On 01/11/2019 09:58 AM, Daniel Jepson wrote:
Hi Kwansik,
The Octoclock datasheet specification is only valid when driving into
a 50 ohm load, and it should be just under 1.4Vpp.
Cheers,
-Daniel
Indeed, which is why the oscilloscope trace shows a bit of distortion--I
see the exact same thing on my oscilloscope
from an Octoclock-G when it isn't terminated in 50 ohms.
Kwansik, I would check for the presence of DC bias on the GPS antenna
connector, and also check your clock connections to your
N210s.
On Thu, Jan 10, 2019 at 7:22 PM Marcus D. Leech via USRP-users
<usrp-users@lists.ettus.com <mailto:usrp-users@lists.ettus.com>> wrote:
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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--
Daniel Jepson
Digital Hardware Engineer
National Instruments
O: +1.512.683.6163
daniel.jep...@ni.com <mailto:daniel.jep...@ni.com>
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