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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