Re: [Discuss-gnuradio] Using external clock with USRP2/WBX

[Damien S.]

Eric Schneider-4 wrote:
> Can you specify how much offset you see?
I see an clock offset of about 2-4 ppm (almost the same with or without
external clock).


Eric Schneider-4 wrote:
> I presume that the transmitter is also using a precision timebase?
I'm recording a signal from a DTV transmitter which is, according to its
owner, synchronized with a GPS receiver. But unfortunately i can't be 100%
sure of that.
And as i'm using a GPS reference in the external clock input i hoped that
the observed clock drift would be null but it's not the case.


Eric Schneider-4 wrote:
> Yes, it works
Ok now i eliminated a possible source for my problem. Remains: transmitter
not synchronized on GPS time, GPS reference not good, error in my
post-processing to find the clock drift... I continue the investigations to
find the probelm source.


Eric Schneider-4 wrote:
> 
> On Sun, 2010-06-13 at 08:12 -0700, Damien S. wrote:
>> Hi all,
>> I'm trying to use the external clock input of the usrp2 with WBX in
>> receive
>> mode. Unfortunately after processing the recorded signal there is still a
>> clock drift despite the fact i'm using a GPS reference clock. So i
>> suppose
>> that i'm making something wrong.
> 
> Can you specify how much offset you see?
> 
> I presume that the transmitter is also using a precision timebase?
> 
>> 1) I'm using a square 10 Mhz reference from a GPS receiver (Novatel OEM4)
>> which has a 300 mV peak-to-peak amplitude -> this amplitude is sufficient
>> ?
> 
> I'm not familiar with that unit, but 300mV should work.
> 
> I've used a Trimble Thunderbolt and a Jackson Labs Fury GPSDO without
> issue.
>  
>> 2) I activated the external clock by selecting the external SMA option in
>> the usrp2_source block in GRC. Could somebody confirm that is method
>> works ?
> 
> Yes, it works.  I just tested the GRC source and sinks with a signal
> generator and my USRP2 definitely tracks the "External SMA" clock.
> 
> --Eric
> 
> 
> 
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