[hpsdr] Boat Anchors..PreAmps

[Lester Veenstra]

As a point of interest, the optimum loading of an A/D is about 1E-6 percent
time in clip (PTIC). The PTIC is percent of time the input voltage peak
bangs on the rails, that is at minimum or maximum values. For an 8 bit
example, the for some time interval, the sum of the number of times you see
a value of 0 or 255, divided by the total number of samples in the period
counted.  The point of optimum loading   
Is the loading that gives the best NPR. More than that loading point, and
the distortion products (including but not exclusively intermodulation) will
degrade the S/N of the signal of interest. Less than that point, and you
will also lose S/N by having your signal closer to the A/Ds quantization
noise than it needs to be.

Adjusting (an AGC function) the loading of an A/D with PTIC instead of RMS
levels below clip has two advantages. First, the optimum PTIC value is
independent of the type of waveform. It is the same for the extreme cases
of a sine wave, through to case of flat white noise. Second, the metric can
be calculated from the A/D output simply by addition, with no need to do the
multiplies needed to get an RMS value. 

 73
     Les

Lester B Veenstra  MØYCM K1YCM
lester at veenstras.com
m0ycm at veenstras.com
k1ycm at veenstras.com


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