HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: OgreVorbis on October 29, 2019, 0045 UTC
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Hi,
Redhat, your mention of c-quam with a PWM transmitter got me interested, so I thought I should make a separate thread about it.
As I understand it, you modulate the PWM as you normally would with left and right channels mixed together. Then for the c-quam, you need a device that will offer a phase modulated TTL square wave and you feed that in where you would normally have your crystal oscillator?
Do such devices exist? Could a regular part 15 100mW c-quam transmitter have this extracted from somewhere in the circuit or do I need to build my own device that produces it?
As a side note: It also got me thinking - what would happen if you got two PWMs and fed each side of the amp separately (assuming push-pull design)? would it make an ISB type stereo?
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All commercial transmitters did C-QUAM at high level just as you described. In practice, there is more to it than that. The exciter is an outboard piece of gear that supplies the transmitter with RF carrier drive replacing its internal carrier source, and an envelope signal, which is left summed with right (L+R). The exciter produces the carrier drive by adding distortion and delay to the signals feeding the internal modulators . These distortions mimic the response of the modulator in the transmitter so that the carrier drive signal and envelope are recombined properly in the transmitter to make a correct signal that can be decoded.
C-QUAM was never intended for high frequency broadcasting, and as such no commercial gear was ever built (to my knowledge) to drive HF transmitters. Some commercial AM gear can be modified for higher frequency operation, but most of the older gear cannot due to the parts and circuit topologies they used. The delay and equalization circuits are missing from most of the low power rigs, and as such usually cannot be used to drive larger high level modulated transmitters without distortion or near total loss of separation.
In theory, you could have two transmitters operating in quadrature to each other, combined, and each fed with sum and difference signals. This would produce a QUAM signal which could be decoded, but would have distortions when demodulated by conventional diode detectors. The cleaver part of C-QUAM was making the signal AM compatible by stripping the amplitude modulation from the composite QUAM signal to produce phase modulation. This signal is then amplitude modulated in the transmitter to produce a compatible signal. Producing ISB with the same system is done by adding a +/-90 degree phase shift to the modulating signals applied to the low level modulators in the exciter producing independent sideband modulation.
The process of doing all this is further complicated by the lack of commercially available test equipment to setup the exciter for a giving transmitter and frequency. The process can be tedious, as many of the adjustments interact. Also due to load and delay changes, everything affects audio performance, including frequency and antenna changes.
+-RH
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From an old motorola book.
(https://i.imgur.com/RvKOCVh.gif)
+-RH
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Mebbe do isb with each sb a different channel?
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What's the use having AM Stereo? Stations don't play nice love songs anymore,
like this: https://www.youtube.com/watch?v=SV_rIY458gA 8)
~
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Mebbe do isb with each sb a different channel?
That's what Kahns' ISB system did, Left on lower, Right on upper. I ultimately chose C-QUAM based on availability of receiver and transmission hardware.
What's the use having AM Stereo? Stations don't play nice love songs anymore,
like this: https://www.youtube.com/watch?v=SV_rIY458gA 8)
Because I can... Request it next time I'm on, I'll play it for you :)
+-RH
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The isb route would be cake with sdrs. Set one vrx to upper, another to lower, feed each output to a different sound card channel et voila, hfbc or ambc stereo on the cheap.
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The isb route would be cake with sdrs. Set one vrx to upper, another to lower, feed each output to a different sound card channel et voila, hfbc or ambc stereo on the cheap.
I've tried that. The demodulation paths do not have equal delay, and you wind up with phasing artifacts that don't sound good. Either build a plugin that does it right from IF, or forget about it.
+-RH