I did some test with a high IF to see if this is a good idea.
I used a 40 MHZ xtal oscillator block, 2 mixers and 3 tuned circuits at 67 MHz.
The idea is to make a transverter from cb to 67 MHz. That transverter/ CB radio combination would be the IF for the HF transverter.
Results were not as good as I had hoped. Id did work but the intermodulation was way too high. Especially at 7 MHz. Beside that The noise was very high even without antenna. That noise was generated by the 40 MHZ xtal oscillator. I replaced it with a signal out of a HP signal generator. That reduced the noise quite a lot. Unfortunately the intermodulation problem could not be solved easily.
I could have exepected this result. After all modern radio's use a roofing filter.
Those filters have bandwidths of 3 to 22 KHz. That solves some of the intermodulation problems.
I could try to get one of those. However it will increase the cost of this project. The filter is not cheap and I also need an xtal oscillator at an non standard frequency.
So I Think I stick to the 27 MHz IF. The transverter will be usable at 21 MHz and below. I already check the 21 MHz signal on the spectrum analyser. The spurious is -50 dbc. It may even be possible to use it at 24 MHz. Some aditional filtering is needed though
