[time-nuts] Jim Miller simple GPSDO
lars.walenius at hotmail.com
Wed Sep 14 04:40:21 EDT 2016
The VCXO sensitivity given is strange as it indicates a far to wide span so I guessed 30ppm and if it is higher it still needs the damping from R2-C2.
For the OCXO I used the figures given. With 2Hz per volt and 8 Volt span you have 16Hz of span. 16Hz divided with 10MHz is 1.6ppm (parts per million) can also be said as 1.6us/s.
>From: Bryan _<mailto:bpl521 at outlook.com>
Sent: den 14 september 2016 03:59
Thank you very much, your explanation was very helpful. I unfortunately don't have a background in electronics other than at a hobbyist level, and really should just lurk in the back as many of the topics discussed are way above me, but I am learning <g>. So forgive this obvious and perhaps dumb question but how are you calculating the oscillator spans, you reference the VCXO at around 30ppm. I suspect this is because the VCXO has a sensitivity of 600-1000hz/v and the OCXO of 1.6ppm has a sensitivity of 2hz/v or 3.2/v at 8v. But how are you arriving at the ppm values?
>> From: lars.walenius at hotmail.com
> To: time-nuts at febo.com
> Date: Tue, 13 Sep 2016 20:44:50 +0000
> Subject: Re: [time-nuts] Jim Miller simple GPSDO
> 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).
> > Bill....WB6BNQ
> > Bryan _ wrote:
> > >Hello:
> > >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?
> > >Cheers
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