Amateur Radio Asked by Autistic on September 27, 2021
The mode of SSB is alive and well. It has relevance for long-distance comms and does not waste spectrum. The orthodox methods of generation are Filter, Phasing and Weaver. These methods can be and are done in DSP or firmware but the fundamentals are the same. The resulting low-level analog RF signal goes to a linear amp possibly via one or more hetrodyning stages. The linear amp has not gotten much more efficient since the 1960s. So the more efficient classes of PA operation are not utilised in practical SSB rigs. Example FM and its digital cousin FSK are fine running class C or the less-documented class E. It has been said that it’s mathematically possible to apply AM and FM to a RF carrier simultaneously to make SSB.
The Amateur who said this may be very very old now. When he was in his heyday it would not have been practical but the question is: Can we do this now?
I could imagine a class D pwm system doing the Envelope and the phase modulated carrier getting clipped and fed into a class E amplifier and then you could run 100Watts with no heatsinks allowing smaller cheaper equipment .ZL4TIY
We have demonstrated a system very much like this. See:
The Polar Explorer: You may never look at your “linear amplifer” the same way again.
In this article, we describe the motivation for and execution of a 55-W SSB transmitter that employs: DSP to generate "baseband" envelope and phase modulation signals; a class E amplifier to increase the envelope signal amplitude for application to the drain of the class E amplifier MOSFET; a quadrature digital upconverter (QDUC) chip to produce the phase modulation gate drive at the carrier frequency.
Of almost equal importance, we demonstrate means of correcting the AM-AM and AM-PM conversion errors in the amplifier system to reduce third-order intermodulation products to the range of -40dBc.
We have made significant advances in the eighteen months since this article was submitted for publication.
Brian Machesney K1LI and
Tony Brock-Fisher K1KP
Correct answer by Brian K1LI on September 27, 2021
It has been said that it's mathematically possible to apply AM and FM to a RF carrier simultaneously to make SSB.
This is true. However, it's equally true of every other modulation — not just SSB. This is easiest to see from the DSP perspective, where we work with complex-valued signals almost all the time. "AM and FM simultaneously" is just the polar (rather than Cartesian) breakdown of the complex baseband signal.
This actually has an application in RF electronics, which is probably the idea you heard about: I've heard it under the name polar amplifier, and indeed it is theoretically more efficient than an ordinary linear amplifier. The major problems of using a polar amplifier are generating the nonstandard driving signals and getting the proper phase alignment between the phase control and amplitude control. (Here's a document I just found discussing the design (in Chapter 4); it calls the thing a "polar modulated power amplifier".)
So, as far as I know you could certainly build a complete SSB transmitter this way. It would be a substantial engineering challenge, and it would hopefully be more efficient.
However, given that in practice the modulation would be implemented by DSP producing the phase and amplitude signals, and the most straightforward way to do that I can think of is to apply one of the standard methods to convert the real audio to IQ (Cartesian) SSB, then convert to polar, it doesn't seem quite right to call this truly a fourth method of generating SSB.
(Also, in DSP, the filter method and the phasing method are essentially the same thing, and the Weaver method is just breaking up the math into slightly different parts.)
Answered by Kevin Reid AG6YO on September 27, 2021
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