On 01/10/2019 08:07 PM, Kwansik Park via USRP-users wrote:
Dear all,
I have been performing experiments of collecting GPS signals and
checking operations of GPS software defined receiver (SDR), using two
USRP N210s.
I used "UHD: USRP Source" block in gnuradio-companion, and both clock
and time sources of the two N210s are set to be 'external'. The two
N210s are synchronized by 'octoclock-g' which distributes 10 MHz
clock and 1 PPS signals.
Until a few days ago, the experiments were successful and I check that
the SDR was able to acquire and track the received GPS signals.
However, from this week, after collecting data using those two N210s,
the SDR cannnot even acquire any GPS signals. Any parameters of the
SDR and blocks in gnuradio-companion were not changed. When I set the
clock sources of the N210s to be 'internal' or 'default', the SDR
worked well.
I found that the LED of the octoclock-g, which indicates status of
GPS-lock by the internal GPSDO, is turned off even though a GPS
antenna is connected to the octoclock-g.
I checked the antenna and the cable using other commercial u-blox
receiver and there was no problem, but, the LED is still left turned off.
In data sheet of the octoclock-g, it says that the 10 MHZ clock and
and 1 PPS signals can be still provided with lower accuracy, although
the GPSDO is not locked to the GPS signals.
Meanwhile, I measured the 10 MHz clock and 1 PPS output using
oscilloscope. In data sheet of the octoclock-g, it says that Vpp
(Peak-to-peak voltage) of the 10 MHz clock is about 1.4V and I found
that the measured Vpp is about 3.5V which is higher than the Vpp
specified in the data sheet.
Is there anyone who can explain why my USRPs don't work well?
Thank you all.
One thing to check for would be presence of the DC bias on the GPS input
connector--somewhere between 3.3V and 5V--if you're using an
amplified GPS antenna (most of them are), then it'll need the DC bias
on the connector to power the LNA in the antenna module.
Can you possibly post an oscilloscope screen shot of the 10MHz signal
from the Octoclock-G ?
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