FM broadcast mirrored to different bands - fault?

Need help installing or figuring out something about your SDR? Ask here.
Post Reply
Na svyazi
Posts: 1
Joined: Sun Nov 11, 2018 9:39 pm

FM broadcast mirrored to different bands - fault?

Post by Na svyazi » Sun Nov 11, 2018 10:23 pm

Hi everyone!
I'm new to SDR and the RTL-SDR particularly. I've just received my RTV-SDR v3 dongle and started playing with it.
The first thing I noticed is the strange behavior at different bands when strong signals appear to be mirrored to different frequencies. But when I change tuning of the receiver by shifting the whole spectrum in SDR# these mirrored signals move to different frequencies.
I noticed this behavior in CB, HAM, 800 MHz, 900 MHz, 2400MHz band etc.

For example,
I can see FM broadcast stations by tuning to about 27MZh (see the attached Screenshot 1). I'm able to listen to the FM stations by moving the cursor to the corresponding frequencies and selecting WFM.
The really strange thing happens when I change tuning of the receiver by moving the whole spectrum to the left or to the right. The mirrored ghost FM broadcast stations move to different frequencies (see Screenshot 3 and Screenshot 4).

The same thing happens in different bands mentioned above. I get ghost signals (not from FM stations but from digital stuff in the higher bands) that change frequencies corresponding to tuning of the receiver.

Is that faulty hardware or is it that the expected behavior?
Can it be something wrong with the mixer,filters etc?
Attachments
1_c.png
Screenshot 1
1_c.png (249.96 KiB) Viewed 2020 times
3_c.png
Screenshot 3
3_c.png (259.17 KiB) Viewed 2020 times
4_c.png
Screenshot 4
4_c.png (256.69 KiB) Viewed 2020 times

rtlsdrblog
Site Admin
Posts: 2163
Joined: Mon Nov 19, 2012 11:54 pm

Re: FM broadcast mirrored to different bands - fault?

Post by rtlsdrblog » Mon Nov 12, 2018 10:50 pm

This behavior is expected with the lost cost 8-bit RTL-SDR dongles. Basically very strong signals will overload the dongle, and this results in those strong signals being imaged to other frequencies.

The possible solutions are:

1) Lower the gain
2) Use an attenuator
3) Use an antenna tuned for the frequency band of interest (i.e not a wideband antenna)
4) Use a bandpass/bandstop filter to remove the strong signals
5) Purchase a more expensive 12-bit SDR which is more resilient to overloading, but can still overload with very strong signals.

For for strong broadcast FM signals we sell a broadcast FM bandstop filter in our store which can help.

tcdev
Posts: 2
Joined: Tue Mar 26, 2019 11:15 am

Re: FM broadcast mirrored to different bands - fault?

Post by tcdev » Tue Mar 26, 2019 11:30 am

I too have this issue. Genuine dongle from verified eBay seller. The strange thing here is, I've had the dongle for a week and a half, same antenna (dipole that came with the kit), and didn't have the issue until today; no WBFM at all in the 27MHz CB frequencies, and now it's very strong. I get imaging of the broadcast FM stations up to about 30MHz.

Happens with SDR#, SDRuno, HDSDR on Windows and Gqrx, CubicSDR on Linux.

I'm assuming it's related to the demodulator (LO) frequency, not the tuned frequency?!?

How would one explain the sudden issue with no changes to the setup?

Question: would a proper HF antenna reduce this problem?

tcdev
Posts: 2
Joined: Tue Mar 26, 2019 11:15 am

Re: FM broadcast mirrored to different bands - fault?

Post by tcdev » Tue Mar 26, 2019 12:21 pm

And to answer my own question...

Hooked up to a 27MHz 1/4 wave whip antenna, I don't get any imaging of commercial broadcast FM stations at all...

...so it's off to buy a bandstop filter I guess...

Post Reply