[time-nuts] GPSDO Design

[Didier Juges]

> -----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