Re: [Discuss-gnuradio] byte-wide ADC transfers

[Eric Blossom]

On Sat, Feb 18, 2006 at 06:10:41PM -0500, Clark Pope wrote:
> In rx_buffer.v I see the 8-bit transfer mode but it only seems to me that 
> it allows doubling the number of channels transferred not doubling the rate 
> on the two channels? I want to get 16 MHz bandwidth into my PC. I am able 
> to run 8 MHz, 16-bit I/Q fine in usrp_fft.py so I figured I should be able 
> to do 16 Mhz, 8-bit I/Q since it is the same byte rate over USB but I still 
> can't get it to work.


> seems to me that selecting 8-bit sends Ch0-3 whereas 16-bit sends Ch0-1?

Please look at the *current* code.  You are misunderstanding what it's
doing.  Both modes are capable of sending [ch0, ch7] across the bus.  The
even channels are the I outputs of the DDCs, the odd channels are the
Q outputs.

> I still haven't tracked down why the make_format isn't showing up either, 
> btw.

You do not have the current usrp code.  I can tell this from the
verilog you posted.  Please updated usrp and gr-usrp from CVS.

The current implementation of the halfband filter in the FPGA requires
8 clocks to process the 31 taps.  This works fine with decimation 8 or
higher.  To run at decimation 4, you'll need to use a build of the
FPGA that does not contain the halfband filter, or you'll need to
reimplement the halfband, such that it uses two multipliers instead of
one.  In addition to the standard 2 Rx (with halfband) 2 Tx build, the
*current* code contains an FPGA image with 4 rx paths (without the
halfband) and with 0 transmit paths.  It's not installed by default.
You can manually copy it from usrp/fpga/rbf/usrp_std_4rx_0tx.rbf.r2 to
PREFIX/share/usrp/rev2/usrp_fpga.rbf.

Using that rbf file and this command (all from current CVS):

 $ ./usrp_fft.py -f 100.1M -8 -d 4

produced the attached screenshot.  Note that it is 16 MHz wide.
I was using the TV_RX daughterboard, which contains an approximately 
6 MHz wide SAW filter, hence there's not much outside of +/- 3 MHz.

Eric

fft-16MHz-wide.png
Description: fft-16MHz-wide.png