I've acquired some Rx and Tx modiuules that formed part of a fixed microwave link and whose frequency of operation comfortably covers our 24GHz band. The line-up in the Rx module consists of Low Noise Amplifier, followed by a 90 degree splitter feeding into two sub-harmonic mixers. The IF output from these, in the region of 900MHz, feed into another 90 degree hybrid which combines them and gives the sideband cancellation / reinforcement needed for a normal image recovery mixer approach. A similar approach is used on the transmitter module, in reverse. About 20dB of image rejection is achieved, which is more than enough for noise cancellation, and spurious signals at the image aren't a problem - on Tx a simple waveguide filter suffices for the final cleanup
Now, for modification for amateur usage, the LO input is replaced by a good quality, low phase noise fixed frequency from a mutiplied-up crystal source. So far so good - no mods required. The snag comes at the IF. We need to cover a small section of the band only, ie the narrow band segment and the beacon subband, and the obvious solution is to use a transceiver that covers the resulting IF band. Just about adequate performance is obtained with a 1296MHz IF and this is the quick-and-easy route taken by others with these modules. Mixer sideband rejection is only about 10dB though. I have tried replacing the IF combiner with one for 144MHz and it did work to an extent, but was rather a bodge due to the board construction of microstrip on alumina/PTFE board making it very difficult to remove the old one, all very fraught with the possibility of damaging the chip-and-bond RF circuitry, and wasn't very satisfactory. Now, for the SDR related question : Will this work? It is easy to get at the Rx and Tx mixer IF ports on their own, with some PCB surgery using little more than scalpel and small soldering iron. So I can bring both of these out to the external world, then connect to the I/Q inputs of an SDR soundcard. But, the LO has to be fixed - tunable LOs at 24GHz with the low phase noise needed for SSB/CW is not an option, so at most an SDR will only give 192kHz band coverage. Barely enough to even cover the Doppler shift for EME / satellites, and certainly not adequate for narrowband operation plus beacon monitoring, So, how about a double conversion I/Q route? Take the quadrature IF and with no additional filtering apply to another pair of mixers fed with equal phase local oscillator signals for a parallel, coherent, non phase shifted second conversion. This second IF can be anything low and feasible, I was thinking in the region of 1 - 3MHz to allow a DDS based second LO. We now have a linear translation of the first IF down to audio, and the fine tuning can be done by altering this second LO. As the 90 degree phase shift generated at the original RF input is preserved through both frequency conversion instead of the usual one - is this the same as a conventional SDR front end? AND... If, instead, of a non-phase shifting second conversion, a quadrature second LO is applied, we don't even need an SDR, a single audio channel will suffice. Seems too good to be true! ...must have missed something obvious, ...it can't be this easy. ...does there have to be some physical RF filtering around the second conversion to remove image sidebands? ...Am I missing the wood for the trees? Forget practicalities, gain control etc, that is hardware and can be dealt with. Is the concept OK? -- Andy G4JNT www.scrbg.org/g4jnt
