[ntp:questions] Re: Question to NTP developers

John Ackermann N8UR jra at febo.com
Thu Aug 24 19:01:02 UTC 2006


Eugen COCA wrote:
> John Ackermann N8UR wrote:
> 
>> I have this issue with my Cs and Rb clocks.
> 
> John,
> 
> could you recommend me a good (and cheap enough)  Rb clock ?
> 
> Thanks !

There are a number of used Rb "bricks" available on eBay that work 
pretty well.  Many of them came out of digital cell sites, where they 
apparently were replaced by GPS units.

There are a couple of Efratom models -- FRK and FRS, I think, that are 
well respected.  They tend to go for ~$300 (give or take a hundred) and 
are quite reliable.  They are little bricks about 3x4x5 inches and run 
off of 24 volts.

However, these are generally 10MHz output only and don't have a clock or 
1pps output so you'll need to add that externally.

Stanford Research sells a unit -- the PRS-10 -- hich is maybe $1200 new 
and has a very interesting RS-232 interface, can be disciplined to GPS, 
and (I think) has a 1pps output.  It's the same form factor as the 
Efratom units.  I've never played with one, but I know several folks who 
have and think they're nice units.

The unit I'm using is a surplus HP-5065A with an "after-market" 
replacement for the HP digital clock option.  The problem is that the 
after-market unit PPS signal is just a little bit shorter than the 
original HP, and thus doesn't like to feed the Linux box.

As an overly nit-picky note for this forum, one difference between the 
inexpensive Rb units and the lab-quality ones like the HP-5065A is that 
the small, inexpensive guys directly FM the frequency standard as part 
of the phase lock detection process.  That results in relatively poor 
phase noise.  The lab units apply the FM separately from the oscillator 
output, and thus are much cleaner.  For timekeeping purposes this is 
utterly irrelevant; for use in RF systems it becomes meaningful.

John



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