Quote from AdminTC on 04/11/2021, 11:26

If i use the SPECTRAN with span = "Full" i have a small peak in the center of my spectrum which moves with frequency. Where is this signal coming from?

This is the so called "DC offset" from the ADC (analog to digital converter).

Nearly every practical SDR with a quadrature mixer has this small center peak.

First of all it is good to know that most modern modulations are designed to have no significant signal in the middle of their spectrum for this reason e.g. look at a typical Wifi.

Engineers work around it in different ways:

• 1) For signals that don't require the total bandwidth you can tune above or below the signal frequency so it doesn't overlap with the DC offset.
• 2) Use smaller spans (1/2 or smaller spans) which will use the sideband instead. This will remove the DC completly and you get a clean spectrum.
• 3) Use high amplification (preamplifier) which will bring the DC offset below the noise floor (e.g. change the Reflevel to -50dBm or lower) and will also remove the DC completly and you get a clean spectrum.
• 4) Use the SPECTRA output of the V6 block. This offers an advanced DC correction algorithm.

 

A typical DC offset within the center of the spectrum in Full span mode and high Reflevel (around 2-3dB above noise floor):

 

Use the preamplifier to remove the DC Offset (in this case change the Reflevel to -50dBm or below):

 

Use smaller spans (e.g. 1/2) to remove the DC Offset: