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 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
