[time-nuts] GPSDO Design

[Bruce Griffiths]

oops I omitted an important phrase.

Bruce

Bruce Griffiths wrote:
> There are 4 principal sources of noise
>
> 1) The GPS receiver
>
> 2) The 4046 Phase detector
>
> 3) The opamp
>
> 4) The OCXO
>
> In the short term the GPS receiver noise will dominate.
The GPS receiver noise will dominate the sort term phase detector output 
noise
>
> In the long term the 4046 phase detector noise and drift together with 
> the OCXO noise and drift will dominate.
>
Again add the phase "the phase detector output noise" to the end of the 
sentence.
> Unless you make an extremely poor choice of opamp the 4046 phase 
> detector noise and drift will be much larger than that of the opamp.
>
This actually depends on the phase detector comparison frequency at 1PPS 
phase detector (plus divider drift) of 1ns/C is equivalent to 5nV/C 
input offset drift at the opamp (about 1/10 that of a chopper stabilised 
amplifier). At 100Hz 1ns/C is equivalent to about 500nV/C at the opamp 
input or about 10x that of a good chopper stabilised (or "zero drift") 
amplifier.
>
> Bruce
>
> John Foege wrote:
>> 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
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to 
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>