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Displacement Modulation - Next-To-Last Mystery Solved

Jul 3rd 2023, 15:09


Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
In my earlier discussions, I mentioned that a mysterious problem was that the modulation could not be obtained when the plate tank circuit was fully tuned to resonance - instead, I had to significantly de-tune from the Peak Efficiency Power tuning point, in the direction of reduced capacitance, for the modulation to get through to the antenna. This has been true in my little homebuilt rig, and also true to almost exactly the same extent on my simple modification of the venerable EICO 723 60-watt transmitter (ca 1960).

So last night, I suddenly wondered, What would happen if we forced the plate tank to resonate at a higher L/C ratio? In other words, push the capacitance radically smaller by radically increasing the coil inductance? This morning, I decided to try this.

In the EICO 723, the plate tuning capacitor is directly tied to the 'bottom end' of the mult-tap coil (a coil about 3.5 inches long and almost 1.5 inches diameter, maybe 25 or so unevenly-spaced turns) - the whole coil is used for 80 meter tuning, which I don't care about. I first removed the wire link from the capacitor to this point. Then, I unsoldered the wire at the 40-meter tap and moved it over to this point at the end of the coil. This means I'm using the former 80-meter coil to tune to 40 meters with the selector switch at the '40' position.

Next, reducing the tuning capacitance: My first stab was a porcelain-bodied trimmer of unknown value between the capacitor and this coil end (meaning that this cap and the tuning variable were in series). When I fired her up, I could never get the plate current to dip - obviously, this series cap must be too small (according to the 723 parts list and schematic, the tuning variable is 140pF max.). I still have a couple dozen 280pF 3KV disks lying around, so I put one of those in the series position. Voila! I was now able to tune the plate tank to resonance.

I was immediately able to observe something I was sure would happen, although the effect was more pronounced than I expected: The change in the tank circuit caused the tuning 'peak' (the sudden dip in plate current at resonance) to become a 'hill' - the current dip was no longer a sharp 'hiccup' but rather more of a 'shallow breath'. What this means is that the Q of the plate tank had been greatly reduced, meaning: (a) much 'softer' tuning to resonance, and (b) much less peaking of efficiency and RF power (similar to the de-tuning I was doing before)!

I now started up my monitor receiver and monitor 'scope (the one with the tunable tank circuit and short antenna on the Vertical Input terminals). When I keyed the transmitter, I was met with the first really interesting discovery of the day, to wit:

During all my experiments with this modulation, I have been plagued by a sizeable AC hum which shows up as a displacement modulation of its own. This varies in degree from time to time, but is always there. It even showed up as hum in my CW signal when I was tuning for that. Always very frustrating, and nothing I've tried has ever removed (or even reduced) this. Now, however, with a much lower Q tank circuit, this is almost entirely gone - the 'scope trace looks almost like a textbook picture CW signal. Fascinating!

So then, I powered up my audio amp and started my 'test tape' of a male speaking voice. To my amazement, the modulated wave pattern looks and sounds flawless, WITH NO DE-TUNING whatsoever! My little Heathkit 10-watt amplifier has its own Volume control, and using this easily controls the precise amount of modulation. I observed no obvious audible distortion. This was exactly like the best results I had ever obtained previously - and WITHOUT the annoying background hum!

IMPORTANT Observation: What this means is, we can now tune the rig in a 'normal' manner, without monitoring a transmitted signal during the process! I regarded this as one of the most egregious flaws that needed resolving to make the method truly practical.

CONCLUSION: For the modulation to make it to the antenna jack, DON'T use a tightly-tuned high-Q plate tank (the kind you would design for highest plate efficiency). Instead, provide a lower Q tank of higher L/C ratio, and accept the lower efficiency and softer tuning to peak power as simply a 'cost of doing business' with this form of modulation.

THE REMAINING MYSTERY: Are there modern, state-of-the-art receivers that can detect the audio out of this signal? My little Regenerative and Direct Conversion receivers handle it easily, but most serious hams would see these as practically toys compared to the full-functioned, multi-hundred-dollar units everybody wants today. So far, I have never had a response to a CQ I've thrown out there. So, are modern receivers just 'blind' to these signals, or am I just waiting for the golden moment when I'll get lucky?

EDIT: Well, I was too enthusiastic about the 60 Hz hum - it's still there, just less obnoxious than it was. Pretty much inaudible when the voice modulation is going, but still noticeable when the voice is quiet. As I said, this effect varies over time - when I saw a really smooth-looking 'scope trace it must have been at a 'quiet moment'. I was a little while ago speculating that the running of the heat pump might have something to do with it, but just now spent a few minutes with the heat pump definitely not running and there was basically no relief from the effect. So, who knows? Sorry I jumped to conclusions on this one.

Yet Another EDIT: Further experimentation resulted in further simplification. With the crystal I got with my first 723, marked '7183 KC' it turned out that the fixed cap in series with the tuning cap is unnecessary - the plate tuning variable has enough range that it reaches 'dull resonance' with the 80-meter coil, simply at a very different spot in its rotation. It was silly that I failed to find this on my first try, since I think we all know that tuning variables cover a huge range of capacitance values.

Also, somewhere along the line I should have mentioned that in getting this to work on the EICO 723, the grid tuning has to be set to a pretty low value, under 1.0 mA, for the modulation to work. I am convinced that most of the problems encountered in my experimentation were not true 'design' problems, but rather, failures to tune things up to agree with some Laws Of Physics that were poorly understood by me.

Larry K0WUQ

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