On 06/24/2011 03:03 AM, Eddie Sun wrote:
Thanks for the reply, but i still have some
questions.
2011/6/21 John Andrews
<address@hidden>
A USRP is a baseband IF receiver. Tune it to the GPS L1
frequency with the right decimation rate so that you have your band of
interest selected. This should give you the IF signal.
The source block that i used is the "UHD:usrp_source block" for
USRP N210 in gniradio companion, after setting the frequency to L1
frequency 1575.42MHz, there is no "decimation" term can be set in the
block(only usrp1_source and usrp2_source block have that term, not
uhd), so should i use the "Rational resampler" block to instead of it?
or other method to complete the decimation.
The flow graph will only be
"UHD:usrp_source block"→"Rational resampler"→"File sink"
is that right?
And I'm still a little confused, why i don't need to down convert the
frequency but just do the decimation, i thought decimation is to
slowdown the sample rate.
Is that mean the flow graph output from "UHD:usrp_source block" is
already a IF signal? If this is true, what is that signal frequency? It
can't still have the 1575.42MHz if it's a IF signal, isn't it?
Thanks,
Eddie
In UHD, you set the sample-rate of the source block, not the
decimation. The UHD code determines the
appropriate decimation to use based on what it knows about the device.
The USRP hardware "stack" arranges for the signal of interest to appear
as *complex baseband* signal,
in which the signal goes from -bandwidth/2 to +bandwidth/2, which
uses the "I" and "Q" signal
representation. The "IF" is 0Hz in this case. You shouldn't need to
re-sample to process the resulting
baseband signal. This baseband "I and Q" signal format is *extremely*
common in modern
DSP systems for RF. For more background, you should look up the
terms "direct conversion receiver",
and "quadrature mixer" on Google.
--
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org