Homodyne Reception

If you think that this is the story of greeting gentlemen of dubious sexual preference then think again. This is probably nowhere near as interesting, but it is something a little different.

The earliest radio receivers used a simple rectifier or “coherer” to detect a signal. Selectivity was almost non-existent and if two or more transmitters were operating in the vicinity it was all but impossible to separate them. Marconi’s application of Sir Oliver Lodge’s “syntony” or the tuned circuit was an improvement, but it wasn’t until the triode or “Audion” amplifying valve was invented by Lee de Forest in 1909 that things really started to look up. By using valves (with their high input impedance- tuned circuits were less highly loaded or damped, but the TRF ­Tuned Radio Frequency receiver still wasn’t too selective especially as the frequency increased. In 1917, Major Edwin Howard Armstrong invented the superhet as a means of improving receiver performance. You will recall that the “supersonic heterodyne” works by converting all incoming frequencies to one common IF or intermediate frequency and that the major amplification takes place at this frequency. In general terms, the lower the IF frequency then the better will be the selectivity but the worse will be the image or second channel problems. This is because at the lower frequencies we can achieve a higher Q or quality factor with simple tuned circuits, but as the frequency rises so Q tends to decrease and the response broadens. Early superhets had IFs of around 150kHz; this gave good selectivity, but once the received frequency got above a few megahertz the “front-end” or antenna tuning circuits haifficulty discriminating between the desired frequency and the co-channel or “image frequency”. (Double, or even triple conversion superhets were designed to overcome this defect, but that was later on in the story-.

Additionally, tuned circuits were not “ganged” for automatic tracking and sometimes the IF circuits had individual tuning controls as well, all of which made the early superhet an interesting piece of equipment to operate. However, needs must when the devil drives, and it wasn’t too long before tuning circuits were ganged thereby greatly simplifying tuning, and the superhet became the standard circuit in all but the very cheapest of receivers. This was fine at the time because broadcast stations were mushrooming and sensitivity and selectivity were the names of the game. The audio side of the receiver was also pretty restricted in its response, especially at the higher frequencies, the upshot of it all being that stations were generally limited to a bandwidth of 9kHz in Europe and 10kHz in Americnd Australasia. Since an AM signal has symmetrical sidebands, this limited the highest transmitted audio to half the bandwidth, or around 4.5kHz. This was fine for voice transmission, but left something to be desired if you wished to hear the finer points of young Mr Yehudi Menuhin tickling his Stradivarius.

By the 1930s Howard Armstrong was experimenting with frequency modulation as a way of overcoming the static, mutual interference and other noises-off so common on AM. Additionally, FM was able to take advantage of the huge strides then being made in audio reproduction, and by the late 1930s he had an FM station up and running in New York, but commercial rivalry with the AM system and then World War 2 got in the way of further expansion of the service. However, when things settleown again, FM broadcasting took off in a big way in the U.S.A. and Europe offering listeners extended audio response to 15kHz together with noise and static free reception. Australia dabbled with the system experimentally, and finally the Government gave the green light in the late 1970s. Many of you will recall the days when the only two FM stations in Sydney were 2MBS-FM and ABC-FM. By the late 1980s, however, most local music broadcasters and, I use the term “music” in the loosest sense, had migrated to the VHF FM band, leaving the AM band mainly to talk-fest type broadcasting, where a bit of sideband cutting, restricted audio response, and the occasional burst of QRN didn’t make much difference anyway!

Interestingly, the AM scene has changed in recent years and some broadcasters are now transmitting signals whicave the potential to rival the quality of FM. The latest transmitters use Harris modulation, a digital system which is virtually distortion free even at 100% modulation, and some stations are now permitted to transmit audio frequencies up to 15kHz, giving a signal width of up to 30kHz. Many transmit in stereo, though this facility seems not to be widely used. In Sydney, Radio 2SM on 1269kHz not only uses the Harris system but also records all its audio digitally on hard-disc and uses a special high-quality dedicated circuit between the studio and transmitter, which is claimed to be much superior to the boosted high-quality Telecom lines usually used. Clearly the limitation is now likely to be the AM receiver with its relatively narrow response due to the pass-band shape of the IF system, and it becomes an interesting exercise to examine ways to recover this audio without chopping off the high frequencies.

Signal demodulation of AM is usually by diode rectification and this itself introduces a degree of distortion due to the non-linear characteristic of the diode action, so something better is needed. In 1947, D.G.Tucker proposed the synchrodyne method of demodulation in which the sideband content of the AM signal is convert directly to audio frequencies. Tucker observed that if the local oscillator in a superhet is made the same as the original transmitter carrier frequency, the mixer output will be the difference between the carrier frequency and the instantaneous sideband frequency, thus giving an audio signal directly, and the highest audio signal will be limited only by the highest transmitted sideband, assuming the absence of any filtering. This, of course, is the familiar direct-conversion receiver and is the basis of the product-detector now found in all SSB receivers, but the problem with broadcast quality reception is that not only must the local oscillator be kept absolutely at the carrier frequency, but also the transmitter oscillator and the receiver oscillator must be kept in phase. For voice reception, minor differences between the two oscillator frequencies are tolerable because the voice appears only slightly higher or lower in pitch; this situation is intolerable with music.

Tucker, and others, dievelop circuits which kept the local oscillator locked in frequency and phase with the transmitter, but these were complex and made the receiver too expensive for domestic use, although some were built for off-air monitoring by broadcast managements. Nevertheless, the possibility exists to recover all sideband information transmitted, though where stations are at the minimum spacing a 9kHz whistle may be a problem. (Stations geographically close are generally quite widely spaced in frequency, so this problem of whistle is more likely at night when ionospheric propagation of distant stations causes the carriers to beat at 9kHz-.

Is it possible to use the carrier of the transmitter itself as the source of the receiver local oscillator? In 1973, J.W.Herbert, ZL2BDB, published an interesting idea to achieve just that, using the complex innards of ICs to create an outwardly simple circuit. After all, if we could create a local oscillator from the original carrier, then such an oscillator would inherently have the same phase and frequency as the transmitted signal. Modern integrated circuits have made this possible in a deceptively simple way, and this is where we finally get to the homodyne system. The idea is to strip the carrier of its original modulation, amplify it as a sine-wave, and then “beat” it with the original signal to achieve direct conversion from modulated RF to audio.

The bandwidth of the recovered audio will be limited only by the response of the audio amplifier, plus any sideband cutting which may occur due to the necessarily having tuned circuits ahead of the product detector. Because the local oscillator is deriveirectly from the off-air signal there will be no tuning whistles or heterodynes such as you get when tuning a conventional receiver with a conventional product detector. Try tuning an AM signal in the SSB mode of your receiver to see what I mean. Additionally,there can be no second-channel breakthrough. Indeed, it is quite eerie at first when tuning with the homodyne; you just slip from one station to the next with no squeals, whistles, sideband hash or anything! Just good, clean audio.

So how is it done in practice? Refer now to the diagram. Signals are amplified by the quite broadly tuned RF amplifier (to minimise sideband cutting-, after which they pass to the detector stage. Here, within the IC the signal is again amplified and split into two paths. One goes directly to the balanced modulator, while the second goes to a limiting amplifier, after which it is fed to a pair of back-to-back diodes. Because of the action of the limiting amplifier and the diodes, we are left with a carrier stripped of its original modulation, but now in the form of a square-wave wave-train. This square wave is now passed to the tuned circuit, tuned to the signal frequency, where it is restored to a sine-wave by the fly-wheel effect of the tuned circuit. This sine wave is fed also to the balanced modulator. If you have become a little lost here, recall that the action of the limiter followed by the diode clipper will be a square wave output at the signal frequency. A square wave consists of an infinite number of odd harmonics of a sine wave, and the harmonics are created by the non-linear action of the diodes. This removes the modulation. Passing the square wave into a high Q tuned circuit selects only the fundamental of the square wave which, as we have seen, is a sine wave. Thus we have available at the balanced modulator the two ingredients needed for direct conversion; a modulated RF signal and a local oscillator which is of exactly the frequency and phase of the original carrier-. Thus the output of the balanced modulator or detector is an audio signal which is directly proportional to the original modulation. In theory we can recover the entire audio spectrum being broadcast from the transmitter, but in practice we may have to employ a notch filter to remove unwanted high frequency beats from adjacent stations, especially at night. Notwithstanding, the recovered audio is substantially better than that from the usual superhet, and there is potential for experiment.

Is it worth all the hassle? Talk-back garbage is still garbage even if the voice is undistorted (but still often heavily compressed-, but 2SM sounds terrific if you are into non-stop country music. You can really pick the difference in audio quality from one station to another, and as an exercise in AM reception building a homodyne was a lot of fun and certainly rewarding in that sense. Yes, it really does work, and if anyone would like to pursue the topic I have a couple of circuits whicave been published at various times.Even if you have no intention of building a homodyne, I hope that just reading about it will have given you some insight into the technique.

I certainly learned a lot: maybe you have learned a little, too.

Clive, VK6CSW, formerly VK2DQE.[via Sam Wright VK6YN (G3CYT)]

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