[time-nuts] GPSDO Design

[Bob Camp]

Hi

Your OCXO is going to have phase noise. It might be easier to think of the
added op-amp noise in the frequency domain. Phase modulation from the noise
voltage is a direct calculation, unlike short term stability from the noise
voltage. As long as the noise from the op amp is more than 10 db below the
phase noise, you probably can ignore it.

If you go the pure analog approach, implementing the time constants will be
expensive. Big plastic dielectric capacitors (Teflon comes to mind) are
about the only way to go. Digital is cheaper and easier. It does have it's
own issues with A/D's and D/A's.

If you want to stick with analog, you could use a short time constant. If
you do, then the noise from the GPS receiver (phase noise) will
significantly raise the noise coming out of your OCXO.

Looking at it like a big low pass filter: The GPS is very noisy beyond 0.001
Hz, so you need to cut out the noise starting at that point. Of course it's
only "very noisy" when compared to a fairly good OCXO. Even a not so good
OCXO only brings it in to maybe 0.01 Hz.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Foege
Sent: Thursday, January 14, 2010 9:08 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPSDO Design

I should point out that I was planning on using a passive loop filter
after the 4096 phase comparator with a long time constant and then
simply using an opamp to scale my EFC voltage after the passive filter
accordingly. However I assume I could make an active lowpass filter
with a good opamp that has the proper gain to scale the EFC voltage.

I am basically going the route that James Miller went with his Simple
GPSDO project. As a first step, I wanted to use a simple passive loop
filter and a scaling amp after it that corresponds to whatever OCXO I
manage to scrounge up off eBay. As a second step, I was thinking about
implementing the loop filter digitally to achieve finer control over
the operation of the GPSDO as well realizable longer time constants.

Thanks.

John Foege

On Thu, Jan 14, 2010 at 8:37 AM, Didier Juges <didier at cox.net> wrote:
>
>
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com
>> [mailto:time-nuts-bounces at febo.com] On Behalf Of John Foege
>> Sent: Thursday, January 14, 2010 5:58 AM
>> To: time-nuts at febo.com
>> Subject: [time-nuts] GPSDO Design
>>
>> Hi All,
>>
>> Quick question for the more experienced members here with
>> GPSDO design/operation. Let's assume I'm using a 4096 phase
>> comparator chip followed by some kind of long time constant
>> lowpass loop filter, whether it be analog or digital, is not
>> of concern for the following question.
>>
>> Obviously using a 74HCT4096 would mean that my EFC voltage
>> range would be approx. 0-5V. If I wanted to use an OCXO with
>> say a 0-8V EFC voltage range, then I would be inclined to
>> simply use an op-amp amplifier with a gain of 1.6 to scale
>> the EFC voltage accordingly.
>>
>> But not just any op-amp would do I take it? High-speed would
>> of course be of no concern. Also low-offset would be of
>> little concern, as the PLL would work to correct this, and it
>> therefore seems to be negligible.
>
> Keep in mind that for the PLL to correct any error, it has to see the
error
> first. So while the PLL will take out offset errors in the long term, a
> thermal change will still cause a transient error in the short term, until
> it is removed by the PLL. The more nutty time-nuts among us think that is
a
> problem that can be avoided by using temperature stable parts and good
> design practices. There is not much point in saving a couple of $ with a
> lousy op-amp in a $100 or more project.
>
>> However, the part that's
>> got me thinking is noise.
>> Obviously any noise at the ouput of the amp would adversely
>> affect the frequency stability of the OCXO.
>
> Think of noise and offset as the same thing, in a different time scale.
>
>> I thought the best way to control this would be to use an
>> extremely low noise op-amp employing a rather large
>> compensation cap to give me a rather small bandwidth, perhaps
>> only a few hundred hertz.
>
> Typically, the bandwidth should be much much less than a few hundred Hz,
> probably more like 0.01 or 0.001 Hz...
> It depends on the stability of the OCXO compared to that of the GPS
> receiver. You want the good short term stability of the OCXO and the good
> long term stability of the GPS. The bandwidth should be set to the exact
> point where they cross.
>
> Because of the very low bandwidth, thermal effects may not be corrected by
> the GPS. If your system is not thermally stable, you will have to increase
> the bandwidth of the loop, and while it will allow the GPS to more
> effectively remove short term thermal effects, that will also allow more
GPS
> noise.
>
> Because the circuit will have much gain at very low frequencies, you want
an
> op-amp that has particularly low noise at very low frequencies (typically
> low flicker noise).
>
>>
>> John Foege
>>
>
> Didier
>
>
>
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