[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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