[time-nuts] Frequency comparison

Neville Michie namichie at gmail.com
Mon Feb 8 23:29:49 UTC 2010


Hi,
I am setting up an LPRO in a steel box (for a little additional  
magnetic shielding).
The power supply is a linear in a separate box (separate the magnetic  
fields).
In the LPRO box is a 5V voltage reference (2 x LT1009) which is used  
across a
pair of trim pots (coarse and fine) with two front panel jacks for a  
high impedance
DVM to set the frequency trim voltage f the LPRO.
The LPRO is insulated with more than an inch of foam plastic and a  
small fan
(on the outside of the box) is used to control the 1/2 inch aluminium  
plate that the
LPRO is bolted to to about 38*C. The temperature control is much  
better than 1/10 degree.
Inside the box is a decimal divider with outputs down to 1Hz, and a  
phase detector
composed of 2 D latches (CMOS) the output of which is filtered and  
fed to a strip
chart recorder or data logger.
I have a TBOLT connected to a decimal divider, so I can compare the  
phase with the LPRO.
At  10kHz I get 1microsecond per month resolution ~ 2x10^12. As  
errors show up
the trim voltage can be adjusted with the DVM by an amount that is  
easy  to calculate.
So with a bit of time and without much in the way of high precision  
timing gear I hope
to be able to get the LPRO adjusted to the limits of its precision,  
and I avoid the short term
fluctuations in the TBOLT output.
After a year or so I will be able to tell you how well it works.

cheers, Neville Michie



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