2011/11/29 Kevin Tien <kvn.t...@gmail.com>
> Hi,
>
> We're using USRP2s with the RFX2400 daughterboards, and we've recently
> switched from the old USRP drivers to the UHD drivers as per general
> recommendation. However, we've run into a few problems that have us
> stumped:
>
> 1) Previously, our receiver gains were set to 0 dB and we had no
> problem receiving a signal with >30 or 40 dB SNR. However, now the UHD
> receiver blocks need specified gain around 30 or 40 dB to achieve this
> same performance; did baseline values change?
>
As far as I remember, there was something about the old driver sending out
signals that were ints
The new driver sends and receives +/- 1, so you will need some more gain
(38 dB I believe somone calculated)
> 2) As far as we can tell through searching the Internet, the sampling
rate in the Tx/Rx blocks has replaced the decimation and interpolation
> fields from the legacy blocks. But what exactly are we specifying with
> the sampling rate now? Is it the sampling rate the rate at which the
> transmitter transmits samples, with interpolation assumed if the data
> going into the block isn't throttled? In which case, would higher
> sampling rate imply lower 'interpolation' rate? Bottom line, we have
> no idea how to play around with the sample rate, or how important it
> is.
>
> The UHD driver sets up the interpolation rate for you.
It will select the interpolation rate most appropriatly close to 100MHz /
requestedSamplerate
With the caveat that it first checks if it is dividable by two(for the
first hb filter), if that result is dividable by two( for the second hb
filter) and if the final result is less than 128 (for the CIC) it will be
set.
If you select an "unobtainable" sampling rate, the output will tell you.
in pseudocode:
%%%%%%%%%%%%%% Mechanism for chosing HB filters and interprate %%%%%%%%%%%
%%%%%%%%%%%%%% root / host / lib / usrp / cores / tx_dsp_core_200.cpp %%%%
%
% if (interp_rate > 128) interp_rate &= ~0x1;//CIC up to 128, have to use 1
HB
% if (interp_rate > 256) interp_rate &= ~0x3;//CIC up to 128, have to use 2
HB
% size_t interp = interp_rate;
%
% //determine which half-band filters are activated
% int hb0 = 0, hb1 = 0;
% if (interp % 2 == 0)
% {
% hb0 = 1;
% interp /= 2;
% }
% if (interp % 2 == 0)
% {
% hb1 = 1;
% interp /= 2;
% }
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Thanks for your help,
> Kevin Tien
>
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>
Best
Paul
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