This is going to be way over the top for "nerd-ishism". Probably not the cup of tea for many........
The question I wanted to answer for myself was: "So.... if we can linearize the SSB generator signal chain so that it produces no appreciable intermodulation products, what would be the capabilities of the transmitter from the final mixer through the PA?" To answer that question I needed a 45 MHz, two-tone signal with no IMD. If I have that and I inject it into the new filter followed by the mixer, then any IMD measured on the output is the result of that chain and not of the SSB generator and the bi-directional amps. In essence, "what is the clean power capability of the PA section of the radio?"
The answer is "About 3 watts."
So I made a 45 MHz two-tone test signal by setting one output of my signal generator to 44.996000 MHz and a second to 44.997000.? Both signals are more or less centered on the 45 MHz filter passband and are the same level, 1 KHz apart. Those two signals are put into a hybrid combiner and then through an attenuator to the injection point in the graphic, the input to the new filter. By varying the attenuator (with RV1 set for maximum) I could control the SSB output of the radio.?
IMD was measured at the 1 watt out level for reference and then the level increased until IMD measured -24 dB. The power level was different on each band but was a little over 3 watts for the lower three bands. For 10 meters (not shown) I was only able to get 2.1 watts at that IMD threshold.
For each band the purple trace shows intermodulation (refer to the table) when the injection level is set to produce one watt. The blue trace shows the power level in dBm that resulted from increasing the injection level until the IMD reached -24 dB.
So what does all of that mean? It means if we can completely clean up the SSB generator, and that looks reasonably possible, we have a 3 Watt SSB transceiver until someone tackles linearizing the transmit power chain. Lots of work to do!
WA8TOD
