[time-nuts] First success with very simple, very low cost GPSDO

Charles Steinmetz csteinmetz at yandex.com
Thu Apr 10 15:08:41 EDT 2014


/tvb wrote:

>I'm suspicious of fast/slow tracking loops. If you want to vary the 
>tracking, perhaps it is best to continuously, transparently, 
>smoothly vary loop parameters according to drift rate rather than 
>use a hardcoded fast/slow algorithm binary switch. I'm sure there's 
>deep theory on this, which I have not read yet.

No reason to be suspicious, they work well if they are designed 
well.  Quite often, the best loop tuning for acquiring lock is very 
different from the desired loop characteristics during locked 
operation.  This is particularly true in very slow loops such as we 
see in GPSDO applications.  In this case, two time constants, each 
with its well-defined application, can work spectacularly better than 
just one.  With just one tc, mutually exclusive design constraints 
pull in two directions and often result in a compromise that 
sacrifices operating characteristics to achieve bearable performance 
during lock acquisition.  Separate acquisition and maintenance time 
constants allow you to have both, and there is no practical 
difficulty in making sure that each one does its job and does not 
interfere with the other.  The fast tc operates only during 
acquisition, and the slow tc operates 99.99% of the time.  Only when 
there has been a big disturbance (in which case the GPSDO should be 
in an alarm status) would the fast tc be recalled.

Having several intermediate time constants raises potential worries 
that the control circuitry will not always choose the best tc for 
current conditions.  There are many instances in which it works well 
(it has been widely speculated that the HP 38xx GPSDOs use this 
technique), although care is necessary in design.  However, if, as 
you posit, it would be beneficial for the loop parameters to vary 
with the drift rate, it would seem clear that several steps of 
piecewise approximation would surely be better than just one 
operating alignment as long as proper care is taken and no harmful 
artifacts are created in doing so.

Best regards,

Charles






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