FM stations appearing in many bands

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john_guitar
Posts: 2
Joined: Sun Sep 04, 2016 8:23 pm

FM stations appearing in many bands

Postby john_guitar » Sun Sep 04, 2016 8:43 pm

Greetings,
I am new to the SDR stuff and I recently purchased the SDRplay. I am quite happy about it.
I have a Diamond D3000N discone antenna connected to it and I use SDR#, SDRuno and SDR console V2
software suites.

I noticed a strange phenomenon (?) while I was scanning all the VHF and UHF frequency range to
listen to interesting signals: in many parts of the spectrum I see the FM signals from the FM broadcasting
band (88-108MHz). I see them over and over again many times from 25MHz to 2GHz.
Is it normal? Or am I doing something wrong?

Here are the links of 2 photos:
1. FM stations appearing in the 406MHz region (SDR#)
Image

2. SDRplay settings (SDR#)
Image
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g1hbe
Posts: 216
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: FM stations appearing in many bands

Postby g1hbe » Mon Sep 05, 2016 11:14 am

John
This is a known phenomenon with the SDRPlay, as it is with other types of SDR. There is no way around it except by using an Band 2 rejector filter in the antenna feed. You may be able to move the FM responses out of the way by choosing a different sampling rate or another LO frequency (look under the 'advanced' tab), but a band 2 rejector is probably the bast approach. There are a few types available and there's quite a lot of talk about them over on the Facebook SDRPlay group.
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Andy

john_guitar
Posts: 2
Joined: Sun Sep 04, 2016 8:23 pm

Re: FM stations appearing in many bands

Postby john_guitar » Mon Sep 05, 2016 9:00 pm

Thank you very much for your reply!!

Today in the morning, I stumbled on an FM Notch filter in the SDRplay Playground and I just
ordered two similar filters from ebay. I hope that those band reject filters will deal with the
problem somehow.

From a technical point of view, what is the cause of this phenomenon?
Is it the overloading of the input stage along with AD conversion aliasing?
Last edited by john_guitar on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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g1hbe
Posts: 216
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: FM stations appearing in many bands

Postby g1hbe » Mon Sep 05, 2016 11:38 pm

Some of the unwanteds are due to the conversion oscillator (just like a normal superhet), but others a traceable back to harmonics of the sampling frequencies. Sometimes you can make an unwanted response shift out of the way by moving the centre frequency (I guess this moves the LO frequency), but other times it can be more stubborn. I added a set of filters to my SDRPlay, manually switched.
SDRUno takes a bit of getting used to after the simplicity of HDSDR and SDR#, but we're getting there (slowly....).
Last edited by g1hbe on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Andy

n9yty
Posts: 1
Joined: Fri Sep 23, 2016 1:00 am

Re: FM stations appearing in many bands

Postby n9yty » Fri Sep 23, 2016 1:18 am

I'm quite disappointed by this, actually... I spend the $130 instead off the $30 for an RTL-SDR expecting it to work better, and yet I am seeing FM radio appearing in the wrong places. In particular, just now, seeing them around 336Mhz... I picked up an RTL-SDR thanks to Amazon Prime, got it quickly, and see that it does not have this problem. So what gives? So you are telling me that I don't really have a wide-coverage receiver, because if it requries putting inline filters in to exclude frequencies outsie what I want that is not only very inconvenient but impossible if you are trying to use it remotely.

:(
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