[time-nuts] Fury GPSDO servo gain

Wed Feb 6 19:22:25 EST 2008

Hello Don, et. al,

forgot to mention:

we recently updated the user manual and place the latest firmware version  
1.17 onto the website for download.

This version has minor improvements such as auto-survey when the Almanac is  
lost due to low-battery voltage, and a couple of new/modified SCPI commands 
such  as sync? gps? servo?.

bye,
Said

In a message dated 2/6/2008 15:37:38 Pacific Standard Time,  SAIDJACK at aol.com 
writes:

Hello  Don,

glad that you are getting the unit to work for you.

We have  tested the unit with many OCXO's, most of which have between  
+/-20Hz  
to +/-40Hz deviation, so have much less variance from unit to unit  than  the 
large number of oscillators that are un-doubtfully in the  hands of the time  
nuts.

One problem is that the  unit-to-unit variance of the Hz/Vtune for  the same 
exact OCXO model  is quite high. For a typical oscillator that we  use this  
specification ranges from 1.2ppm to 4ppm, that's almost a range of 4  to  1 
unit-to-unit on the same part number!

Getting good  gain numbers also depends on the stability of  the oscillator, 
with  less stable oscillators requiring higher gain settings than  more 
stable  
oscillators.

So a universal gain number that works for all  oscillators with the same  
Hz/Volt slope is not possible. [One  comment on the 10811's: the best way to 
use  
these is to mechanically  tune these to have a "natural" control voltage of  
+2.5V. The Fury  can do +/-2.5V adjustments in this case, with 2.5V   average]

Please keep in mind that the DACGain value is not one that we  would  expect 
the user to vary dynamically. We would expect the user  to set  this value 
once 
into a ballpark figure based on the table,  then vary the  EFCScale and 
PHASECOMP values to establish the best  loop performance.

An equation that can be used to understand how the  control voltage is  
calculated is (very!) roughly this (excluding  Tempco and Aging  
compensation):

ControlVoltage =  DACGain * [(EFCScale * OffsetError) +  
(Integral[Phasecomp *  OffsetError])]

This is very similar to a traditional PI control  loop.

In our experience the easiest way to establish good working  parameters  for 
a 
given OCXO is:

1) Set Phasecomp to 0.0, that  removes the PhaseComp (integral) component  
from the  equation.

2) set DACGain to an interpolated number from the  table

3) set EFCScale to some initial value, around 1.0 to  5.0

4) Wait for the unit to lock

5) Give the loop a jolt by  (carefully) turning the OCXO over (2G  Tiltover 
test), and wait for  the unit to lock again. Alternatively, you can also  
change 
the  Coarsedac setting by a couple of steps with the SERV:COARSEDAC  command, 

this is similar to giving the loop a (Dirac pulse) jolt.

6) Using  GPSCon, Ulrich Bangert's Plotter, or a similar program to  graph 
the  
control voltage, check to see if the unit is under- (ringing) or   
over-damped 
(response is too slow). Adjust EFCscale up or down based on  this  result. If 
the unit oscillates, the EFCScale parameter is too  high.

7) Once an acceptable EFCScale value is found, then start  adding  
Phasecompensation. Start with Phasecomp between 10.0 and  25.0. 
PhaseCompensation  is the 
"force" that pushes the UTC offset to  0.0ns. Without it, the unit would  
settle in to some arbitrary phase  offset from 0ns.

8) If the unit oscillates, reduce the  Phasecomp.

With this method it is possible to quickly get an  understanding of  what 
values result in minimum overshoot (if any)  and minimum ringing.

What we are hoping is that the Time Nuts users  will share their experiences  
and post Parameters that worked for  them here on this forum since all of you 
can  do many more and much  more varied experiments and investigations than 
we 
can  do.

In  summary, setting the three values DACGain, EFCScale, and PHASECOMP does   
take some time, and depends on:

* the OCXO's Hz/Vtune  sensitivity
* the OCXO's aging rate
* the  OCXO's thermal stability
* the OCXO Vtune  Hysterisis

and other parameters, so it's as much an art-form as a  science to get  the 
absolute best parameters. We hope that as many  users as possible can share  
their settings and experiences here, and  the example values we give 
hopefully  
will result in a users' unit to  lock well so that experimentation can  begin.

Hope this  helps,
bye,
Said

In a message dated 2/6/2008 13:38:29  Pacific Standard Time,  
true-cal at swbell.net  writes:

This  request is primarily directed to Said concerning  the Fury OEM board 
that  several of us bought a few months back. I  have been working with the 
board 
and  trying several different OCXOs.  Is there a table or graph that would 
help 
determine the SERVO:DATG index  if the Hz/volt slope of the oscillator is  
known. The simple table in  the Users Manual (rev. 1.4) that lists four  
different data points is  a start but it is very difficult to interpolate  
intermediate 
points.  Additionally, the Max Frequency Deviation for the  oscillator is a  
bit ambiguous since some are based on -5v to +5V (10811) and  others  just 0v 
to 
+5 (MTI). The table lists an index of 250.0 for the 10811  @  2Hz (-5v to 
+5v) 
but Paragraph 4.7.1.2 states 150 for a +-2v EFC.  Several  index points 
referenced to Hz/volt would help save a lot of  experimentation.  

Regards... 
Don  J.

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