Re: [Discuss-gnuradio] USRP Gain

[Richard Jaeger]

Brian,

More questions.

Q1:  I had second thoughts about the CIC gain.

If I am going to use a 4th order CIC to give an overall decimation of  
R, then the decimation of each stage will be
the fourth root of R, and the gain of the fourth-order filter will be

g = (R^0.25)^4 = R    -   back to equaling the overall decimation  
factor.

However, the decimation distribution is more complicated since R^0.25  
is seldom an integer.
Take my case where the decimation is R = 222 = 1 x 2 x 3 x 37  where  
37 is a prime number.
On the other hand, R = 256 = 4^4 is convenient for the formula above,  
however I think the
asymptotic formula is really valid only for large R.

(Note that 222^4 = 2.4 x 10^9, a huge gain.  A 1 uV signal would give  
a 2400 V output...)

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Q2:  More questions on gain.  Here's my measurement.

Agilent signal generator output at 1.8 MHz:  10 mV
Attenuator setting 46 dB
PGA setting: 0 dB
fft display output:  0 dB (= 1 V^2) - Checked separately

So the input signal is 0.01V/200 = 50 uV and the overall gain is 1V/ 
50uV = 20,000
of which 222 can be accounted for by the CIC filter.  So, I am missing  
a factor of about 100.
Is there a gain inherent in the channel filter?  I have a further  
decimation by a factor of 6 there.
Even with that, I am still off by a factor of more than 10.
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Q3: MDS measurement

The signal disappears into the noise floor with the attenuator at 66  
dB or 10 mV/2000 = 5 uV input signal.
I believe the USRP ADCs are 12 bits with VFS = 2V, so the LSB is 2V/ 
4096 or about 488 uV.  How is the
system resolving signals that are 20-40 dB below the LSB size?

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Dick...