[time-nuts] Advise on building a DIY GPSDO?
Will
zl1tao at gmx.com
Sat Apr 9 18:20:57 EDT 2016
Hi all,
I'm fairly new here and might not fully understand things.
Earlier in this thread it was suggested that one lock an 8Mhz signal to a 10 Mhz signal by analogue methods.
To quote A Plummer:
"and it is relative easy to make 10MHz from 8MHz with analog frequency
manipulation, which generates less jitter
73"
and H Poetzi asked the same thing as I am:
"On Mon, Apr 04, 2016 at 06:07:54PM -0700, Alexander Pummer wrote:
> and it is relative easy to make 10MHz from 8MHz with analog
> frequency manipulation, which generates less jitter
Could you elaborate on this a little if time permits?
I'm more a 'digital person' but it sounds interesting.
Thanks in advance,
Herbert"
I have not seen how that is done as suddenly the signals are 24Mhz and 10 Mhz and digital dividers and multipliers are used.
One other point. Attila mentioned using "LEA-M8T". I assume the T suffix relates to Time rather than the plain GPS. What is the difference? Apart from 50% higher cost.
Cheers
Will
ZL1TAO
> Sent: Sunday, April 10, 2016 at 7:00 AM
> From: "Bob Camp" <kb8tq at n1k.org>
> To: EWKehren at aol.com, "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Subject: Re: [time-nuts] Advise on building a DIY GPSDO?
>
> Hi
>
> Averaged over a long enough time (and without any hanging bridges) the frequency accuracy
> will be fine. The frequency accuracy of a 1 pps output on a GPS is “fine” on the same basis. Since
> 200 KHz is a “round division” off of any of the likely TCXO’s you will not have any jitter or spurs in the “static”
> case.
>
> Bob
>
>
> > On Apr 9, 2016, at 10:07 AM, Bert Kehren via time-nuts <time-nuts at febo.com> wrote:
> >
> > I do not know what U blox does but I know when we use 200 KHz out of the 1
> > pps output on a $ 10 ublox 6 we consistently get better than 1 E-10 closer
> > to 1 E-11 out of the Morion have the data
> > Bert Kehren
> >
> >
> > In a message dated 4/9/2016 10:01:05 A.M. Eastern Daylight Time,
> > kb8tq at n1k.org writes:
> >
> > Hi
> >
> >> On Apr 8, 2016, at 9:39 PM, timenut at metachaos.net wrote:
> >
> >>
> >> Hello Bob,
> >>
> >> Friday, April 8, 2016, 6:13:07 PM, you wrote:
> >>
> >>> Hi
> >>
> >>> If you start from a 24 MHz TCXO (different modules use different TCXO’
> > s):
> >>
> >>> On an 8 MHz output, most of the time you divide by three.
> >>
> >>> On a 10 MHz output, you need to divide by 2.4. The net result is that
> > you
> >>> divide by 2 sometimes and 3 other times.
> >>
> >>> In the 10 MHz case, there is a *lot* of energy at 12 MHz and 8 MHz,
> > along with
> >>> the 10 MHz output.
> >>
> >>> In the 8 MHz case, most of the RF energy is at 8 MHz.
> >>
> >>> ====
> >>
> >>> To correct the output by 1 ppm on the 8 MHz output, you need to either
> > drop or
> >>> add one pulse out of every million pulses. Effectively you divide the
> > 24 MHz by
> >>> 2 or by 4 when you do that. You get a bit of 12 MHz or a bit of 6 MHz
> > as a result.
> >> If you know you are doing a 24Mhz and a 10Mhz, why not divide the first
> > by 12
> >> and the second by 5 and then phase lock the resulting 2Mhz? Or divide by
> > 24
> >> and 10, respectively and lock the 1Mhz? That way, everything is exact.
> >
> > The bigger problem is that the 24 MHz is *not* exact. It is simply a free
> > running TCXO
> > that happens to be in a GPS module. It has a basic accuracy of +/- 1 ppm
> > or something
> > similar. It is no better or worse than any other TCXO you could buy.
> >
> > To make it accurate they have two choices:
> >
> > 1) Put a voltage control input on the TCXO and turn it into a TCVCXO, then
> > lock it up
> > with a loop.
> >
> > 2) Let the oscillator free run and “fix up” the output.
> >
> > For a variety of reasons, none of the small GPS modules go with option
> > number 1. They
> > all go with option number 2. The 24 Hz error on the (maybe) 24 MHz gets
> > taken out by dropping
> > 24 edges every second. That’s not a lot of edges, it’s not going to turn
> > the output into absolute
> > garbage you can see on a scope. It is plenty of nonsense to mess up a
> > radio or a piece of test gear.
> >
> > One easy way to look at it: You have ~1 ppm jitter on the output (in the
> > example of 1 ppm of error). A
> > phase locked GPSDO with only simple filtering of a 1 pps would get you
> > down to 0.01 ppm of jitter.
> > A sawtooth corrected 1 pps would get you to 0.01 ppm. A good filter would
> > get you to <0.00001 ppm.
> > Yes, I’m using a very hand waving definition of jitter here, but it does
> > illustrate the point. You could
> > look at the jitter on the pulse drop as 0.04 ppm.
> >
> > Bob
> >
> >>
> >>
> >> Mike
> >>
> >>> That can be filtered out with a RF filter. The same is true with a
> > (somewhat more
> >>> complex) filter on the 10 MHz output.
> >>
> >>> In addition to the “big” RF spurs, you get a low frequency component
> > to the output
> >>> modulation. You are “phase hitting” the output eight times a second.
> > That gives you
> >>> an 8 Hz sideband along with the further removed stuff. Since it’s not
> > simple / clean
> >>> phase modulation, there are more sidebands than just the few mentioned
> > above.
> >>
> >>> What messes things up even more is that you never are quite doing one
> > ppm. You are doing
> >>> corrections like 0.12356 ppm this second and 0.120201 ppm the next
> > second.
> >>> The pattern of pulse drop and add is not as simple as you might hope.
> > The low
> >>> frequency part of the jitter (and it will be there) is no different
> > than the noise on
> >>> a 1 pps output. You still need to do very long time constant (or very
> > narrow band)
> >>> filtering to take it out.
> >>
> >>> Bob
> >>
> >>>> On Apr 8, 2016, at 7:06 AM, Herbert Poetzl <herbert at 13thfloor.at>
> > wrote:
> >>>>
> >>>> On Mon, Apr 04, 2016 at 06:07:54PM -0700, Alexander Pummer wrote:
> >>>>> and it is relative easy to make 10MHz from 8MHz with analog
> >>>>> frequency manipulation, which generates less jitter
> >>>>
> >>>> Could you elaborate on this a little if time permits?
> >>>> I'm more a 'digital person' but it sounds interesting.
> >>>>
> >>>> Thanks in advance,
> >>>> Herbert
> >>>>
> >>>>> 73
> >>>>
> >>>>> On 4/4/2016 4:27 PM, Attila Kinali wrote:
> >>>>>> On Mon, 4 Apr 2016 17:56:29 -0400
> >>>>>> Bob Camp <kb8tq at n1k.org> wrote:
> >>>>
> >>>>>>> The variable frequency output on the uBlox (and other) GPS
> >>>>>>> receivers has come up many times in the past.
> >>>>
> >>>>>>> If you dig into the archives you can find quite a bit of
> >>>>>>> data on the (lack of) performance of the high(er) frequency
> >>>>>>> outputs from the various GPS modules. They all depend on
> >>>>>>> cycle add / drop at the frequency of their free running TCXO.
> >>>>>>> Regardless of the output frequency, that will put a *lot* of
> >>>>>>> jitter into the output.
> >>>>>> That's why you should put the output frequency of the ublox modules
> >>>>>> to an integer divisor of 24MHz. Ie 8MHz works but not 10MHz.
> >>>>
> >>>>>> Attila Kinali
> >>>>
> >>>>
> >>>>> _______________________________________________
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> >>
> >>> _______________________________________________
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> >>
> >>
> >>
> >> --
> >> Best regards,
> >> Timenut mailto:timenut at metachaos.net
> >>
> >> _______________________________________________
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