[time-nuts] LPRO-GPSDO

[Bob Camp]

Hi

Simply heating the existing crystal might help things a little. 

One minor note:

The VCXO in the LPRO is a 20 MHz oscillator. They divide it by 2 to get 10
MHz. They use the 20 MHz in their synthesis scheme to get to the rubidium
transition frequency.

There are a lot of 10 MHz OCXO's out there that do the same crystal at 20
MHz / output at 10 MHz trick.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Miles
Sent: Friday, January 08, 2010 4:19 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] LPRO-GPSDO


> 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.

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.

-- john, KE5FX



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