[time-nuts] Austron PRR-10 GPS discliplined Rb...
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Fri Jan 26 19:58:58 EST 2007
David I. Emery wrote:
> The interesting thing about these units (which ceased production
> in July 2005 - possibly because of the abandonment of the Oncore
> receiver family by Motorola) is that they are the second kind of GPS
> disciplined clocks - namely phase microstepper based designs which
> accept a reference 10 mhz input and use a DDS chip to create a phase
> rotated and frequency corrected version which is used to phase lock a 20
> mhz VCXO and from that generate a new 10 mhz and 1 PPS. This is in
> distinction to the Lucent RFTGs which adjust the C field of the LPRO RB
> to phase lock it to the 1 PPS input.
> Apparently the firmware measures the frequency offset of the 10
> Mhz reference input (in my unit generated by a LPRO 101) and its
> behavior over time and temperature and uses this to generate a phase
> step correction for the DDS which results in a precise 10 mhz output and
> 1 PPS used to compare with the GPS timing receiver 1 PPS and adjust the
> correction and its derivatives over time for optimum tracking.
> This means they can take a slightly off frequency but stable 10
> mhz and make a precisely on frequency and even more stable 10 mhz locked
> to GPS when GPS is available and open loop corrected to the last GPS
> offset values when GPS is not using both measured frequency offset and
> change of frequency offset with time (and I think temperature).
This technique is suitable for use with any of the ultrastable oscillators on the surplus market whose frequency has drifted outside the efc and manual adjustment range. Since 48 bit DDS chips are readily available, adequate adjustment range and resolution is available to discipline oscillators that are 100ppm or more off frequency. If a mix and divide technique like that in:
is used then the residual phase noise floor and spurs from the DDS output can be significantly attenuated, particularly if a regenerative divider is used in the last stage of the mix and divide chain.
Another option is to use a low noise reference OCXO that has no frequency adjustments using either varactors or trimmer capacitors, perhaps enhancing the oscillators stability somewhat.
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