[hpsdr] DDC Noise Figure

[h-hartfuss at t-online.de]

Hi Edson, 

If the noise figure of the ADC is dominated by quantization noise, it's
noise figure Fadc is given by
Fadc=((DV)^2)/(6kTRfs)
with DV the voltage quantization step and fs the sampling frequency (see
for example Frerking's book "Digital Signal Processing in Communication
Systems").
So only by reducing the quantization step size (more bits) or by
increasing the sampling rate fs you can reduce the noise figure of the
ADC. You cannot improve this figure with measures in the signal
processing pass.

In the Mercury case the effect of increasing the sampling rate and/or
the number of bits will, however, be very small since a preamp is
involved which has 20dB gain.

Generally, when cascading stages characterized by noise figures F1 and
F2 the overall total noise figure Ftot is always:
Ftot= F1 + (1/G)(F2-1)
where G is the gain of the first stage. The noise figure F1 of the first
stage dominates the overall behavior and that the more the higher its
gain G is. 
It is completely independent of what you are cascading.

73, Hans, DL2MDQ.

PS: In this context my paper on Mercury noise temperature measurements
in TAPR / psr109 might be interest for you.




-----Original Message-----
Date: Mon, 28 Mar 2011 01:06:16 +0200
Subject: Re: [hpsdr] DDC Noise Figure
From: Edson Pereira <ewpereira at gmail.com>
To: KA2WEU at aol.com

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