[Elecraft] MDS practical measuring?

[Alan Bloom]

I have always disliked the term "Minimum Discernable Signal (MDS), 
because it really is a misnomer.  Obviously different people can discern 
a signal at different levels, and what does it mean to "discern" a 
signal anyway?

The technical definition of MDS is simply the effective noise level in 
the receiver.  It is typically measured by injecting a low-level CW 
carrier.  When the signal plus noise is 3 dB (twice the power) compared 
to the noise alone, then the signal level equals the noise level, which 
is the MDS.

A receiver's MDS is a strong function of bandwidth.  If you increase the 
bandwidth by 10x, the MDS goes up (gets worse) by 10 dB.  The standard 
bandwidth for receiver testing is 500 Hz.

Since an SSB receiver is nothing more than a frequency translator from 
radio to audio frequencies, you can measure the signal to noise level at 
the audio output.  However, theoretically you need a true RMS voltmeter 
to do an accurate measurement.  A typical multimeter reads the average 
voltage of the rectified AC waveform, which is a little different from 
the RMS value.  The error is small, however.  You can correct for it by 
adjusting the signal generator for a 3.2 dB increase (1.445 voltage 
ratio) instead of 3.0 dB.

A bigger source of possible error is the automatic gain control in the 
receiver.  Disable it if possible to make sure it doesn't affect the 
measurement.

Recent editions of the ARRL Handbook (2012 or later) have a thorough 
explanation of how to measure MDS, in Section 25.5 "Receiver Measurements."

Alan N1AL (author of Chapter 25)


On 05/04/2014 05:34 AM, Brian Alsop wrote:
> It was time to drag out the test gear and give my older K3 a going over.
>
> One thing of interest was Minimum Detectible Signal.
>
> There is a WIKI definition for it but the equation doesn't help me a bit.
>
> I just thought I'd attach a calibrated signal generator and keep
> reducing the level (for a 500 Hz bandwidth) until I couldn't hear it.
> That doesn't seems subjective since the signal generator is a constant
> tone and not information to decode.
>
> Then I tried looking at Spectrogram and defining a minimum S/N ratio
> which would define MDS.  The MDS values derived this way were much lower
> for a 6 dB S/N ratio.  In fact, I couldn't hear the signal at this S/N
> ratio!  The integration time constant used for averaging clearly was
> helping with detection.  So what time constant would be appropriate for
> normal CW?
>
> How indeed is MDS measured quantitively?
>
> 73 de Brian/K3KO
>
>
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