Re: [Discuss-gnuradio] USRP Gain

[Eric Blossom]

On Thu, Sep 04, 2008 at 10:04:22AM -0500, Richard Jaeger wrote:
> 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...)
>
> -------
>
> 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.
> -----
>
> 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?
>
> -----
>
> Dick...

Re CIC gain, there's a also a decimation specific shifter involved in
the path.  Please take a look at

  usrp/fpga/sdr_lib/cic_decim.v and cic_dec_shifter.v

Eric