Re: [Discuss-gnuradio] Disabling Decimation/I&Q Mixing in USRP Firmware

[Jeffrey Lambert]

Okay,

So I think I've answered my own question with regards to the disabling 
the NCO/Mixer and the CIC decimator stages in the receive chain.  It DID 
require modifying a patch that I found thanks to Google and editing the 
source code so that Quartus would not give an error on compiling.

Without the decimation stage, there should not be any decimation.  
Running the custom firmware suggests I was getting an approximate sample 
rate of something like 25.334 MHz.  I repeated the experiment using 8 
bit samples and the number improve to a more respectable 32 MHz.  This 
seems to suggest that interleaving is still being done; the o-scope 
display suggests that half of the data stream from the USRP is still 
discarded (very possibly from buffer overflows?).  Why am I getting 
these results?  Does disabling the NCO remove I/Q interleaving and 
provide only real values over USB to the host?

If anyone could answer I would greatly appreciate it; I am trying to get 
the highest sample rate possible from the USRP into a PC for capturing 
some very short (nanosecond scale) pulses.


On 3/4/2011 5:23 AM, Jeffrey Lambert wrote:
> Hello,
>
> I would like to compile a custom firmware to disable the decimator in 
> the USRP.  I have two questions: 1) Is it as simple as commenting the 
> "`define RX_CIC_ON" line in "common_config_4rx_0tx.vh" and then 
> compiling the firmware?  2) USB Bandwidth is limited; by disabling the 
> decimation, where are the extra samples going?
>
> Similarly, is disabling the NCO and I/Q mixing as simple as commenting 
> the line "`define RX_NCO_ON"?


--
~Jeffrey Lambert, K1VZX