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Not at all impressed with sdrplay

Posted: Sat Nov 28, 2015 4:25 am
by enigma
running with either hdsdr or sdrconsole, there is an audio line right in the middle of the waterfall.
I have tried two different virtual audio cables, shut off the wifi, turned of all computer except the one running the sdr, and then
swap the sdr to another computer and shut down the one it was just on.

No matter what freq I change it to, it always right there in the middle.
Originally I had it tuned to 1296 MHz. That is when I noticed the problem.
I know very well I was not picking up the EME beacon.
No antenna connected.

HDSDR display the audio signal on the freq as 700 hertz.
Remember it states right in the center of the waterfall no matter what freq the sdrplay is set to.

My SDR-Play is about two weeks old.

Re: Not at all impressed with sdrplay

Posted: Sat Nov 28, 2015 5:51 am
by Tech_Support
Dear Enigma,
What you are seeing is the effect of DC offsets in the I/Q signal paths. This is a function of the direct conversion architecture and will be present in all receivers using this architecture. This can largely be removed by applying post processing DC offset compensation. In HDSDR, make sure that you have DC offset compensation on. Select Options [F7] -> Input Channel Calibration for Rx -> Rx DC Removal Mode: on

Alternatively, you can select Low IF mode in the ExtIO interface and the DC offset will disappear as there are no DC offsets when using a non-zero IF.


SDRplay Tech_Support

Re: Not at all impressed with sdrplay

Posted: Sat Nov 28, 2015 3:28 pm
by enigma
Implementing one of the solutions individually or both at same time, does not stop it altogether.
Using both solutions together helps somewhat, but that line is still there and can still hear it.

I also now have two sub bands, one either side of this audio noise, that is hotter than the static noise,
but not as hot as the audio signal. This two sub bands also stay on the screen when either increasing/decreasing
the LOa.

Re: Not at all impressed with sdrplay

Posted: Sat Nov 28, 2015 5:55 pm
by Tech_Support
Dear Enigma
If you get a chance, please send a screen grab of your HDSDR spectral display along with the waterfall so we can see what you are seeing to:

Please also send a screen grab showing your ExtIO settings. In Low IF mode it should be architecturally impossible to get a DC centre spike. That is because the de-rotation to I/Q samples is performed in software and so is completely insensitive to any DC offset effects. In ZIF mode, even with DC offset compensation, there will still be a small spur, but it should be very low. As I said previously, a DC centre spike is a fundamental artefact of the ZIF architecture and receiver using the same architecture will show the same effect. It only takes a few uV of DC mismatch at the I/Q mixer outputs to give a sizeable spur in the centre of the spectrum.


SDRplay Tech_Support

Re: Not at all impressed with sdrplay

Posted: Sat Nov 28, 2015 6:14 pm
by enigma
Three images sent.
1296 MHz
1296 MHz w/extio
51 MHz

Re: Not at all impressed with sdrplay

Posted: Tue Dec 01, 2015 2:00 am
by enigma
After doing everything support told me to do, including the ferrite beads on the USB cable, this is what I get with no antenna connected.

Well forget the image.
I am not going to find somewhere to host it.

Short description, I now have multi straight line signals on the waterfall, 16 to be exact.
Actually now I have about 200 spread over displ

Taking the gain reduction to 102, I still have two single freq signals, and one taking up some bandwidth that keeps changing position, but still same bandwidth.

This is on the 14 meter band.

Re: Not at all impressed with sdrplay

Posted: Wed Dec 02, 2015 9:07 pm

I have the same issue with misstuned antennas and strong out band signals.

Example: I use my 2m/70cm/23cm triplexer as a preselector for VHF/UHF and have strong 900MHz and 1800MHz (GSM) and 2100 MHz (UMTS/3G) signals only 250m away from my antenna in direct sight and a strong FM and DAB tower approx 4km away, also in direct sight. When i tune my SDRplay to 435 Mhz and connect my Diskone antenna to the 70cm input, everything is fine. As soon as i connect the same antenna to one of the other inputs or connect the antenna directly, i have the same problems: 1/f noise, DC offset and mirrors.

I have the same issues with other SDR frontends and think this simply is behaviour by design and not a problem specific to SDRplay. The only solution is a preselector. Remember all professional wideband receivers have complex and expensive preselectors to solve this issue.

Maybe you´ll check your SDRplay with a dummy-load or a simple 50 ohms resistor connected to the antenna input.