One of the earlier instances where the "trick" of getting linear signals through an ostensibly non-linear signal path was brought to the attention of the amateur radio community back in the mid 1970s with the launch of the OSCAR-7 Satellite.
In that case, a method called "HELAPS" was used - and has since been used in one form or another on other amateur radio satellites equipped with linear transponders since then:? A bit of history may be found here:? ? (There's a scan of the HELAPS design document from the '70s out there, but I was unable to find it on a search just now.)
In short, you do this:
- Take the analog signal source - whatever it is, convert it to a lower frequency (I believe that they used 10.7 MHz) and band-pass filter it, then split it into two signal paths.
- On one signal path you do a hard-limit (as is done for analog FM reception) of the signal to eliminate the amplitude component.
- On the other signal path you do an envelope detection to eliminate the frequency component.
- The 10.7 MHz signal from the limiter was frequency converted to the final frequency and applied to the drive of the final amplifier.
- The supply voltage of the Class-C final amplifier was modulated - using PWM (switching supply) techniques with the envelope-detected voltage that we derived earlier.
The result, if done perfectly, will be a faithful recreation of the original signal.? In this case, it was a transponder with many SSB and CW signals - but it could be combination of any type of signal, of any mode.
In the case of the QMX we have the basis of the above:
- A phase/frequency modulatable source of some sort.
- A means of amplitude-modulating the final amplifier.
When I spotted the comment about possible inclusion of non-linear modes (such as SSB) in the QMX manual two questions crossed my mind:
- Is the bandwidth of the drain-voltage modulator of the PA sufficient to permit modulation at audio frequencies?? I thought this noting the presence of C506, the 1uF capacitor.
- Is it possible to phase/frequency modulate the Si5351 at a rate sufficient to permit the production of the spectral components of speech?? I know that there are limitations related to PLL tracking in the '5351 but if one can work around those (e.g. phase adjustment, dividers) this might be doable.
If both of the above are true, it's possible to do this, given sufficient processor power (and it probably doesn't take terribly much:? If Hans has pulled off doing Hilbert transforms of 80 taps or so in the receive path, he can do this!)? If if it turns out that speech isn't practical for whatever reason, other modes involving some sort of envelope modulation (certainly PSK31 - which I have done on a lowly PIC by modulating frequency/phase and amplitude separately) certainly are.
73,
Clint, KA7OEI
