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 On Thu, Nov 20, 2008 at 6:17 PM, dan s <[EMAIL PROTECTED]> wrote: > 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 > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
