[time-nuts] GPSDO design

[Bob Camp]

Hi

A couple other interesting component issues:

If the tuning diode in the OCXO is decoupled with "typical" value resistors,
the KTB noise in the resistors will be greater than the voltage noise of a
good op amp.

If you go the active filter route and use a 1 meg ohm resistor in the time
constant section, it's KTB noise would be 123 nV in a 1 Hz b.w. You can get
op amps quite a bit better than that for voltage noise. Of course you only
see the whole voltage resistor KTB below the cutoff of the filter.

There are lots of things way more important in a reasonable GPSDO than the
noise in the resistors or op-amps.

For under $100 you can get a Tbolt and not have to chug through any of this.
They seem to work very well ...

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Thursday, January 14, 2010 4:12 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPSDO design

The noise of the internal zener used to bias the OCXO EFC varicap is 
likely to exceed the output noise of any amplifier unless the filter 
series resistance is sufficiently large that noise due to amplifier bias 
current is excessive or the amplifier gain is very large.

Bruce

Dick Moore wrote:
> John, I wouldn't bother with amplitude scaling, just unity gain with a DC
offset to center the control range to the 10811A to zero volts or
thereabout, and choose inverting or non-inverting to suit the
characteristics of the control voltage slope vis-avis the 10811A's slope,
which is negative if I remember right. About a 1Hz upper band limit works
pretty well, so amplifier noise really isn't a problem as you'll LP the
output pretty severely anyway.... my 2 cents.
>
> Dick Moore
>
>
> On Jan 14, 2010, at 4:00 AM, time-nuts-request at febo.com wrote:
>
>    
>> Message: 2
>> Date: Thu, 14 Jan 2010 06:57:39 -0500
>> From: John Foege<john.foege at gmail.com>
>> Subject: [time-nuts] GPSDO Design
>> To: time-nuts at febo.com
>> Message-ID:
>> 	<888d55281001140357s6021cfa9i4227741eedb2f633 at mail.gmail.com>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> 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. 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.
>>
>> 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.
>>
>> Anyone have experience with this? Assuming I have an OXCO with a max.
>> pulling range of 1ppm or 1e-6 over a 10V range, then I effectively can
>> pull 1e-7 per volt. This translates to 1e-10 per millivolt and 1e-13
>> per microvolt. Assuming that is a logical conclusion, then for a good
>> OCXO, in which I can at best hope for 5e-12 stability for tau=1s (e.g.
>> HP10811A), I would strive to to keep the noise at such a level that it
>> is an order of magnitude better than the best short term stability
>> figure. Accordingly, then I should shoot to keep any noise under 1
>> microvolt?
>>
>> I don't have much experience with noise calculations. I know it is
>> specified in nV/sqrt(Hz) generally. Translating this to something
>> practical is basically the assistance I'm looking for here.
>>
>> I would appreciate anyone being able to teach me a bit more about this.
>>
>> Thank you in all in advance.
>>
>> Sincerely,
>>
>> John Foege
>>      
>
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