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[Discuss-gnuradio] Problems transmitting a simple IF sample upconverted

From: Kris Lists
Subject: [Discuss-gnuradio] Problems transmitting a simple IF sample upconverted on a USRP2
Date: Mon, 5 Oct 2009 20:51:31 -0600

I’m attempting and failing to use a USRP2 w/RFX1800  to transmit a known good L1 GPS signal sample to a COTS receiver for some repeatable hardware testing work. The code transmits without error messages however the receiver doesn't “track” any satellites from a cold start with a sample of sufficient length and I’m not sure exactly what I’m doing incorrectly so I’ll document what I’ve done so far in hopes that someone will point out something obvious to persons with more DSP background than myself.

Input Sample (simple amplitude IF @ 5Msamples/sec and a IF frequency of 1.25Mhz)

I convert this to a complex sample file using some C code setting Q to 0 for all samples and scaling I from a signed char (-127:128) to a float between (-1:1).

I then build a basic flow in python from the sample file to a USRP2 sink setting the center frequency CF to 1.57542G, and a interpolation of 20 to convert the 5Msamp/sec to 100Msamp/sec. The waveform transmits for the expected 60 sec.

 Do I need to offset the CF to compensate for the IF being @ 1.25Mhz rather than baseband? Any other areas of gotchas, filters etc that a DSP noob is missing?

Thank you in advance.

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