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Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion
Date: Wed, 03 Nov 2010 22:46:28 -0400
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On 11/03/2010 06:57 PM, Markus Heller M.A. (relix GmbH) wrote:
> I kindof have a similar problem. Guess I havent understood this yet. 
> I want to generate a very basic signal at 10.000000 MHz. A little up or
> down is not relevant, but I want to hear a signal at all. I have a very
> sensitive HF receiver right next to my dummy load, so that should work
> with a very faint rest. 
> In GRC I have a signal generator and a USRP2 sink. 
> Can you please tell me reference values for 
> * sample rate
> * interpolation on the usrp2
The maximum interpolation rate on the USRP2 is 512, which means that the
narrowest baseband
  signal you can squirt over the Ethernet to it is 100Msps/512, or

So, the sample rate for your (complex) signal should be at a minimum,
195312.5Hz, and then
  request an interpolation rate of 512 when setting up the USRP2 sink.

Although, it may be "tidier" to use an interpolation of 500, and thus a
sample rate of 200KHz.

Create a 1Khz sinusoidal signal, sampled at 200KHz, send that to your
USRP2, tuned to
  10.0000MHz.  You should see that on your HF receiver in CW mode,
somewhere around
  10.001MHz or so (+/- handwaving).

The interpolation tells the FPGA in the USRP2 how to "expand" the
bandwidth of your signal to
  match the fixed sample rate of the DAC that feeds the analog Tx chain.

The sample rate of your signal source had better match what you told the
USRP2 for interpolation,
 or your signal won't be interpreted correctly.

Different types of signals have different inherent bandwidths, and you
chose sample rates and
  interpolation rates to "fit" the signal at hand.  A typical amateur
radio SSB signal, for example,
  is only a couple of Khz wide, but remember that the *minimum*
bandwidth you can squirt over
  the Ethernet, and get interpolated up to the DAC rate is 100Msps/512. 
Which means that
  narrow-band signals tend to get oversampled by quite a bit inside the
flow-graph before
  they end up being presented to the USRP2.

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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