Hi,
I'm trying to figure out an impedance problem with my USRP/LFRX system. 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.
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.
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).
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.
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)
Are there any other circuitry changes I can try in the lab?
Thanks,
Dan
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