As a SDR-IQ user, this has been a very educational thread, thanks for taking the time sharing your knowledge and experience.
I really enjoy my SDR-IQ, but my biggest complaint is the noise and the non working noise blanker with SV. I thought by interfacing with a receiver, I would have the best of both worlds? (extra filters, noise blanker etc.)
OK, first, what kind of noise problem do you have? In general I have not found the SDR-IQ to be that much more susceptible to noise than any other radio. Of course, you can SEE the noise easier, so it is often more easily noticed.
But for sure, when you have noise, the NB in SpectraVue only works with certain types of noise. It is not the best NB I have used, by far, but also not the worst.
Using the -IQ behind the R-75 is going to give you coverage in the 30-60 MHz range. Not sure why you want extra filters, what are you trying to filter out? What other benefits you get will depend on exactly where you tap into the R-75 at. Your choices are going to be limited to grabbing the signal in the 2nd IF or the 3rd IF, but there are a couple places in each IF that could be used.
The best protection, filters, noise blanker, etc, would be found in the 3rd IF, 450 to 456 kHz depending on the mode the R-75 is in. Probably sometime after Q891 or the Q911/912 pair. Of course, at this point the signal has passed through at least 3 or 4 filters and then you are going to be limited to a 15 kHz or less bandwidth, down to 2.4 kHz depending on the R-75 switch settings, so really no reason for using an SDR at that point.
If you grab the signal in the 2nd IF, about the 9.01 MHz IF, then you are still after the Noise Blanker of the R-75, but also after 2 or 3 filters. After buffer Q801 I think your bandwidth would be limited to about 15 kHz. Again, do you really need an SDR to use that bandwidth? You MIGHT be able to grab the 2nd IF right after the NB circuit and have a bit more bandwidth (not sure how much, the bandwidth of the proceeding filters are not specified on the drawings, I suspect about 30 kHz), but as there is no buffer between the NB and the filters you would have to come up with your own buffer circuit, something that will not load or add noise to the R-75. Grabbing it after the buffer in that stage also means grabbing it after yet another band pass filter.
Since no matter where you grab the IF out of the R-75 you are going to be killing the wide bandwith display, and thus the point and click capability, of the SDR it seems to me that the only advantage to putting the SDR behind the R-75 is to use the sharp skirts and adjustable bandwidths of the SDR filters.
Any SDR used behind a traditional superhet design is best employed when you can grab an IF before heavy bandwidth limiting filtering. In the R-75 using the first IF, right after the Q441,442 pair, would be best, but since the IF at this point is 69.01 the SDR-IQ can not use it directly. You could tap it here and provide your own mixer and LO to bring it down to a range the SDR-IQ can use. This is exactly what RFSpace had to do, using the RFSpace IF-2000 daughter board, to make the SDR-IQ work with the Yaesu FT-2000 and FT-950 radios as these radios have a first IF up around 69.450 MHz. All of the other IFs were post filter.
However, when done correctly the SDR behind the traditional radio is really good, I have an SDR-IQ behind my FT-2000 and it is simply a fantastic combination. The good RF front end of the FT-2000 and the point and click operation and fantastic filters of the SDR are a very good combination.
T!
