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[Discuss-gnuradio] LFRX not picking up close to DC

From: Karthik Vijayraghavan
Subject: [Discuss-gnuradio] LFRX not picking up close to DC
Date: Wed, 29 Oct 2008 03:41:19 -0700

I am trying to use the LFRX daughterboard to record very low frequency signals in the range of ~DC to about 10Hz. To test this I hooked up a function generator to the USRP with an output impedance of 50 ohm. Since I wanted to get 4 channels simultaneously I loaded the "std_4rx_0tx.rbf" image and looked at the signals after recording to a file. The daughterboard seems to be having a DC blocking behaviour where the amplitude of received signals decreases with lower frequencies. My rx gain is set to 0 and Vin = 1Vpp with 0 DC offset. At 10Hz I get an amplitude of 400 from the ADC. At 1 Hz it drops to 40 and I get nothing at DC. I have tried fiddling around with the gain, but it doesn't help much.

This seems to be weird behaviour since the LFRX is supposed to work upto DC. Am I missing something? I have read all the previous posts on this issue and they weren't helpful for my problem. Any hints/directions would be greatly appreciated. I have spent quite a bit of time on this can't figure out the problem.


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