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Why Displacement Modulation Is NOT the Easiest Thing In the World

Jun 23rd 2023, 16:12


Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
There are many pitfalls to trying to get Displacement Modulation to work. Interestingly, many difficulties have to do with capacitance; others are simply variations on imperfect tuning of the amplifier output circuits. Here's a summary of the difficulties I have found, all based on my experiments at 7 MHz (40 meters):

1. Instrumentation for RF amp tuning: In my opinion, it is not possible to be sure you are getting good Displacement Modulation propagated without monitoring the wave 'on the air'. Simply looking at what's happening in the RF amp's plate tank circuit won't do, because it's easy to lose the modulation in the antenna or antenna tuner. I have found that TWO monitoring systems are functionally necessary: a) A 'remote' receiver tuned as precisely as possible to the carrier frequency being used; and b) A 'remote' oscilloscope set up with an antenna, ground and small tank circuit of reasonable Q, to reveal the ether wave shape. In both cases, 'remote' means at least a few feet between the instrument and the transmitter.

The receiver by itself cannot guarantee that the modulation is being achieved, because if you're generating an AM signal, it will be detected (and thus, will sound) exactly the same. What listening to this receiver WILL tell you is if distortion is present.

Similarly, what the 'scope will show you is that Displacement Modulation is in fact what's propagated - the wave is completely different from AM and totally unmistakable; BUT, it cannot show you whether the propagation is a 'clean' signal. If there is ANY distortion of the wave, some AM is being created and you will consequently be generating sidebands (though their amplitude will probably be small). The creation of sidebands is exactly what we're trying to eliminate, of course.

The combination of seeing the modulation on the scope and hearing a clean, undistorted audio output is exactly what's needed to assure that you're getting exactly what you want. This combination is the only method I've seen that establishes exactly what's being propagated.

2. Tricky tuning method: Here's the method I've found that gets the job done every time:
- First, get monitor receiver and monitor 'scope going.
- Next, start up your transmitter and do a 'key down' tuning to maximum efficiency - exactly as you would for CW operation.
- Now, reduce RF power to about 60 percent. This is to provide 'headroom' and 'footroom' for the modulation, exactly as we would have to do for unclipped AM operation.
- Next, with the key 'locked down', start audio input (probably best using a sine wave generator, though a spoken voice recording can be used).
- Tune the monitor receiver for the best reception possible. If the audio is heard at all, it will be heavily distorted. The received signal on the 'scope' will probably be a 'string of beads' pattern, i.e. equal amplitudes above and below centerline. just like AM.
- De-tune the RF amp plate tank circuit in the direction of reduced capacitance. The audio heard in the monitor receiver should gradually clarify, and the wave shape on the 'scope should be something like a 'dental X-ray' shape - unequal modulation above and below centerline. Eventually a 'flat-top' or 'flat-bottom' modulation should be reached, while the signal heard through the receiver may sound nearly undistorted - you're almost there!
- Continue de-tuning until you get a 'scope trace where the top and bottom of the wave appear to deflect together in the same direction the same amount, and the detected audio seems undistorted. Note that it IS possible to over-modulate even without the 'scope trace showing any visible signs, such as clipping of the peaks!

That's about it. This makes it sound a lot harder than it really is, and it actually becomes fairly natural after a couple of hundred tries. Note that the de-tuning may cause ancillary problems such as stronger harmonics.

There HAS to be a better way of doing this. More experimentation required ...

3. SWR / antenna tuning problems: If you have bad antenna tuning or just plain bad impedance matching in the output, this shows up as the 'string of beads' wave pattern on the 'scope and somewhat distorted audio (if you can hear any detected sound at all). All you can do is correct the problem with the usual methods.

4. Excessive system capacitance: If your amp somehow has a lot of 'stray capacitance', the modulation will not be achieved. Capacitors such as by-pass and DC blocking caps should be the smallest values that will work. One thing this means is that on a multi-band transmitter, the highest bands may not support the modulation at all.

5. Plate tank circuit capacitance-dominated: In my experiments, I found that a plate tank L/C Ratio of less than about 0.06 uH/pF would not carry the modulation - so, if you just can't seem to make it work, this could be the issue. Unless you're home-building, this may not be easily correctable. Note that this was discovered at a 40-meter carrier frequency; the critical ratio might be totally different at a different one. More experimentation required ...

Doesn't this sound like fun?

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