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 <gnu.f...@gmail.com <mailto:gnu.f...@gmail.com>> > > > 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
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