Re: [Discuss-gnuradio] information regarding the code

[Chuck Swiger]

At 09:16 AM 6/9/2005 +0530, you wrote:
> Hi
> folks,
>
>        
> I went through this SSB implementation code and I have certain
> doubts&.
>
> I need to what is
> USRP_INTERP and why he has taken it as 80 and also USRP_DUC

This code is my single sideband transmitter - 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 3615Khz at 128,000,000
data rate.


> I
> also would like to know whether we can implement MIMO(Multiple i/p and
> multiple o/p)
>
> What is FIR_INTERP and how
> exactly it is
> calculated??
>
> And also the lpf1_taps
> which is present and also about the function
> gr.firdes.low_pass

gr.firdes.low_pass is a function to calculate the fir taps used in the
actual filter. You give firdes your filter
type, low_pass, high_pass, band_pass, etc, and the cutoff frequencies and
the transition band skirt width
and it calculates the taps for the filter.
As far as I know you can implement MIMO.

--Chuck