[time-nuts] Designing and building an OCXO and GPSDO
WB6BNQ
wb6bnq at cox.net
Sun Aug 10 23:26:48 EDT 2008
Hello Philip,
I agree with Bruce about the digital stuff and semiconductor
temperature sensors, etc. From your commentary I think you should do
some reading before proceeding. Here are some suggestions;
The first is a series of Application notes from Agilent (old hp test
div) called AN-200. A total of 5 App notes comprise the AN-200
series. If you go to the following Web page and enter AN-200 at the
top of the page in the search box, you will get a return of all the
AN-200 booklets in PDF that can be downloaded. The BIG one is
AN-200-2, but it would be to your advantage to collect all of them.
You need to paste in the entire link below if your browser doesnt see
the whole thing when clicking on it.
[1]http://www.home.agilent.com/agilent/facet.jspx?t=80030.k.1&cc=US&lc=
eng&sm=g&pageMode=TM
Next is the AN-52 series, also at the above site. The original,
produced in the 1960's is AN-52. Later, in the 1970's, they rewrote
and split this App note into two titled AN-52-1 & AN-52-2. There is
also AN-52-4, but that does not cover your interests at the moment. I
would suggest downloading ALL of them, including the original AN-52.
An-52 does have historical perspective and a few things not included in
the later rewrites.
That should keep you busy for a while. A lot of stuff on the WEB, some
good and some not so good, just take it with a grain of salt ! NIST
(the old NBS) has several things worth reading, however, most of that
deals with the measurement process or is a rigorous mathematical
analysis of one thing or another.
In my experience, an inexpensive metal can crystal and a decent
oscillator circuit will hang in there under 10ppm in a regular room
with a stable ambient temperature. HP used such a crystal in their
60KHz receiver because it was controlled in a loop from the 60KHz, thus
approaching the accuracy and stability of the transmitted signal.
Second it does not take much to get parts in 10^-7 range. Temperature
compensated crystal oscillators easily handle that level. With care, a
crystal oscillator in a well designed circuit can reach parts in 10^-8
with a bit-bang oven control. HP did that in the late 1950's. From
that point the difficulty is logarithmic.
Bill....WB6BNQ
Philip Pemberton wrote:
Hi folks,
I've been following the mailing list for a few weeks using
Pipermail (the
web-based archive) and I figured now was a good time to jump in (so
to speak).
I'm working on a GPS-disciplined oscillator, based on a Trimble
SVeeSix GPS
receiver, and a homebrew OCXO. I've got a pair of 10MHz 50-degree-C
oven
crystals, and have a pretty good idea how to handle the temperature
regulation.
What I'm planning to do is mount the crystal on a copper plate
with two
power transistors, using heatsink compound between the copper and
transistors/crystal case, and fit a temperature sensor to the top
side of the
crystal case. I'm planning to use a copper bracket to hold the
sensor onto the
crystal, and in turn mount the crystal to the copper base.
As far as temperature regulation goes, I'm going to use a PIC
microcontroller (one of the 8-pin chips with an A/D converter) to
monitor the
temperature of the crystal, and use a PID loop to control the two
power
transistors to maintain a temperature of 50C +/- 2 Celsius (the
accuracy spec
of the temperature sensor). I also have other higher-accuracy
sensors (Dallas
DS18S20 and DS18B20) that I can calibrate with; these are accurate
to around
half a degree Celsius with a resolution of 0.5C.
The whole thing is going to be mounted in a metal box lined with
1/2in
thick polystyrene, with all external connections made via Molex KK
connectors
and standard hookup wire. If there's any advantage to doing so, I
might use
RG174 cable for the oscillator output, but otherwise I'll stick to
the KKs and
maybe twist the OUT/GND wires together.
What I'm stuck on is the oscillator itself. The crystals are
standard
parallel-resonant parts, with a load capacitance of 30 picofarads.
I've got a
few varicap diodes (varactors) that I'm planning to use to allow
external
trimming of the frequency, on top of what the ~20pf "coarse" preset
will
allow. So on one side of the crystal I'll have a 33pf capacitor, and
on the
other a 20pf load capacitor, the varicap and a low-value DC-blocking
capacitor
for said varicap.
The standard oscillator circuit for TTL seems to be a pair of
74HC04
inverters and a few passives, or a transistor version that outputs a
sine-wave. Are there any particular types of oscillator that are
more suitable
for high-accuracy timing?
What I'd like to do is use this oscillator to calibrate frequency
counters
and check the calibration on oscilloscopes and similar. Being able
to lock
function generators (a mix of custom DDS sine generators based on
Analog
Devices DDS chips and FPGA-based complex-signal DDSes) against the
oscillator
would be very useful as well. Should I be going for a 1V sine output
and then
convert this to TTL in the generators (which are easy to retrofit
with adapter
boards) or output TTL from the reference and leave it at that?
What design parameters should I be optimising for, and how?
Given that a standard crystal is good to roughly 100ppm, and most
commercial OCXOs are specified to be within 1x10^-9 or better, I'm
aiming for
around 1ppm to start with. Is even this realistic for a homebrew
device?
There seems to be quite a bit of difference between just building
a 4MHz
oscillator to run a PIC MCU to building an accurate frequency
reference source...
As far as parts are concerned, I'm planning to use either a BB153
or BB148
varicap, a Microchip TC1047AVNBTR temperature sensor, a National
Semiconductor
LM4040CIM3-4.1 voltage reference for the PIC's A/D, two BD139 power
transistors and a PIC12F683 microcontroller.
Thanks,
--
Phil.
lists at philpem.me.uk
[2]http://www.philpem.me.uk/
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References
1. http://www.home.agilent.com/agilent/facet.jspx?t=80030.k.1&cc=US&lc=eng&sm=g&pageMode=TM
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