[hpsdr] Boat Anchors..PreAmps

[Lester Veenstra]

The value of PTIC was arrived at experimentally rather than empirically. So the mathematicians out there can settle back down, unless one decides to prove the result is impossible.

The simplicity of this method is that you do not need to actually calculate the percentage. Just count for a long enough time to actually see about 1E-7 percent, based on the sample rate. If before that time interval, you are counting values, you are overloading and should drop the AGC gain a step, and restart the count. If by the end of the period that should give the PTIC target, you have counted noting, increase the AGC gain a notch and try again.

The relationship between the PTIC value and the NPR value is a steep curve. That is, the PTIC will change a decade for a small (say 1/2 dB) change in the NPR, so a crude estimate of the PTIC is sufficient to get you in the right loading region. It is not necessary to measure until you have a statistically significant number of samples. In practice, the target is no counts in the expected period, step a half db and count some more. If you get counts, drop a half dB.  The period necessary to get an expected count will be long enough to ensure you have a long slow and easy AGC function. Yes you will bang up and down in succession to keep you in a known area, but the output will not have perceptible artifacts.  

And to respond to an earlier question in this thread, optimum is optimum. What is optimum based on the total input to the A/D, all the power in all the nyquist or less bandwidth, is optimum for any small signal in the bandwidth. In other words, with a wide band A/D, you cannot get a better loading that will improve the S/N of a narrow band demod process after the A/D.



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

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-----Original Message-----
From: lstoskopf at cox.net [mailto:lstoskopf at cox.net] 
Sent: Tuesday, April 12, 2011 8:39 PM
To: 'Tom Holmes'; phil at pharman.org; lester at veenstras.com
Cc: hpsdr-request at lists.openhpsdr.org; hpsdr at lists.openhpsdr.org
Subject: RE: [hpsdr] Boat Anchors..PreAmps

.........  Do you have a reference for those of us with 50 year old college math?

So we sample 10E6 samples...no 255, raise the gain, I'm guessing if you get 255 before you are at .37E6 counts..drop the gain.  And in there somewhere must be a calculation of how much to raise. .......


N0UU

---- Lester Veenstra <lester at veenstras.com> wrote: 
> 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.   

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