Since this doesn't seem to work as expected from my experiments, I'm throwing it open to discussion rather than keeping this Limbricks' Enigma idea under wraps as some sort of 'stealth' transmission mode.
All the following are with a 2v peak RF carrier with just a dummy antenna resistor, so technically not transmitting.
I chose to use a 7.023 MHz carrier and AM modulated with a 32.768 kHz sine wave from a watch crystal oscillator, gated at 5 Hz (20wpm).
I used another watch crystal oscillator as the reference.
Why 32 kHz? The experiments with watch crystals confirmed research on-line, these can be ultra stable. Several days with a only a few degrees of phase shift at identical temperatures between a pair of crystals, extremely low jitter. The Q of these tuning fork crystals is incredibly high (they will carry on ringing for several seconds after initiated by another crystal oscillating nearby). The exact frequency is irrelevant, the need is for a reference, not a time keeper.
If pre-determined drift on receive of the reference is intended until some part of the signal is picked up, a PLL can take over, like a key in a lock.
As far as extracting the AM from the carrier, I tried a Direct Conversion Receiver (my own ultra low noise homebrew). Instead of a CW tone at say 700 Hz, it is 32 kHz.
The AD630 outputs the 5 Hz when buried in antenna noise, but only to about the same threshold as the ear can just detect anyway(when receiver retuned to audible beat) . Not very stealthy then!
I tried a high Q bandpass before the AD630. It didn't help (obviously the lock-in amplifier is very high Q effectively, as it's working principle). What I did see on the scope of this filtered signal was other passing 32 kHz signals. Harmonics of SSB? CW?.
These are the only explanation for the random 'blips' which prevent higher gain of the low pass AD630 output.
Antenna noise is not 'white noise'. Which I have always suspected, looking at spectrum plots. Some significant part must be signals overlapping in a mess of 'straw'. Certainly, there is a dip in spectrum where SSB transmissions are filtered below 300 Hz.
So more selectivity is needed, an IF would have to account for the 32 kHz bandwidth. Is that correct, 5 Hz or DC to 32 kHz?. A massive segment of the 40 m band.
Cannot see how to use the unique signature frequency of the reference to discriminate the very narrow wanted part of all those 40 m signals!
I can get almost the same range by just amplifying the pick up across the room of the 32 kHz oscillator signal using a few inches of wire as 'antenna', with RF module disconnected.
The 32 kHz can be divided down, but then the advantage of keeping the reference away from the clutter of ordinary phone and CW would be lost.
Plus the advantage of the low pass filter of the lock-in amplifier reduces with relative frequency - the datasheet example is 400 Hz carrier with 0.1 Hz 'data' for 100dB discrimination through the noise.
Any thoughts?
If this could be got to work, in spite of twiddling a knob to adjust propagation phase shift, it opens a whole new field of QPR.
All the following are with a 2v peak RF carrier with just a dummy antenna resistor, so technically not transmitting.
I chose to use a 7.023 MHz carrier and AM modulated with a 32.768 kHz sine wave from a watch crystal oscillator, gated at 5 Hz (20wpm).
I used another watch crystal oscillator as the reference.
Why 32 kHz? The experiments with watch crystals confirmed research on-line, these can be ultra stable. Several days with a only a few degrees of phase shift at identical temperatures between a pair of crystals, extremely low jitter. The Q of these tuning fork crystals is incredibly high (they will carry on ringing for several seconds after initiated by another crystal oscillating nearby). The exact frequency is irrelevant, the need is for a reference, not a time keeper.
If pre-determined drift on receive of the reference is intended until some part of the signal is picked up, a PLL can take over, like a key in a lock.
As far as extracting the AM from the carrier, I tried a Direct Conversion Receiver (my own ultra low noise homebrew). Instead of a CW tone at say 700 Hz, it is 32 kHz.
The AD630 outputs the 5 Hz when buried in antenna noise, but only to about the same threshold as the ear can just detect anyway(when receiver retuned to audible beat) . Not very stealthy then!
I tried a high Q bandpass before the AD630. It didn't help (obviously the lock-in amplifier is very high Q effectively, as it's working principle). What I did see on the scope of this filtered signal was other passing 32 kHz signals. Harmonics of SSB? CW?.
These are the only explanation for the random 'blips' which prevent higher gain of the low pass AD630 output.
Antenna noise is not 'white noise'. Which I have always suspected, looking at spectrum plots. Some significant part must be signals overlapping in a mess of 'straw'. Certainly, there is a dip in spectrum where SSB transmissions are filtered below 300 Hz.
So more selectivity is needed, an IF would have to account for the 32 kHz bandwidth. Is that correct, 5 Hz or DC to 32 kHz?. A massive segment of the 40 m band.
Cannot see how to use the unique signature frequency of the reference to discriminate the very narrow wanted part of all those 40 m signals!
I can get almost the same range by just amplifying the pick up across the room of the 32 kHz oscillator signal using a few inches of wire as 'antenna', with RF module disconnected.
The 32 kHz can be divided down, but then the advantage of keeping the reference away from the clutter of ordinary phone and CW would be lost.
Plus the advantage of the low pass filter of the lock-in amplifier reduces with relative frequency - the datasheet example is 400 Hz carrier with 0.1 Hz 'data' for 100dB discrimination through the noise.
Any thoughts?
If this could be got to work, in spite of twiddling a knob to adjust propagation phase shift, it opens a whole new field of QPR.