[hpsdr] Noise Floor Calculation/Measurement Of Legacy System

[Helmut Oeller]

Hi John, 

some time ago I wrote a little paper for a SDR meeting explaining how easy
MDS or noise figure can be determined without external instrumentation. As
it's in German language, so I'll try a short guide:

1.       Calibrate your S-Meter with an accurate signal source using the
setting tool of PowerSDR.

2.       Select 'SigAvg' in the S-Meter settings and e.g. a SSB-bandwidth of
2400 Hz

3.       Terminate your RX socket (ATT = 0) with 50 ohms and read the MDS at
the S-Meter in dBm

This value corresponds with the equation:

                         Noise figure F dB = MDS dBm + 174 dBm - 10 log B dB

 

A typical result of a Hermes board on the 20m band was -131, 3 dBm @2400 Hz.
That give a noise figure of 

 

F = - 131,3  dBm + 174 dBm - 33,8 dB = 8,9 dB

 

You can verifiy this result using the noise line of the Panadapter. In this
case you have to calculate with the FTT bin width (Hz) and select 'Average'
detector in the settings. 

 

73, Helmut, DC6NY 

 

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