Something from old GNU radio :) but still informative

The interpolation factors control how we get the data rate from 32000 samples per second to 128,000,000 samples per second and in what stages. I arbitrarily chose an audio rate of 32000, and an intermediate frequency of 1600000. That means that the second audio low pass filter (lpf2) interpolates the signal by 1600000/32000 or a nice even number of 50.  We have to get the data rate up to 128,000,000 for the hardware DAC, and it turns out that 128,000,000 / 1,600,000 is another nice even number of 80. So the data rate path looks like:

audio in => (32,000 sps) X50 (1,600,000 sps) X80 (128,000,000) => RF out

BTW, linux has a neat utility called 'factor' - if you just type 'factor 128000000' you get

128000000: 2 2 2 2 2 2 2 2 2 2 2 2 2 5 5 5 5 5 5

that's  13 2's and 6 5's.

the 32000 audio rate factors to 2 2 2 2 2 2 2 2 5 5 5, which leaves us with 5 2's and 3 5's to account for somewhere.   

50 uses 2 5 5,  and 80 uses up the other 4 2's and the remaining 5.  At least that's one way to look at it and apportion the factors wherever you want, but they must add up in the end.

USRP_DUC is for setting the center frequency of the digital upconverter. I set the duc to 3.6 MHz, and send it a signal at 15 Khz (at the 1600000 IF data rate) and get an end result of 3615 KHz at 128,000,000 data rate.

** The USRP samples at 64 MS on receive.  You then decimate that down by a factor of 8 to 256.  Then the rest is done in software i.e. in GNU Radio.