[time-nuts] EFC info on Trimble 34310-T OXCO
Charles Steinmetz
csteinmetz at yandex.com
Sat Aug 23 00:39:35 UTC 2014
Bob wrote:
>The GPSTM is not as tweak friendly (no filter changes allowed) as
>some of the other GPSDO's.
And that is a major problem. The correct filter settings for a Rb
local oscillator are very different from the settings for an OCXO,
which in turn are different from the correct settings for a TCXO.
As a general matter, almost all of the DIY GPSDO designs I have seen
use PLL loop filter settings that are not optimal. Many are not even
close (several orders of magnitude, or more, from optimal).
Generally speaking, the PLL loop filter cutoff should be set
approximately where the GPS xDEV curve intersects the local
oscillator xDEV curve. That puts the better device (GPS or local
oscillator) "in charge" of the composite xDEV at all tau -- the local
oscillator at short and medium tau, and the GPS at long tau. Optimal
crossover tau will generally be in the range of seconds for a TCXO,
hundreds of seconds for an OCXO, and hours to tens of hours for a Rb.
Sometimes, there are good reasons to depart from this general
rule. In particular, if a speedy recovery from holdover is required,
then one might choose a PLL filter cutoff tau that is lower than
optimal. The default crossover tau for the Trimble Thunderbolt is
chosen quite low, presumably for this reason. See, for example,
<http://www.leapsecond.com/pages/gpsdo/>, where the GPS contributes
significantly to the ADEV way down at tau = 1 second, where the local
oscillator is clearly much better than GPS and continues to be for
more than two decades. The Miller DIY GPSDO on that page is crossed
over about 3 decades lower than optimal. (The Miller GPSDO uses a
Shera DIY controller; I presume the Shera has the same crossover tau.).
Compare this to the HP z3801A and Jackson Labs Fury on the same
page. The HP crosses over about 2 decades higher than the
Thunderbolt and Miller GPSDOs, but that is still premature by about
two decades given the very high quality of the OCXO in that
particular unit. The Fury crossover is set well, but the overall
ADEV is let down by the low stability of the OCXO in that particular
unit. (Note that the crossover in commercially produced GPSDOs must
accommodate the range in production ADEVs of the local oscillators
used, and are likely set a bit lower than optimal for most of the
actual OCXOs on this account.) If the filter parameters are
adjustable -- as they are in the case of the Thunderbolt -- then a
time nut can tune his or her individual sample to get the best
possible performance that particular oscillator can deliver.
As I have mentioned before, rather than just setting the time
constant low to speed up holdover recovery, a better solution is to
implement a switchable PLL loop filter. A GPSDO designed this way
uses a suitably long time constant for normal locked operation to
minimize xDEV at all frequencies, and a faster time constant for
turn-on warmup and holdover recovery. It is rumored that the z3801
is designed this way.
Best regards,
Charles
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