Frequency correction/calibration in SDRsharp for RTL R2832/R820T Tuner SDR dongle

04 Jul 2014 04:39 7
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SDRsharp/SDR# software needs initial frequency calibration while using with R2838U or R2832U with R820T Tuner USB SDR dongle. This is done from the configuration settings by correcting the receiving frequency offset. Usually by increasing the frequency in ppm [parts per million]. If Transmission freq. of a station is 145.825 MHz, 1 ppm [ 1 part per million ] would be equivalent to 145.825 Hz.
145.825 MHz = 145825000 Hz
1 ppm = 145825000/1000000 = 145.825 Hz
100 ppm = 14582.5 Hz i.e. 14.5825 kHz
73 ppm = 145.825 x 73 =10645.225 Hz
i.e. 10.645225 kHz
So arbitrarily if we assume that the dongle is 100 ppm offset; this implies an offset of 14582.5 Hz, i.e. 14.5825 kHz. NFM [ Narrow Band FM ] transmission has a bandwidth of 10 kHz. If the dongle is a few kHz off [say more than 10 kHz] the receiving frequency in SDRsharp, it would not zero beat on the frequency of the transmitter [received by the dongle] for the TX to be audible on SDRSharp. So the SDRsharp needs calibration. The ppm correction value in SDRsharp software usually vary from 50 to 70 for most of the R2832/2838 [with R820T Tuner] . In my case it is approx. 73 ppm, i.e. the dongle has an offset of 145.825 x 73 = 10645.225 Hz, i.e. approx. 10.645 kHz which is compensated in SDRsharp by frequency correction/ppm correction.

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