AM signal. Maybe the oscillator would lock well enough to the incoming carrier to allow synchronous demodulation, but I suspect that it would be more a demonstration of the inherent difficulties in getting the synchrodyne system to work, particularly with fading and Doppler-shifting HF signals. Still, RF injection to synchronize the oscillator was what was proposed in an August, 1948 Wireless World article on the synchrodyne by “Cathode Ray”.

Cheers,

Re: Homodyne/Direct Conversion
 

When I read this I wondered if I was confused. However, after checking it seems that some people use homodyne in this sense (i.e. LO derived from incoming signal) while others use it as a synonym for synchrodyne. It has also been used for receivers which use the oscillator from an associated transmitter e.g. Doppler radar speed gun.

Whatever you call it, the problem remains of extracting/producing a signal with the right frequency and phase. A PLL can be complicated. A filter requires accurate tuning. You can ease requirements if you can guarantee that AM negative peaks will never completely remove the signal.

Re: Homodyne/Direct Conversion
 

Hello all,

Interesting thread. I built a homodyne similar to the one here: http://www.vintageradio.me.uk/radconnav/transtrf/ (March 1972 The Radio Constructor, G. W. Short). I chose this as it seemed simple!

It worked ok but needs a fairly strong signal and <90% modulation or it can go a bit odd. The sound is a bit "quacky" and adjacent channels can be a problem. However I built it as a receiver for NBTV signals where - even though it sounds poor it outperforms all the other receivers I've tried. Presumably due to its wider bandwidth and the fact that it doesn't muck about with the phase of the signals - important for vision....

Next receiver I make for NBTV I intend to try a crystal ladder filter at say 1MHz to extract the carrier and a PLL type arrangement with local oscillator to demodulate - not sure how well this will work with VSB signals but it should be interesting to find out...

Dom

Re: Homodyne/Direct Conversion
 

I suspect that the homodyne works well for NBTV for essentially the same reasons that homodyne (quasi-synchronous) demodulators become very commonplace, and eventually more-or-less standard in domestic colour TV receivers once suitable IC technology was available and economic.

The PLL approach would seem to work with VSB signals as evidenced by the existence of TV IF/demodulator ICs such as the LM1823. In the latter case, the carrier is not filtered prior to feeding the PLL control circuitry. Rather, the PLL itself sets the effective carrier channel bandwidth, and at a much lower level than would likely be possible with a filter. I am not sure if the fact that the incoming carrier is on the Nyquist slope of the IF bandwidth curve has much effect on the quality of the reference carrier generated by the PLL. But my understanding is that National developed the LM1823 in part to allow production of a sufficiently “clean” intercarrier signal for stereo sound purposes, something not achieved with the standard homodyne demodulators, and which more frequently has resulted in the use of quasi-split sound techniques.

Many years ago I had the good fortune to have some dialogue with Ed Forster, the designer of and patent holder for the Liniplex HF receivers. In summary, the use of appropriate PLL circuits (in this case a tracking type) was seen as a better choice than the use of filters, although it did require accurate AF phase-shifting circuits to derive the USB and LSB signals post-demodulation. In connection with an item early in this thread, Ed noted that the Liniplex (and similar) receivers could not follow any phase modulation that might be on AM signal, such as with the BBC Droitwich LF transmitter, so its presence would result in some low-level distortion.

When homodyne demodulators are used with AM signals whose negative modulation depth can approach 100%, really hard limiting is required to develop a sufficiently clean reference signal. But it seems to be possible. As far as I know, Motorola used a homodyne demodulator – with unfiltered carrier - to obtain the envelope signal in its original C-QUAM AM Stereo station monitor chain, so one assumes that it worked well enough with the heavily modulated (and heavily compressed) signals that were/are characteristic of North American broadcasting practice. The Hershberger/Popular Electronics homodyne/synchrodyne demodulator design I mentioned in my earlier posting probably also had effective limiting for the homodyne section – I think that Hershberger was the main architect of the Harris AM Stereo system, so presumably knew what he was doing. It is reasonable to assume that ICs such as the SL625 and TDA1071, both having HF receiver applications, had adequate limiting. From what I can deduce about the TDA1071 configuration, it might make the basis for a simple TRF receiver with homodyne demodulator.

Homodyne demodulators have also been used for P-type television systems with positive vision modulation and AM sound, where negative modulation depth can approach 100%. The TDA2542 IC is a vision IF amplifier and demodulator, I think the P-system counterpart to the TDA2541, and incorporates the customary tank circuit between the limiter and the demodulator multiplier. The TDA2543 is a sound IF amplifier and demodulator for AM TV sound, and likewise incorporates a reference carrier tank circuit. On the other hand, the “multistandard” TDA3845, with both FM (quasi-split) and AM channels, does not include a tank circuit on the AM side. So I remain somewhat mystified as to the reasons why the tank circuit is used in some cases, but not in others.

Where limiting is not quite adequate for heavily modulated signals, the homodyne demodulator would tend to act as a squarer, which would introduce distortion. Adjacent channels also cause some distortion, as they modify the parameters of the limited reference carrier. Hence the need for adequate pre-demodulator selectivity in the homodyne case.

Cheers,