# [time-nuts] Jim Miller simple GPSDO

Tim Shoppa tshoppa at gmail.com
Wed Sep 14 08:07:51 EDT 2016

```There are special "wide-pull-range" VCXO's where a 10MHz unit will indeed
have sensitivity of 600Hz/V or more. e.g.
http://www5.epsondevice.com/en/products/vcxo_standard/vg4231ca.html

I don't know exactly what Epson does inside that particular unit, but a
trick to get wide pull range with discrete circuits is to put two or more
crystals in parallel.

Tim N3QE

On Wed, Sep 14, 2016 at 4:40 AM, Lars Walenius <lars.walenius at hotmail.com>
wrote:

> 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.
>
> Lars
>
> >From: Bryan _<mailto:bpl521 at outlook.com>
> Sent: den 14 september 2016 03:59
>
> >Lars:
> 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?
> -=Bryan=-
>
> >> 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:
> > http://www.eevblog.com/forum/projects/my-u-blox-lea-6t-
> >
> > Lars
> >
> > >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?
> > http://www.jrmiller.demon.co.uk/projects/ministd/dcamp.gif
> >
> > -=Bryan=-
> >
> > >> 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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