# rtlsdr - hearing an FM broadcast on different frequencies

There is an FM station in my city which transmits at 105 MHz.

Today I discovered that I can also hear it quite well if I set the LO to 32.6 MHz and then shift the signal a bit. I'm using an rtlsdr with RT820T2 and gqrx.

Some of the combinations that work are:

LO: 32.6 MHz Broadcast: 32.4 MHz
LO: 32.7 MHz Broadcast: 32.8 MHz
LO: 32.8 MHz Broadcast: 33.2 MHz


So, an LO increase by 100kHz shifts the signal by 400kHz. How is this effect called and what is the formula to compute the frequencies?

Edit 28.12.2020
I was able to research this a bit more today.

First, I found a lot of other posts describing the same effect, for example:

The problem is that none of the answers in those threads satisfies me. They usually throw around terms like "overloading" and "intermodulation" without further explanation. I want to know exactly why, for example, 103.0MHz mirrors at 32.0MHz, and 106.0MHz mirrors at 33.0MHz - these are nice, exact integers, so I suppose that some rather simple formula which explains them should exist.

Second, I created a more detailed table of some of those frequencies in the range 32.0MHz - 33.0MHz (The range where this happens is much larger however, for me it's 26.6MHz - 33.7MHz):

LO Freq Shift Real Freq
32.0 31.4 -0.6 103.6
32.0 32.0 +0.0 103.0
32.0 32.8 +0.8 104.0
32.1 31.4 -0.7 104.0
32.1 31.8 -0.3 103.6
32.1 32.4 +0.3 103.0
32.2 31.8 -0.4 104.0
32.2 32.2 +0.0 103.6
32.2 32.8 +0.6 103.0
32.3 31.4 -0.9 103.0
32.3 32.2 -0.1 104.0
32.3 32.6 +0.3 103.6
32.3 33.2 +0.9 103.0
32.4 31.6 -0.8 105.0
32.4 32.6 +0.2 104.0
32.4 33.0 +0.6 103.6
32.5 31.6 -0.9 103.6
32.5 32.0 -0.5 105.0
32.5 33.0 +0.5 104.0
32.5 33.4 +0.9 103.6
32.6 31.8 -0.8 105.6
32.6 32.4 -0.2 105.0
32.6 33.4 +0.8 104.0
32.7 31.8 -0.9 106.0
32.7 32.2 -0.5 105.6
32.7 32.8 +0.1 105.0
32.7 33.6 +0.9 106.0
32.8 32.2 -0.6 106.0
32.8 32.6 -0.2 105.6
32.8 33.2 +0.4 105.0
32.9 32.6 -0.3 106.0
32.9 33.0 +0.1 105.6
32.9 33.6 +0.7 105.0
33.0 33.0 +0.0 106.0
33.0 33.4 +0.4 105.6

Third, this is a screenshot as I move from 32.5 to 32.6MHz. Things that I want to note:

1. The strange way the frequencies move - they move up in frequency as the LO moves up - which is the opposite of what usually happens. You can see other weaker signals moving the usual way to the left.
2. These signals are really strong - the audio quality is actually great.
3. The amplifier gain is set to auto here, but I can hear audio if I set it as low as 20dB, and on some frequencies I can go down to 2.8dB and still hear the station.
• Depending on the settings in the RTL control program there is a first conversion to zero-IF or near-zero, or low IF, offset from zero. The front-end mixer for the range 24 MHz and higher is a mixer with a variable LO that converts the FM broadcast frequency to that lower IF frequency. The LO contains harmonics and at one third of the LO frequency there is a spurious reception.
– user16925
Commented Nov 27, 2020 at 12:02
• Back to your question: the basic effect has a name: harmonic mixing. Formula to compute the spurious frequencies: N * F_lo plus or minus the IF can result in spurious reception. The actual IF, as said already, depends on the settings of the SDR control program.
– user16925
Commented Nov 27, 2020 at 12:02
• Fifth order IM? I don't think so.
– user16925
Commented Dec 1, 2020 at 23:36