[time-nuts] Jim Miller simple GPSDO
lars.walenius at hotmail.com
Tue Sep 13 16:44:50 EDT 2016
As I have understood it the change of VCXO gain is the reason that R2-C2 can be omitted. With the VCXO with a large span the damping is needed otherwise it will oscillate.
The XOR phase detector has a range of 50us with 10kHz in. The VCXO has maybe a span of 30ppm (us/s) and with R1-C1 time constant of about 16seconds the phase shift will be close to 180 degrees.
With the OCXO with a span of 1.6ppm (us/s) the apparent time constant will be about 32 (50/1.6) seconds and the 16 seconds time constant of the R1-C1 will act more as a low pass filter at gain cross over with a phase shift much below 90 degrees.
Sorry for the bad explanation but what I try to say is: If the phase detector range divided with the VCXO span is larger than the R1-C1 time constant R2-C2 can be omitted.
This thread on EEVblog might be interesting for those that think of using the Miller-style GPSDO:
>From: Bryan _<mailto:bpl521 at outlook.com>
>Sent: den 12 september 2016 10:49
>Thank you for the reply.
Yes, R1/R2/C1/C2 is what I was referencing. I was not sure as the values in the schematic are referenced when using the C-MAC (now RAKON) VCXO. Further into the material the author switched to a Isotemp 134-10 OCXO and used a DC amplifier to compensate for the 0-8v for the EFC, but stated that R2 and C2 are not needed when using this OCXO. Not sure why they are omitted, is it because of the DC amplifier or because of different specs of the OCXO?
>> Date: Mon, 12 Sep 2016 01:25:53 -0700
> From: wb6bnq at cox.net
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Jim Miller simple GPSDO
> Hi Bryan,
> No ! Assuming you mean R1/R2/C1/C2 of the Miller schematic, those
> values are already set for the comparison frequency (10KHz) of the PLL
> phase comparator (U2).
> Bryan _ wrote:
> >I have been following the Jim Miller simple GSDO build project at http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htm I have a few OCXO's kicking around, but wondering what would the appropriate components be for for R1,R2, C1, C2 to provide the PLL filter. I assume the PLL filter needs to be designed to accommodate a specific oscillator specifications, or maybe it doesn't really matter and can use the default values in the schematic?.
> >Was also considering using a picdiv instead of the 2- 74HC390, not sure if that would be an advantage or disadvantage in terms of operating performance?
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