John Miles wrote:
I control my lpro101 with trimble thunderbolt .
now is more then one month , work good very stable .
But I am not happy , sure long time is more stable of original
thunderbolt with ocxo , sure is more stable in holdover ( we need in
holdover ? i never lost gps signal ) .
But short time is worst then original ocxo . I have 4 lpro101 , all four
have short time as datasheet near 2.5 E-11 .
Maybe I am not lucky and my all lpro are not the best in short time ,
but because my primary interest is short time , I need to drive my
spectrum analyzer and my microwave generator with clean and stable
reference , the lpro not respond to this needs .
GPSDOs and atomic standards like the LPRO are not that different inside.
There are different bells and whistles, but they are both just simple PLLs
at heart with a more-or-less noisy reference that contributes long-term
stability to a quartz oscillator, of varying quality, that in turn is
responsible for short-term stability.
The LPRO is about 10-12 dB cleaner at 1 Hz than my older Thunderbolt, but
about 20 dB worse at 1 Hz than the newer Thunderbolts that most people have.
Why? Open the LPRO up and you'll see that the 10 MHz crystal is a small,
ordinary-looking part soldered to the board and exposed to the environment.
It's reasonable to expect worse short-term performance than the OCXOs in
GPSDOs, which tend to be better-than-average parts.
It probably wouldn't be too tough to replace the 10 MHz oscillator in the
LPRO with an external OCXO, as can be done with a GPSDO. It would be
necessary to measure the V/Hz figure of the existing oscillator and
construct an attenuator or amplifier to reconcile it with the new
oscillator. Or you could leave the LPRO intact and use its existing 10 MHz
output to discipline an external OCXO in a slow (> 100s) loop.
On that note...
I am currently reverse-enginering the FEI FE-5607E Rubidium, which is a
variant of the FE-5607A, but with the internal OCXO removed such that a
larger and better OCXO may be used. Interestingly enought, the EFC is
always available on the 13-pin circular connector. The variations with
10 MHz output (FE-5607A), 5 MHz input (FE-5607D) or 10 MHz input
(FE-5607E) is just what SMB connector hooks in where and wither an OCXO,
Doubler or nothing is in the OCXO location. The unit I have is partly
damaged, but I've got the basic plot figured out fairly well.
If someone do have the service manual for this one, I would be greatful
ofcourse.
I agree that some care would need to be taken in ensuring similar V/Hz
figure, as it will not only control the PLL loop bandwidth, it will also
affect the Q of the resonance, and you do not want to raise it to insanity.
The latter is basically what will happen anyway in many test instruments
that accept an external 10 MHz reference, so it may not actually be a
problem. You should try to get a feel for the loop bandwidth of any
instruments you want to drive from an external reference, so you can avoid
wasting time making improvements that won't be noticeable.
You mid-tau properties should improve by implanting an oscillator, if
done correctly. If you don't need it, then it certainly is unnecessary.
Cheers,
Magnus
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