Impedance Issue with the USRP/LFRX


#1

Hi,
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I’m trying to figure out an impedance problem with my USRP/LFRX system.Â
I am using the LFRX daughtercard to measure background noise from a
hydrophone (underwater microphone). I am using a +40dB gain
preamplifier to amplify the signal from the hydrophone and then I feed
it into the LFRX channel A.
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I am pretty sure that even with a preamplifier, the 50ohm input
impedance of the LFRX is too low.  I am using a dynamic signal analyzer
(with 10Mohm input impedance) to measure the output of the
preamplifier/hydrophone. If I use a Tee SMA connector and then a coax
line in to the LFRX, I notice a voltage drop factor of about 3.22 in my
dynamic signal analyzer response. This test was performed by using a
transducer in a bucket to generate a single frequency tone.
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I performed this test for an array of frequencies and the voltage drop
factor varied from 3.22 (at 10khz) to 4.48 (at 100 kHz).
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Does anyone know how I could overcome this impedance mismatch? When I
test this out with the hydrophone and no preamplifier, the effect is
even more drastic because the hydrophone cannot drive the 50 ohm input
impedance of the LFRX. I am confident the preamplifier is working
properly.
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I tried removing the R9 50ohm resistor. When I tried measuring a signal
with that resistor gone, I measured absolutely no response (zeros in my
data file using usrp_cfile_rx.py)
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Are there any other circuitry changes I can try in the lab?
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Thanks,
Dan


#2

What is the output impedance of your preamplifier? I do similar data
acquisition with the LFRX and I pass my signals through an op-amp
buffer and through a 50ohm series resistor at the output so I get a
gain of 0.5. That has always worked for me.

You could test this out by hooking up a signal generator with a 50ohm
output impedance directly to the LFRX or through the opamp buffer and
sending in a known voltage.

Karthik