# Signal in the center frequency of my SDR spectrum always

I'm pretty new to SDR, but I understand a little bit about RF.

On all 3 of my SDR device (two RTL one HackRF) The center frequency is always full of interference from something unknown.....it doesn't appear to be static, because if I use a chirpchat demodulator, it's picking up data (even if the protocols wrong, I'm assuming there is still some sort of bit information)

Any idea what this interference is? How do I fix it? Thank you!

• First, welcome to the site! Clarification: is it always the center frequency or do you mean specifically what's tuned in the image, 92 MHz? – webmarc Dec 22 '20 at 15:43
• @webmarc always, sorry if that wasn't clear, this was just an example image – Danielle Dec 30 '20 at 1:51

## 2 Answers

That is DC offset, either in the analog to digital converters, or their driving circuitry.

The average voltage at the ADC's input is ideally exactly in the middle of the ADC's range, since this maximizes the maximum voltage swing up or down before clipping. The DC offset is the difference between the ADC's "middle" voltage and the average voltage present at the ADC's input. The analog circuitry in the SDR will contain biasing circuitry designed to minimize the DC offset.

But, the biasing circuitry and the ADC are subject to manufacturing and temperature variation among other things. It's theoretically possible to reduce the DC offset to an insignificant amount, but in practice this would require such precision that the expense would be unacceptable.

So, nearly every practical SDR with a quadrature mixer has this issue, and engineers work around it in other ways. Many modern modulations designed to be received by SDRs are designed to have no significant signal in the middle of their spectrum for this reason. For signals that don't require the total bandwidth of your device you can tune above or below the signal frequency so it doesn't overlap with the DC offset.

• @phil-frost-w8ii thank you so much that makes perfect sense now. I'm assuming its the DC offest happening from either the 3.3v the device works at, or 5v usb level (depending where the DAC is in the chain). Is it just noise? or is it digital feedback (either from the DAC or UART) – Danielle Dec 20 '20 at 0:50
• @Danielle see edit – Phil Frost - W8II Dec 20 '20 at 15:20

I want to mention that the technique described at the end of the accepted answer can be seen very clearly in the LuaRadio documentation: https://luaradio.io/examples/

Each of the examples defines a variable tune_offset, whose value is slightly larger than the bandwidth of the desired signal. In the first example, which is an FM receiver, the bandwidth of interest is 200kHz, and tune_offset is -250kHz. Say that we want to receive a signal at 101.00MHz:

1. The source block will tune the SDR to 101.00MHz - 250kHz = 100.75MHz. Now in the baseband the signal that we care about will be centered at 250kHz (from 150kHz to 350kHz) - far away from the DC spike at 0Hz.
2. After that, the tuner block will shift all frequencies down by the same (250kHz) amount - now our signal will move where we want it to be - centered around 0Hz, and the DC offset will appear at -250kHz. Next, the tuner block will also filter everything above 200kHz, so the DC offset won't interfere with the subsequent processing.