[Discuss-gnuradio] Impedance Issue with the USRP/LFRX

[dan s]

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