[time-nuts] GPS noise reduction
magnus at rubidium.dyndns.org
Sun Apr 6 07:17:58 EDT 2008
From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] GPS noise reduction
Date: Sun, 06 Apr 2008 11:16:36 +1200
Message-ID: <47F80854.7040007 at xtra.co.nz>
> > Bruce,
> > The data I am trying to determine is the GPS short-term phase error
> > based on the results from comparing the same receiver to multiple
> > higher short-term stability sources. If that could be determined
> > then you would have an idea of the short-term noise being added
> > by the receiver and could possibly correct for it.
> > The two oscillators in the system are both quiet, but have opposite
> > age rates allowing me to see a difference, otherwise I wouldn't
> > be able to tell a difference in the data sets at all. That's why
> > additional testing was done to insure the two weren't injection
> > locking.
> > Thanks,
> > Richard
> Your assumption that if the 2 oscillators didnt drift in opposite
> directions you wouldnt be able to see any differences in the datasets is
> Unless off course, the time intervals being measured by the TICs are
> sufficiently long that the short term instabilities of the 2 100MHz TIC
> oscillators mask the TIC quantisation noise.
> If your oscillators are sufficiently unstable for a fixed isolation
> between the 2 then injection locking will not occur.
> The amount of isolation required to prevent injection locking increases
> dramatically as the frequencies of the the 2 tuned circuits approach one
Actually, in this application will a bit of interlocking not hurt, as the two
oscillators should longterm have the same frequency and interlocking will pull
them together to their average frequency. If you further aid this interlocking
by externally couple them together for higher frequencies (up to say 10-100 Hz)
then you can sum their outputs and get a reduced noise response, a 3 dB
improvement. The noise processes internal to the oscillators will be
uncorrelated where as the locking causes the signals to be in phase.
If we go back to the original problem, the two oscillators is to be steered to
the same frequency... so any interlocking between them is as such not a problem
as it achieves part of the goal. However, the bandwidth of the interlocking is
of interest if we also wants to measure the noise sources. Below the
interlocking bandwidth (above the interlocking tau) then oscillators will
behave more and more as one oscillator and differences will change. Above the
interlocking bandwidth, the individual noise sources is more distinct.
Still, you need to measure the A-B to make conclusive measures. Also, the TIC
resolution is indeed problematic for high frequency noise analysis.
More information about the time-nuts