Rx Comparisons

[M0DTS]

I've done some tests in the shack recently with DVB-S and DVB-S2 using a couple of the Software Receivers...

I thought it maybe useful for others playing with weak signals and what to expect.
This came about after my poor 70cm experiences this last week, i was seeing signals at 10-20dB over the noise but no decode with minitioune due to qrm.
The guys at Dwingloo were using SDRangel with good success so i had to take another look!

SDRangel:
DVB-S appears to be approx. 2.5dB down on sensitivity compared to Minitioune Software.
The benefit here is that it locks instantly (once above threshold) providing data for the video decoder, ideal for QSB but quite peaky tropo where as Minitioune takes many seconds before it locks on.
It may be the saviour on 70cm for me with all the qrm up here, hope to try it next time i'm out...
Note that 5/6 and 7/8 FEC DVB-S would not work on both of my test systems.
DVB-S2 is many dB's down compared to Minitioune Software, but this is because it has no error correction, fine for local strong signals though.

dvb-s_gui:
It's fine for stronger more stable signals but rather difficult to get to lock on weak signals so not ideal for Dx!

Required S/N:
If you are looking for signals on an an analyser or SDR, these are independent of SR:
At 1/4 FEC DVB-S2 we need about 2.5dB over noise to have a chance of decoding.
At 1/2 FEC DVB-S2 we need about 4dB over noise to have a chance of decoding.
At 1/2 FEC DVB-S we need about 4dB over noise to have a chance of decoding. (that's 6.5dB/noise required on SDRangel)


Some tips for using SDRangel:
I spent a long time trying to figure out why i could not decode a signal, it turns out that you need to adjust the frequency slightly on the DATV Demodulator panel after clicking the location for the decoder on the main spectrum display, If you don't do this then the demodulator does not update - bug reported, hopefully that's just not me!
Don't have your signal around the centre of the spectrum, always offset so that your wanted signal does not overlap the centre point, this appears to give a noticeable improvement on rtlsdr and Airspy receivers - i've not tested any other sdr.
eg. if using 333Ks at 437MHz, set the centre frequency to 437.2 and place the demodulator 'area' over the wanted signal (-200KHz) which is off to the left.
Keep the spectrum at least twice the width of the DATV signal you are receiving but don't go too wide as it'll use more cpu. Use the Rate and Dec on the Rx control panel to change the span.



Minitioune wins as far as stable weak signals go, if you have nearby QRM or if signals are QSB then it's not as good but probably still the best option.
If you have strong but short bursts of signal then SDRangel will be useful.
The Software DVB receives have a big benefit of having customisable narrow filter, the Minitioune NIM's are ~10MHz wide without any filtering beforehand.
I used to have trouble with strong signals at 145.78MHz blocking my 146.5MHz Rx, The SDR rx would have overcome this problem.

I hope that's useful information to someone!


73
Rob

[radiogareth]

Thanks Rob, useful reminders about SDR Angel. Have you tried the Minitiouner hardware with Longmynd on Portsdown? Less data about the signal (not an issue TBH) but generally very quick to deliver the goods.
The only issue I have is the need to upconvert from 10/6/4 with proper band-pass filtering.
Gareth

[M0DTS]

I've used LongMynd/Ryde and it does lock quicker.
I don't find it as good on low SR's though, 250/125/66Ks just not as good as using Minitioune software.

I do need to do more testing there though.
When I eventually get round to boxing up yet another pi i will do some more tests with 'real' signals when portable.

It has reminded me that i should have made some recordings on the SDR of signals this last week for reference/playback tests.

Rob