API 1.8.1 / ExtIO 3.7 - observations

[DaveB]

Well done for making the change to the way the overall gain stays the same when LNA is turned on/off - but still giving a wider range of IF gain. At the very least it can mean not going into overload when engaging the LNA and it also gives the benefits of the LNA at a greater Gain reduction.

A word of warning - with a pre-amp in front of the SDRPlay itself you can overload the Mixer when the LNA is on which results in inter-modulation products popping up

With regard to the effectiveness of the I/Q compensation - I've carried out extensive checking and done them on this new release too. It is brilliant for SDR-console, which had no effective I/Q balance compensation at least of WFM- and I estimate it gives a 15dB improvement in image rejection. I need at least another 10dBhowever and compared to SDR# build 1361and HDSDR it is not as effective.

Yes you can disable the SDRPlay's internal I/Q compensation, but unfortunately I've found that even then the effectiveness of the manual compensation in HDSDR is degraded and it becomes very difficult to push (Heart 102.8) completely into the noise. I carried out the comparison a couple of times.

With a 70 dB signal - and these figures are rough:

No compensation: 25-30 dB image signal 3 MHz away from centre frequency - approx 45 dB suppression
SDR# build 1361 - SDRPlay built in compensation: 10dB residual image signal - approx 60 dB suppression
The next figures are with API 1.5.0 - to avoid interactions:
SDR# build 1361 - own compensation: 5 dB residual image signal - approx 65 dB suppression
HDSDR - optimised manual adjustment: 0dB residual signal - approx 70 dB suppression.

And SDR# isn't quite as good as I thought it was. I would use HDSDR for the most critical weak signal reception - but find there are a lot of spurious spikes across the spectrum and this tends to degrade the sensitivity by masking the weakest signals. SDR console also appears to have quite a few spurious spikes.

In my opinion it is pointless to try SDR# post build 1404 as all the good bits have been locked out - so I've not bothered to test the new API on that. I posted elswhere that it looked like something had been done to the squelch so weak signals wouldn't appear - but it looks like the DC compensation is now not working (it was working on build 1404) - but if anyone insists on using it the new API will sort that out. But IMO the best that gives you with SDRPlay now is a transistor radio.