Brooke
One simple way I'veused to eliminate the two problems you brought up is by
Greatly restricting the tuning effect of the Dac voltage.
Only big downside is you then may have to manually reset the nominal Osc
freq every once in a while if the Dac gets near a limit.
With a little thought an 8 bit Dac is more than enough to make a great GPSDO
out of a good Osc,
as long as the Osc also has a course freq tuning method like the HP 10811
does.
But what I was suggesting is not a GPSDO, but an open loop freq compensator.
Simplified example:
Make the analog output of the PIC equal to (K1 times measured temperature) +
(K2 times lapsed time)
With that, K1 can be used to cancel the Osc tempCoef and K2 the daily ageing
rate.
Having fun, always
thanks
ws
**********
Hi Warren:
I looked into using a PIC to make a GPSO and there are two problems:
(1) is the number of bits you can get in the D/A converter. The best
solution I've seen is used in the Quantic Q5200 GPS receiver which has
what amounts to an internal GPSDO with 48 bits in the oscillator drive.
It's in their US Patent 5440313 (link on Q5200 web page).
http://www.prc68.com/I/Q5200.shtml
(2) Noise on the control voltage.
When I was at HP/Agilent a neighboring engineer was THE man on their
4352 VCO/PLL tester. The D/A converter in it and in the combo boxes
(4395 & 4396 among others) not only had a lot of bits it was also
extremely quiet. Getting both of these is far from simple.
http://www.prc68.com/I/4395A.shtml
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.End2PartyGovernment.com/
WarrenS wrote:
What may make a nice little nut-project is a simple PIC processor type
"Oscillator Buddy" circuit that would reduce the effects of environmental
and time caused oscillator frequency drift errors using the oscillator's
EFC analog input.
What makes a nice "Time-Nut Oscillator" candidate for this project is one
that is repeatable AND predicable.
I am continually ageing and testing several good oscillators including
single and dual oven HP10811s and LPRO Rb to find the most predicable&
repeatable ones.
On these type of selected Oscillators, after minimizing the frequency
uncertainty errors due to basic things like power supply effects and RF
load changes and keeping them in a reasonable stable environment, the main
errors I see are:
1) ADEV noise at tau = 1sec for OXCOs and at 100 sec for RBs (typical
range is from 0.2e-12 to 2e-12)
2) Temperature coefficient (the typical range is from +- 1e-13 to 1e-10
per degC)
3) Ageing rate (typical range is + - 1e-13 to 1e-10 per day)
4) Barometric pressure (Typical effect TBD)
I have found it is possible to reduce the environment and time caused
freq drift errors by 10 to 1 or more on some of the better selected
oscillators using open loop correction techniques.
Disciplining the Osc with GPS in a closed loop (aka Tbolt GPSDO) is the
typical and most effective way to reduce the effects of #2, 3,& 4 above,
but that does have some limitations and the better the disciplined osc is
open loop then the better the closed loop results.
Any thoughts and comments on an "Oscillator Buddy" project.
ws
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