[hpsdr] Janus Lives!!

[Robert McGwier]

A typical receiver measurement in the ARRL labs consists of donig the 
MDS in 500 Hz.  This is the correct measurement to ask for if we are to 
compare the results to others.

If this were calibrated and we saw -159 dBm in 500 Hz ( the meter is the 
integrated noise power in the 500 Hz bandwidth in PowerSDR),   then at 1 
Hz,   if we could do such a filter,  we would get approximately 26.9 dB 
less power in the 1 Hz bandwidth  (  10* log_10(1/500)).   This is below 
the theoretical noise floor since 1 Hz cannot be less than that for room 
temperature since this is not in a system, it is the naked A/D.  It 
cannot be calibrated.  Gerald is right.

Bob



Alberto I2PHD wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Gerald Youngblood wrote:
>   
>> There appears to be one problem with the measurements though.  The noise
>> floor in the screen shots cannot be accurately calibrated since the 500 Hz
>> RMS noise measures -159 dBm.  If you subtract 26 dB for the 500 Hz
>> bandwidth, you get a noise floor of -185 dBm/rt Hz.  This is below kTB of
>> -174 dBm/rt Hz.  That would equate to a negative NF (-11dB).  I would be
>> interested in seeing the 500 Hz bandwidth shot after calibration with the
>> radio attached.
>>
>> 73,
>> Gerald
>>     
>
> Hmmm, IMHO those are not -159 dBm at a bandwidth of 500 Hz.... the bandwidth that must be considered is that 
> corresponding to the bin width of the FFT that has been used to compute the spectrum.
> The 500 Hz are the bandwidth of the audio filter, which has nothing to do with the FFT bin size.
>
> 73  Alberto  I2PHD
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>   


-- 
AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
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