[time-nuts] A man with one used rubidium standard knows what 10MHz is, more than one, not so much...

Don Barr don.barr at gmail.com
Wed Feb 22 20:15:18 UTC 2012


(sorry to Tvb for stealing a good subject line)

I'm just getting into clocks and oscillators, so please excuse any
ignorance.  I like probably many new list watchers saw Dave Jones of
EEVBlog fame with his used FE-5680A and thought - geez I didn't realize
those were so cheap, I'd like an "atomic clock" too, and promptly ordered
three from three different vendors off of ebay.   I figured I'd get one out
of three that showed up and worked, but I actually got all three, and all
three appear to operate.  I also saw on leapsecond that many time-nuts use
the Trimble Thunderbolt as a GPSDO, and as they were cheap, I ordered one
of those from ebay as well.

Now that I have all of these, and I've cobbled together a power supply to
power everything I hooked the Thunderbolt and the 5680's to the one piece
of test equipment I currently have - an oscilloscope.   Maybe out of
ignorance I decided that the GPSDO would be the "most accurate", so I set
the trigger of the scope to the channel that the thunderbolt was connected
to.   I figured the changing relative waveform phase of each unit would
give me idea of how in line each unit was at a gross level.

To my delight the GPSDO and one 5680 is very close, and the relative phase
of the GPSDO and that unit remain relatively constant.   Two of the other
units are "off", and I'm curious as to why.  Each of the "off" units goes
in and out of phase with the GPSDO (i.e., 360 degrees goes by) in about 20
seconds, which I think is +/- 0.05Hz,  which I think, is 5x10e-9 at 10MHz
(correct me if I'm wrong).    (All units left to "warm up" over 12 hours,
the GPSDO has clear view of the sky and self survey was completed, etc.)

I've tried to do a bit of research on Rubidium aging, which seems to cause
the oscillator to slow over time (
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA485423) when exposed to air
(helium), so I expect the 10 years or so of service would make sense
assuming about 5*10e-11 a month, over that period of time.  I presume
that's what the DDS in the unit is there for to allow the units to be
corrected for aging (

So I guess my questions are:
Does my line of reasoning make sense?
Should the GPSDO, for lack of a better standard, over the "eyeball averaged
over 20 seconds" be the best frequency source?

As someone just getting into this, feel free to tell me to go do my
homework and point me at a link!  I just wanted to make sure I was on the
right path of discovery.

Don Barr KA2YDX

<time-nuts at febo.com>

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