Re: [time-nuts] need recomendation for a portable 10mhz reference oscilator
In a message dated 12/11/2007 11:41:02 Pacific Standard Time,
eric.fort@gmail.com writes:
I'm looking for a fairly basic, relatively simple 10 Mhz PORTABLE
reference (probably quartz based) with enough stability over a period
of a week of outdoor temp extremes to keep a 47Ghz transmitter locked
within 100hz while mountaintop contesting. Suggestions are
appreciated. something that is small and ran off 12 volts dc (car
battery) would be a definite plus.
Thanks,
Eric
Hi Eric,
if my math is correct, you will need about 26.3 microseconds holdover
performance per day to achieve that requirement.
A good (and well-aged) double-oven OCXO with aging compensation should be
able to provide that type of performance. The problem is: how much physical
movement will the unit experience? What are the temperature extremes that are
expected, and how fast does the ambient move between the temp extremes? Can it
be GPS locked during that time?
Our Fury double-oven OCXO GPSDO may be a bit expensive for this application,
but if you can keep it within the 0C to 70C temperature range of the OCXO
(by proper thermal shielding) then it would fulfill all of your needs.
The electronics themselves are rated to operate down to -20C, so only the
OCXO needs attention to be well shielded thermally so as to keep it
self-heating at a comfortable temperature.
Aging in holdover is typically better than 7 microseconds per day at room
temp, and it runs from a Lead Acid car battery, all the way down to 11.0V.
If you can use a GPS antenna, the performance would be better than your
requirement of course.
Expect it to draw about 5W, depending on temperature, so a typical 80Ah
battery (derated for low temperature to say 40Ah) would last about 40Ah/0.45A =
3.7 days to 7.4 days depending on the performance of the Pb battery. But you
should really use a marine deep-discharge battery, your car batt may die when
deep-discharged, especially at low temp.
At low temperatures Pb batt's have much lower voltage than at room temp, so
you will need to be able to go down to <12.0V.
A small solar array (2W to 5W) and a bit of sun may extend that operating
time significantly.
bye,
Said
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(http://food.aol.com/top-rated-recipes?NCID=aoltop00030000000004)
A good (and well-aged) double-oven OCXO with aging compensation should
be able to provide that type of performance. The problem is: how
much physical movement will the unit experience? What are the
temperature extremes that are expected, and how fast does the
ambient move between the temp extremes? Can it be GPS locked during
that time?
Assuming you don't GPS lock it...
How well will two units from the same manufacturer/batch track?
I'm assuming they will see roughly the same temperature profile and that you
start with them running at the same frequency.
Has anybody made a triple oven setup? I'm thinking of a big box with a few
holes and a temperature controlled fan. The idea is to let the oven (and
electronics) keep itself warm when the ambient temperature gets very cold.
I'm picturing an open hill top at night with clear skies so there would be
lots of radiation cooling. Of course, with an open hill top, GPS recption
would be easy.
--
These are my opinions, not necessarily my employer's. I hate spam.
Hal Murray wrote:
I'm picturing an open hill top at night with clear skies so there would be
lots of radiation cooling. Of course, with an open hill top, GPS recption
would be easy.
As great as GPSDO are, the problem with using them in a portable
environment is the time it takes to do a survey and then get to final
lock; mountain-toppers may not be in one place long enough.
One thought I've had is to use a cheap Rubidium for long term stability,
and phase lock a good OCXO to it for short term stability and phase
noise (ie, use a loop time constant of perhaps 100 seconds). That would
make a portable package that would meet both the short and long term
stability requirements.
John
Hal Murray wrote:
A good (and well-aged) double-oven OCXO with aging compensation should
be able to provide that type of performance. The problem is: how
much physical movement will the unit experience? What are the
temperature extremes that are expected, and how fast does the
ambient move between the temp extremes? Can it be GPS locked during
that time?
Assuming you don't GPS lock it...
How well will two units from the same manufacturer/batch track?
I'm assuming they will see roughly the same temperature profile and that you
start with them running at the same frequency.
Has anybody made a triple oven setup? I'm thinking of a big box with a few
holes and a temperature controlled fan. The idea is to let the oven (and
electronics) keep itself warm when the ambient temperature gets very cold.
I'm picturing an open hill top at night with clear skies so there would be
lots of radiation cooling. Of course, with an open hill top, GPS recption
would be easy.
Hal,
A triple oven is a particularly bad idea, its just too difficult to
achieve a wide operating temperature range without losing temperature
control or cooking everything in the innermost oven unless one uses a
watercooled peltier device to control the outer oven temperature.
A well designed single oven with feedforward (or the outer oven of a
dual oven system if you cant redesign/rebuild the inner oven) can easily
achieve a thermal gain of more than 1E5 see:
http://tf.nist.gov/timefreq/general/pdf/595.pdf
A bootstrapped oven design like that used by Wenzel (not as new as he'd
have you believe - its been in use in one form or another for the better
part of a century) will also help in improving performance. The optimum
solution is of course to combine the 2 techniques.
http://www.wenzel.com/documents/Sub-pico%20Multiplier.pdf
With a thermal gain of 1E5 for the outer oven temperature fluctuations
at the outer shell of the inner oven will only be a 1 millikelvin or so
(assuming the ambient temperature range is less than 100K).
Bruce
John Ackermann N8UR wrote:
Hal Murray wrote:
I'm picturing an open hill top at night with clear skies so there would be
lots of radiation cooling. Of course, with an open hill top, GPS recption
would be easy.
As great as GPSDO are, the problem with using them in a portable
environment is the time it takes to do a survey and then get to final
lock; mountain-toppers may not be in one place long enough.
One thought I've had is to use a cheap Rubidium for long term stability,
and phase lock a good OCXO to it for short term stability and phase
noise (ie, use a loop time constant of perhaps 100 seconds). That would
make a portable package that would meet both the short and long term
stability requirements.
John
John
Why not lock it to a GPS timing receiver output when its at home, and
allow the controller to learn the aging and tempco characteristics etc
of the oscillator being disciplined and use this information to correct
for temperature variations etc at the remote site?
The model used can be as complex as required to achieve the desired
performance and a PC can be used to derive optimum correction
coefficients, the micro only has to perform the simpler task of applying
these corrections.
Bruce
Hi John, Andy Talbot G4JNT developed the a Jupiter based GPSDO for just this
purpose, several are used by hill-toppers in the UK.
http://www.scrbg.org/g4jnt/freqlock.htm
The idea was that it would settle and be accurate in the time it took to
seet up the microwave part of the station.
I hope that helps/
Alan G3NYK
----- Original Message -----
From: "John Ackermann N8UR" jra@febo.com
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Tuesday, December 11, 2007 8:56 PM
Subject: Re: [time-nuts] need recomendation for a portable 10mhz reference
oscilator
Hal Murray wrote:
I'm picturing an open hill top at night with clear skies so there would
be
lots of radiation cooling. Of course, with an open hill top, GPS
recption
would be easy.
As great as GPSDO are, the problem with using them in a portable
environment is the time it takes to do a survey and then get to final
lock; mountain-toppers may not be in one place long enough.
One thought I've had is to use a cheap Rubidium for long term stability,
and phase lock a good OCXO to it for short term stability and phase
noise (ie, use a loop time constant of perhaps 100 seconds). That would
make a portable package that would meet both the short and long term
stability requirements.
John
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