[time-nuts] Updated Shera controller

Richard H McCorkle mccorkle at ptialaska.net
Wed Jul 28 23:05:46 UTC 2010


Paul,
Interesting you should point this out as the PICTIC was a spin-off
of a 5-year effort to modernize the Shera controller for my own use.
The 42ns TIC in the Shera was a weak point and a different TIC with
higher resolution was needed if sawtooth correction was desired.
My GPSMAX controller uses a smart divider to supply a synchronized
1PPS to the panel and a delayed 500K to the PICTIC front end allowing
the 1PPS timing to be varied by adjusting the controller setpoint.
Just the TS274 portion of the PICTIC II interpolator is used for a
gain of 400 with the gain reduced to 100 to give 1ns resolution.
  Over 100 serial commands allow remote operation and monitoring
over a serial link, and all process variables can be displayed or
adjusted on command. Accumulation times of 30, 60, 120, and 240
seconds were added for higher stability sources. Accumulated
sawtooth data from a Motorola GPS is used to correct the accumulated
TIC value prior to subtracting the setpoint and sending the error
to the Shera filter. The filter output passes thru a scaler and is
added to a correction register, then the correction register value
is sent to a 23-bit DAC similar to the 22-bit design used in the
SRS PRS10 Rb.
  The correction register can be loaded directly by serial command
to set the frequency offset, eliminating the offset trimmer and
associated loss of sensitivity and noise pickup. The scaler allows
loop gain adjustment for differing oscillators by software command
without changing resistors and allows a direct to DAC connection
for many oscillators. A temperature sensor and 1PPS fail detection
were added along with age and temperature compensation in holdover.
The direct to DAC connection and the higher resolution of the
interpolating front end adds sufficient gain for use with most
Rb sources.

Richard


> In message <4C5092FA.2030602 at xtra.co.nz>, Bruce Griffiths writes:
>
>>Instead of copying the Shera controller albeit with higher resolution
>>its probably more cost effective to choose a microprocessor with built
>>in time stamping capability.
>
> Uhm, isn't this exactly where you want to use the
> still-smelling-like-brand-new-car PICTIC II with a good DAC and
> a microcontroller ?
>
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>





More information about the time-nuts mailing list