[DSP-10] Something I don't understand about Alt/A box in LTI mode.

[Bob Larkin]

Hi Courtney,

Great to hear from you.  Let me take a crack at what the A-box in LTI is 
supposed to be.  It is different from the S-meter number, over by the 
frequency display.

The noise power, if the effective temperature was 290K, would be -174 
dBm/Hz.  If it is 600 K, this adds about 3 dB, to give -171 dBm/Hz.  The 
noise bandwidth per bin  is displayed in the upper right corner of the 
screen, and is for your case, 9.4 Hz.  This adds 9.7 dB to the noise power 
in a bin, bringing the noise to  about -161.3 dBm.  This is the noise 
power, referenced to the antenna terminals and a power of  7.41 x10^-17 
milliwatts in our bandwidth.

Next,  (S+N)/N is what we see on the top display when a signal is present, 
where we really mean S and N as powers, as opposed to dB.  So if (S+N)/N 
was 6.4 dB, this would be a power ratio of  4.37. The power ratio of signal 
to noise is one less or 3.37. Thus the signal must be 3.37 x 7.41x10^-17 or 
2.50 x 10^-16 milliwatts.  This same signal level in dBm is -156.0, which 
is shown as the Cntr Sig. This agrees with what you see in LTI

For really weak signals, this is a good way to estimate the power, better 
than the s-meter system that requires knowledge of the analog 
gain.  However, the LTI method does require that we can separate the signal 
spectrally to allow measuring noise without the presence of any signal. In 
LTI this is done away from the center signal, and I don't remember the 
details<g>!

Note also that increasing the averaging (the 1000 points) improves the 
quality of the (S+N)/N estimate, but does not change the mean value.

Does that help??

73, Bob  W7PUA

At 05:32 PM 11/24/2007 -0800, you wrote:
>I'm looking at the N6NB beacon which is coming in pretty well tonight.  I 
>have Te set to 600K, the noise floor in the main spectral display is about 
>10, and the beacon, on peaks, is about 20 which it says is -140 
>dBm.  Sampling is 9.4 Hz. so this all more or less adds up given 
>Boltzman's Constant, -198.6 dB-Hz / K.
>
>In the LTI Alt/A box, it says about 1000 points, 15.0 dB.  Same Te. Cntr 
>Sig = -155.9 which is 6.4 dB (S+N)/N.  This agrees with the yellow line.
>
>How is the Cntr Sig value calculated and what does it mean?
>
>Courtney, n5bf/6
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