On Wed, Oct 29, 2008 at 03:41:19AM -0700, Karthik Vijayraghavan wrote: > 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. > > Karthik
Karthik, you'll want to turn off the ADC dc offset correction loop that's running in the FPGA. (From usrp_basic.h) /*! * \brief Enable/disable automatic DC offset removal control loop in FPGA * * \param bits which control loops to enable * \param mask which \p bits to pay attention to * * If the corresponding bit is set, enable the automatic DC * offset correction control loop. * * <pre> * The 4 low bits are significant: * * ADC0 = (1 << 0) * ADC1 = (1 << 1) * ADC2 = (1 << 2) * ADC3 = (1 << 3) * </pre> * * By default the control loop is enabled on all ADC's. */ bool set_dc_offset_cl_enable(int bits, int mask); In python, it's a method of usrp_source_*. Eric _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
