Thursday, 25 February 2016

Initial results of LNA destructive testing

I've repaired a few VLNA preamps for customers where the noise figure has gone up noticeably whilst the gain remains at the original level.  Because the gain remains normal it can be difficult to diagnose without noise measurement, as everything appears nornal except signals are weaker.
As part of my tests of failure levels in VLNA preamps at 23cm I have now managed to duplicate this problem. This, with other test results, will appear in the proceedings of the EME2016 Conference, later this year.
The problem is due, not surprisingly, to RF overload, probably causing increased gate current to flow.
The signal level it took to cause the problem is the big surprise!


SDRPlay

CHaving read the review of the SDRPlay in Radcom I thought that I'd buy one. Now it has arrived I needed to connect it to my TS2000X in order to monitor 23cm. I am able to do this already via my SDR-IQ and a PAT board connected to the 10.695MHz second IF.  However, I find this most unsatisfactory as the 'span' is limited to about 30kHz and it reverse tunes (LO is high).
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......

Sam


Thursday, 4 February 2016

G3PYB -SK

I am off to Havant in Hampshire tomorrow. Peter's funeral is on Saturday morning and to avoid an early start from over here in Suffolk I thought I would stay over Friday night at the hotel where the post funeral reception was to be held.  It turns out that a number of other microwavers and ATV enthusiasts had the same idea. I only just managed to get a room!
There will be, among memorial items, a QSL board where we will be able to place our QSL cards. 
This seems to be a popular idea and has been seen at several memorial receptions over the last few years. 

I haven't been in the Havant area for many years and I'm looking forward to seeing it again, albeit in somewhat sad circumstances.