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Displacement Modulation - Using an SSB Receiver

Jul 23rd 2023, 15:19


Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
Well, last week I got myself a "new" receiver via eBay - a quite nice looking Heathkit HR-1680. The Copyright date on the book I downloaded says 1976, so this design is almost 50 years old, practically state-of-the-art by my standards. It's very clean except for just a thin film of oxide on the RCA connectors in the rear, which was easy to fix up.

After connecting this to my outside antenna and ground to check out its basic operation and get used to the controls, I moved it into the workroom and set it up in place of my MFJ Regenerative Receiver (my transmission monitor). This means it has a good ground to a driven rod outside, but only has about a 3ft dangling wire for an antenna, and is physically a stone's throw from my EICO 723 transmitter (maybe 14 or 15 feet). OK for preliminary testing.

Today I took this receiver out to 'the old barn', a ritual I refer to as 'portaging the new receiver'. This is an old corrugated steel shed that can't keep out rain, wind or snow but DOES have these critical features: 120VAC power, an 8ft driven ground rod AND connectivity to The Giant Antenna. This antenna is usually called the 'electric stock fence' and comprises maybe a quarter mile of bare wire located 3ft above ground level on average and is well insulated from ground except for contact with about a thousand weeds of variable moisture content. I just disconnect it from the fence charger and use a couple of AWG 14 insulated hookup wires to connect it and the ground rod to whatever receiver I'm working with. The closest point of the antenna to the station is approximately 1 wavelength away (40 meters in my case, or just over 130 feet) - not exactly DXing, but better than just testing from the next room in my basement.

This receiver is quite sensitive and is very simple to set up and use. Both testing locales produced almost identical results, even in terms of signal strength and background noise (which was pretty horrendous). I first ran the transmitter without voice modulation - finding it on the dial was easy due to the incessant 60 Hz hum that I have never been able to keep out of my transmitters. (Someday when I'm gone all anyone will remember about my voice online will be that '60 cycle hum'.)

The most surprising (to me) and disappointing finding was that I am able to tune the signal as both upper and lower sidebands, at good signal strength. Sidebands just aren't supposed to be happening with this modulation! But, there they are. When I found this on the short haul testing, I really hoped they would disappear after portaging the receiver out a good distance (i.e. hoping this was a 'local' effect due to proximity), but not so. The recorded voice sounded just like 'sideband voice' as you tune into it. Nuts.

On CW, I got exactly what I hoped I would get - as you tune onto the station, the CW whistle (from the carrier) drops down and as you get close to 'zero beat', the voice quickly rolls into place. When you get it at just the right spot, the voice quality is really good with no noticeable distortion. Although the sideband voice was intelligible, this CW version was almost full fidelity, the two flaws being that (a) the eternal background hum is noticeable, and (b) strangely, you don't seem to be able to quite get to 'zero beat' - just down to a very low growl and then the tone heads upward again on the other side. I suppose this second effect could be a receiver alignment problem.

Two interesting controls on this receiver have noticeable effects on the whole process. The Preselector control is probably the most 'front end' control on the unit, and is supposed to be readjusted any time you change bands, to get optimum reception. At 40 meters, the 'sweet spot' is just past 50 percent rotation - roll it to either side of that and the audio quality drops rapidly (as does the background noise).

The RF Gain control is more interesting in terms of my modulation. It's a very sensitive adjustment - turn it counter-clockwise just beyond a certain point and ALL audio just 'rolls off the table'. However, just to the right of that spot there's a little magic zone where the background noise drops off way more than the demodulated audio - so, though the audio is pretty low level, it stands out clearly and you can really hear the quality of the spoken voice. Fascinating. (Remember, this is in the CW Mode setting.)


Ordinary amateur voice communication could be carried out with this modulation today. All that's required on the transmit side is a slightly modified CW transmitter fitted with a small audio amplifier, mic, mic preamp and an old-fashioned audio output transformer. For receiving, apparently all you need is a classic SSB receiver with a good CW mode setting.

Of course, nothing is proven about the 'punch' of such a signal in competing with noise, nor of its ability to interfere with other traffic. Having it show up as tunable sidebands is certainly the greatest disappointment. As usual, more experimentation required ...


Of course, as often happens, my observations might be a lot better than my conclusions. I am very ignorant of how SSB actually works on the receiving end, and I ASSUMED that a receiver in its LSB or USB mode can ONLY tune that kind of signal. Again, silly me - that's probably not how it works.

In another thread a long time ago, someone said that an SSB receiver had no problem tuning an AM signal. I assumed that was because it would just tune onto one of the sidebands (which would always be part of such a signal) - but maybe he was really saying that even in a 'sideband' mode the carrier would be detected and demodulated. If that's the case, then the above-stated 'evidence' of sidebands on my signal is simply bogus.

One thing I recently equipped myself with is a little device called TinySA - a really neat piece of gear. However, looking at discussions about it, it doesn't seem to be able to show the presence of sidebands on a modulated signal. It just can't slice the universe fine enough to separate them out. What kind of device would be needed to do this? Or, is there another testing technique that would actually prove the presence or absence of generated sidebands?

This is a crucial question, because the narrow bandwidth of the Displacement Modulation signal is the only 'selling point' that makes it worth the bother of doing this.

Larry K0WUQ

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