SDRplay Ghost signals

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Khitryy Lis
Posts: 3
Joined: Sun Jun 12, 2016 11:38 am

SDRplay Ghost signals

Postby Khitryy Lis » Sun Jun 12, 2016 12:03 pm

Dear viewers of this forum.
Since two days do I own a SDRplay. I purchased this device because I wanted a SDR that has a better quality than the RTL-SDR.
Currently I have connected my 50 MHz yagi antenna to the SDRplay. Now I have some problems with Ghosting signals from dab stations that are broadcasting between 170-220 Mhz. I was really surprised since the RTL-SDR doesn't have this problem and DAB stations should be filtered out by the bandpass filter. I have made a small video to show this problem. For me it's very important that the SDR should work well between 40 - 80 MHz. If there is anybody with a solution I would be really thankful.

https://youtu.be/W7eTC943gJ8
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g1hbe
Posts: 224
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: SDRplay Ghost signals

Postby g1hbe » Sun Jun 12, 2016 4:46 pm

Hi Khitryy. The way the RSP works means it has spurious responses just like any other. Sadly, the RSP has a spurious DAB response in the 70 MHz area and even though the front-end filters reduce this, it does not go away completely. I'm not sure whether the response is due to an unwanted mix in the tuner chip or if it is due to harmonics of the sampling frequency as I'm not sure exactly what goes on inside, but if it is a result of sampling frequency harmonics, you could try selecting a different span width or sampling frequency to see if that moves the interference out of the way.
Whatever is causing it, the best answer would be to add some external bandpass filtering that passes only the band of frequencies you are interested in. Perhaps one of the SDRPlay team will be along soon to help you.
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Andy

jon
Posts: 302
Joined: Tue Jan 06, 2015 10:48 am

Re: SDRplay Ghost signals

Postby jon » Sun Jun 12, 2016 5:32 pm

Khitryy Lis wrote:Dear viewers of this forum.
Since two days do I own a SDRplay. I purchased this device because I wanted a SDR that has a better quality than the RTL-SDR.
Currently I have connected my 50 MHz yagi antenna to the SDRplay. Now I have some problems with Ghosting signals from dab stations that are broadcasting between 170-220 Mhz. I was really surprised since the RTL-SDR doesn't have this problem and DAB stations should be filtered out by the bandpass filter. I have made a small video to show this problem. For me it's very important that the SDR should work well between 40 - 80 MHz. If there is anybody with a solution I would be really thankful.

https://youtu.be/W7eTC943gJ8


Hello Khitryy,
Other members of the SDRplay team will be able to comment further but the problem may be made worse by the very high gain settings you had - I saw on the video you had the LNA on and also only 36dB of Gain reduction. The RSP bandpass filters provide some protection but strong DAB transmissions can be a problem. We need to also rule out the possibility of a hardware fault in your case as well. For speed of interaction please can you email support@sdrplay.com with your email address. many thanks, Jon (marketing)
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RichardG
Posts: 11
Joined: Sat Jun 11, 2016 5:19 pm

Re: SDRplay Ghost signals

Postby RichardG » Mon Jun 13, 2016 9:33 am

Not wishing to hijack this thread but I'm also seeing strong broadcast FM signals up at around 340MHz on my new sdrplay. For instance classic FM is at 341.5Mhz, is this normal? I'm using CubicSDR on a Mac.

Thanks Richard
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Tech_Support
Posts: 223
Joined: Mon Jun 01, 2015 7:00 pm

Re: SDRplay Ghost signals

Postby Tech_Support » Mon Jun 13, 2016 2:48 pm

Dear All,
The spurious DAB response at 70 MHz occurs when the selected LO frequency is 1/3 of the DAB carrier frequency. That way the third harmonic of the LO mixes with the DAB signal producing an on frequency spurious response. The problem is more acute at 70 MHz because the transition from the SW band filter to the VHF band filter occurs at 60 MHz. In the VHF band an additional LNA is used boosting both the gain and sensitivity thereby making the spurious responses more obvious. If this represents a serious reception problem then the easiest solution is to add additional RF filtering to reduce the level of DAB signals present at the input to the RSP.
The subject of spurious FM signals around 340 MHz has been covered in detail elsewhere in this forum, but is a result of the use of a block frequency converter to cover the range 250 MHz to 420 MHz. There are basically two ways to deal with this. Either try changing the first LO frequency or add an external FM band band-stop filter.

Sincerely

SDRplay Tech_Support
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Khitryy Lis
Posts: 3
Joined: Sun Jun 12, 2016 11:38 am

Re: SDRplay Ghost signals

Postby Khitryy Lis » Tue Jun 14, 2016 10:01 pm

Al right, thank you all for the replies in my topic. To make some things clear I have tried to turn off LNA and adjust the gain but whatever I do I can't get rid of these spurious. I have checked the frequencies of the DAB radio stations and the original frequencies are between:
182.90 MHz - 184.40 MHz
226.60 MHz - 228.10 MHz
251.90 MHz - 253.40 MHz
219.60 MHz - 221.10 MHz

I previously used the Perseus SDR for HF and the rtl-sdr for VHF. I choose to buy the sdrPlay since I wanted to buy a SDR that has a betterr quality than the rtl-SDR another requirement was that it covers both HF and VHF.
I expected the quality of the SdrPlay to be equal to the Perseus but unfortunately that to turned out not to be right. I am a little bit disappointed that on the most important band for me there are spurious. So if I'm right the only way to get rid of this signals is to add an external bandpass filter since the internal filter is not sufficient. SO what I will do is to build a low pass or maybe even bandpass filter and connect it to the SDR. Thanks all for your help and support and I will keep you posted!
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F1BJB
Posts: 108
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: SDRplay Ghost signals

Postby F1BJB » Thu Jun 16, 2016 8:29 am

Khitryy Lis wrote:I expected the quality of the SdrPlay to be equal to the Perseus but unfortunately that to turned out not to be right.

Hi
But the price and the frequency range are not the same :)
One very simple test that can easily be done is this:
Tune to the spurious signal and lower the gain to have it barely rising above the noise.
Without changing any adjustment tune to the real frequency of the spurious signal.
See how stronger it is .
My guess is that in most cases it should be at least 40dB stronger and this is not a bad result
taking into account the very wide frequency coverage of the SDRplay and it's price.
In reality this is improved by using a suited aerial but additional filtering may be required.
Depending on your location and band of interest instead of various bandpass filters a simple band notch can
do wonders.
In my case notching the FM band does wonders but in HF I use various low pass filters.
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Khitryy Lis
Posts: 3
Joined: Sun Jun 12, 2016 11:38 am

Re: SDRplay Ghost signals

Postby Khitryy Lis » Fri Jun 17, 2016 7:40 pm

@F1BJB Thanks for your information, the DAB station is indeed very strong here.

As experiment did I make a small low-pass filter with two coils and two capacitors. See the attached files for the results.
I still have some problems with signals from HF so next project will be to build a bandpass filter. But for now I am already very happy with this result.
Thanks for all your help.
Attachments
2016-06-17 19.43.29.jpg
2016-06-17 19.43.29.jpg (62.1 KiB) Viewed 2846 times
before.jpg
before.jpg (180.85 KiB) Viewed 2846 times
after.jpg
after.jpg (170.96 KiB) Viewed 2846 times
Last edited by Khitryy Lis on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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F1BJB
Posts: 108
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: SDRplay Ghost signals

Postby F1BJB » Sat Jun 18, 2016 12:49 pm

F1BJB wrote:My guess is that in most cases it should be at least 40dB stronger and this is not a bad result

Hi glad you had an improvement
It seems that ,as far as DAB signals around 60Mhz are concerned, I was a bit optimist in the above statement.
DAB is rather weak here but reception is only attenuated by 15 dB.
So there may be room for a low cost design improvement of the SDR play here :)
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