[time-nuts] Gravity and OCXOs

Robert Atkinson robert8rpi at yahoo.co.uk
Sat Aug 15 12:43:01 UTC 2009


When applied to meter movements this is called geotropism. 

Robert G8RPI

--- On Fri, 14/8/09, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> Subject: Re: [time-nuts] Gravity and OCXOs
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Date: Friday, 14 August, 2009, 9:02 AM
> Murray Greenman wrote:
> > Rick is right.
> > 
> > The effect you see when turning your GPSDO upside down
> will be
> > predominantly the direct gravitational effect on the
> OCXO crystal and
> > its mountings. You see this with any OCXO, and the
> good ones will have a
> > '2G turnover' or other G rating quoted.
> > 
> > For example, the well known HP 10811A specification
> says 'Gravitational
> > Field: <4 x 10^-9 for 2g static shift (turnover).
> That's fairly typical.
> > 
> > I just measured the effect on a good C-MAC 10MHz OCXO
> (similar to those
> > used in many recent GPSDOs) and measured 40mHz p-p, or
> exactly 4e-9.
> > 
> > It's complicated in the GPSDO because the correction
> mechanism (via GPS
> > 1pps) is much slower than the rate at which you can
> change the
> > gravitational effect. Rb references simply have a
> faster loop and will
> > correct the G effect more quickly.
> 
> If you care about that speed, you use GPSes which has
> higher speeds and you don't use a PPS since as you pointed
> out is unfit for it. Also, if you care about it, you'd use
> sensors to sense the gravity and use that for the bulk
> adjustment, if a low sensitivity crystal isn't enought or
> available in matching specs.
> 
> The PPS is just an arbitrary limit.
> 
> Cheers,
> Magnus
> 
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