Connecting an SDR to the TS2000X first IF (135.495MHz) turned out to be surprisingly easy as there are a number of MMCX connector test points on the 23cm module board. CN8 is a light tap (1pF) between two 135MHz tuned circuits and after the first mixer (dual gate fet). CN7 is after the 135MHz monolithic filter and first IF AGC controlled stage. I really didn't want to have the SDR connected after an AGC controlled stage as it meant high level signal level measurements would likely be wrong as the AGC activated. I was not sure whether the IF at CN8 would be clean enough. It was.
I had a short UF/L to SMA cable end jack available, purchased originally to use with a small GPS module. The cable is about 15cm long and very thing, but as this is 135MHz it doesn't really matter.
This cable turned out to be long enough and thin enough to loop around the 23cm antenna flying lead as it emerges from the TS2000x. By carefully drilling a 7mm hole in the removeable (2 screws) cable clamp plate I was able to fit the SMA bulkhead jack in place next to the cable lead. There is not quite enough room to do up the SMA nut, but a dab of superglue soon fixed that!
With this IF tap cable in place I was able to do some tests through the front end. Previously ARRL Lab had measured the TS2000X noise figure as 6dB as quoted in a QST review. From 23cm RF cable input to CN8 cable IF output I measured a very acceptable 4.1dB, consistent across the whole 1240 to 1300MHz range with a centre band gain of 14dB falling by about 1dB at either end of the range. I felt this was adequate for connecting the SDRPlay without using one of the G4HUP PAT boards, although this might be fitted at a later date.
Checking with my spectrum analyser there were no spurious mixing signals within the 135MHz IF range down to at least 70dB below my -40dBm 1296MHz input test signal.
Using HDSDR I was able to see noise from the SDRPlay up to about 3.5MHz either side of centre band (IF) and able to zoom right into signals such as the Martlesham 23cm beacon. There is no centre (DC) spike even using the zero IF option!
If ever the SDR application teams ever get round to adding Continuum mode to this or any of the other SDR programes they will become really useful for measurements rather than just looking at signal spectrum (Spectravue does have Continuum but doesn't run on the SDRplay!)
Back to destroying 23cm preamps in the name of determining safe input levels......