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Why Displacement Modulation Is the Simplest Thing In the World

Jun 23rd 2023, 14:51


Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
I have continued to experiment with Displacement Modulation, through many mis-steps to be sure, but always eventually propagating the wave successfully (at 40-meter frequencies). But, I just realized I've never explained how to set it up. Technically, it's about the simplest mod in the world if you have an old vacuum tube transmitter or amplifier sitting around. Unless you have a lot of experience under your belt, you'll probably need the schematic. With that in hand, proceed as follows:

1. Find the point near the control grid (G1) of the final RF amplifier stage where the DC bias AND the RF input come together. This is a 'summing point' (to borrow an Op Amp term) where the DC bias voltage and the RF voltage are being algebraically added for input to the tube.

2. At this summing point, solder on an RF choke, sized for a good high reactance at the carrier frequency you're using. This choke will prevent the RF input from being drained away to ground through the audio connection.

3. At the other end of this choke, solder on a fair-sized electrolytic capacitor. Our audio current will be small, so even 30 uF is big enough (we're handling a single human voice, not a symphony orchestra). Voltage should be at least two times the bias voltage (cathode-to-grid DC bias) and the NEGATIVE end should be soldered to the choke (because, typically, the grid will be negative in relation to the cathode - this is important!). This capacitor keeps our audio setup from altering the amplifier's DC bias.

4. Set up a pair of terminals (binding posts or whatever you like) on a convenient location at the rear of the chassis. These will be the AUDIO INPUT terminals. Now, solder an insulated lead from the positive end of the electrolytic just installed to a one of these terminals. Next, solder a lead from the cathode of the tube to the other audio terminal. This completes the RF amp modification.

What this should tell you is that the audio will be applied between the control grid (G1) and the cathode of the tube. This would be considered a very high impedance input - a fact important to remember.

5. Outside the chassis, set up a tube-type Audio Output Transformer (often known as a loudspeaker transformer) so that its HIGH impedance side (usually labeled the Primary) is attached to the two terminals just installed. It will probably work best if a resistor (somewhere in the range of 9K to 15K) is also installed ('shunted') across the two terminals to provide a reasonable load.

The other side of the transformer (i.e. the Secondary) is used as the primary here. It will be attached to the speaker output of any small audio amp that can be set up to handle your microphone input (you will need a small mic preamp if your audio amp isn't designed for microphone input). Even one side of a small stereo amp can be used, since very little output power is required.

That's all there is to it. Modulation level is readily controlled by the volume control setting on the audio amp or preamp. Simplicity itself.

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