[time-nuts] Cs stability
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Jul 16 17:21:12 EDT 2007
Pablo Alvarez Sanchez wrote:
> ); SAEximRunCond expanded to false
> Errors-To: time-nuts-bounces+bruce.griffiths=ra.co.nz+bruce.griffiths=xtra.co.nz at febo.com
> I am curious about the total stability of Cs clocks. Normally producers give you an initial accuracy after 30 minutes of power on and a table with the Allan deviation for different measurement intervals.
> After that they give you the environmental and physical specifications. For the hp5071 you have:
> General environment
> Operating 0°C to 55°C
> Non-operating -40°C to 70°C
> Humidity 0 to 95%RH (45C max)
> Magnetic field dc, 55, 60Hz 0 to 2 gauss peak - any orientation Atmospheric pressure £1E-13 change in frequency for pressure down to 19kPa (equivalent to an altitude of 12.2km) Shock and vibration Mil-T-28800D, Type III, class 5 Hammer Blow Shock Test, Mil-S-901C, Grade A, Class 1, Type A Mile-STD, 167-1 (phase noise)
> EMI: Conducted and radiated emissions per CISPR 11/EN 55011, Group 1, Class A
> EMC: per MIL-STD-461C, Part 7, Class B dc magnetic field up to 7.8 Gauss
> My questions are:
> Are the Allan deviation specs also valid for all the environmental range, including shock and vibration, or only for lab conditions?
> In the article "OBSERVATIONS ON STABILITY MEASUREMENTS OF COMMERCIAL ATOMIC CLOCKS", Pekka Eskelinen claims to have measured a phase temperature coefficient of 100ns/degree for commercial Cs clocks in 1999.
> http://ieeexplore.ieee.org/iel5/6762/18075/00840739.pdf (If you cannot read it I can try to send you a copy by email)
> Has any of you ever measured such a coefficient?
A Cs clock uses a frequency lock loop to control the frequency of the
local crystal oscillator, the crystal oscillator phase is arbitrary. The
phase of the output with respect to the crystal depends on the
propagation delay of any intervening electronics including amplifier and
filter phase shifts. The phase shift of tuned circuits and other narrow
bandwidth filters has a relatively high temperature coefficient and this
is perhaps what has been measured. Modern isolation amplifier designs
eschew the use of tuned circuits and other narrow bandwidth filters and
consequently have much lower phase shift temperature coefficients
(typically a few picosec/C).
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