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

Azelio Boriani azelio.boriani at screen.it
Wed Feb 22 22:25:06 UTC 2012


Yes, the "you need more stuff" is the most dangerous step. Soon you will
need a time interval counter and you'll start to hunt for an SR620.

On Wed, Feb 22, 2012 at 11:03 PM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> Simple answers:
>
> 1) Your GPSDO is more accurate than your Rb
> 2) Your Rb is more stable for short periods of time than your GPSDO
> 3) Now you need more stuff...
>
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Don Barr
> Sent: Wednesday, February 22, 2012 3:15 PM
> To: time-nuts at febo.com
> Subject: [time-nuts] A man with one used rubidium standard knows what 10MHz
> is, more than one, not so much...
>
> Hi,
>
> (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 (
>
> http://www.ko4bb.com/Manuals/05%29_GPS_Timing/FEI/FE-5680A/FE-5680A-tuning.p
> df)
>
>
> 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.
>
> Thanks,
> Don Barr KA2YDX
>
>
>
>
>
>
> <time-nuts at febo.com>
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