LFRX not picking up close to DC

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

On Wed, Oct 29, 2008 at 03:41:19AM -0700, Karthik Vijayraghavan wrote:

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.
  • 
    
  • The 4 low bits are significant:
  • ADC0 = (1 << 0)
  • ADC1 = (1 << 1)
  • ADC2 = (1 << 2)
  • ADC3 = (1 << 3)
  • 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

On Wed, Oct 29, 2008 at 3:55 AM, Eric B. [email protected] wrote:

signals
and
you’ll want to turn off the ADC dc offset correction loop
*
*/
bool set_dc_offset_cl_enable(int bits, int mask);

In python, it’s a method of usrp_source_*.

Eric

Eric,

Thanks, that worked! For archival purposes here is the piece of code
that
worked for me. It disables the DC offset correction from all 4 ADCs

self.rx_src = usrp.source_c()
self.rx_src.set_dc_offset_cl_enable(int(0),int(15))

I hooked up a 2Hz sine wave with no DC offset to the USRP and I see a
rather
large offset after the signal comes into the USRP. This offset value
changes
every time I run my program and remains constant during the course of
the
program. The offset is different for different ADCs / channels. I also
measured an open circuit voltage of 106mV between the inputs of all the
ADCs. Is this variable offset due to some registers not getting cleared
when
a program finishes running?

Thanks,
Karthik

also measured an open circuit voltage of 106mV between the inputs of all the
ADCs. Is this variable offset due to some registers not getting cleared when
a program finishes running?

Thanks,
Karthik

After measure the signal offset I adjusted the adc offset from python
till
the offset went away. When I use something like
self.rx_src.set_adc_offset(3,int(273)) I was able to get the offset
completely removed. Also, for some reason after putting this line in,
the
variable offset that I was seeing went away. I checked with restarts,
USRP
power off etc and this seemed to work perfectly.

Karthik

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