[time-nuts] Advise on building a DIY GPSDO?

Alex Pummer alex at pcscons.com
Fri Apr 8 23:05:07 EDT 2016


Sorry, I forget that 24MHz must be divided by 12 not by 6  to get 2MHz....
but here in the attachment is an old Fairchild app-note  -- from1981 -- 
which shows how to make a 50% duty cycle divide by 12 circuit with a 
9316 TTL device, which is  the predecessor of the 74HC or AC 161 the 
circuit works well with the new chips to
73
KJ6UHN
Alex






-------- Forwarded Message --------
Subject: 	Re: [time-nuts] Advise on building a DIY GPSDO?
Date: 	Fri, 8 Apr 2016 17:39:16 -0700
From: 	Alex Pummer <alex at pcscons.com>
To: 	Discussion of precise time and frequency measurement 
<time-nuts at febo.com>



"Could you elaborate on this a little if time permits? I'm more a
'digital person' but it sounds interesting. Thanks in advance, Herbert "

Yes Herbert
here is;
first divide 24MHz by two you get a very good quality absolute 50% duty
cycle 12MHz, than you feed that 12MHx into mixer [which could be a
transitional gate like HC4066 ] the other --LO input of the mixer you
need to drive with a 10MHz oscillator, the output of the mixer will be
2MHz, which you filter amplify and using to drive a freq multiplier, you
have to multiply by 5 to get 10MHz  that is your 10MHz input of the
mixer's LO port,
That was a very old style but very reliable way to do and since you have
the LC filters you will not have to much jitter issue.
Also you could go a more modern way; divide the 24MHz by 6, but make it,
that you have 50% duty-cycle  [you could make it with a CMOS device,  by
loading during the counting a hex counter ] use one 10MHz better quality
crystal oscillator, of its output has to be divided by 5 also with 50%
duty-cycle output! than use one EX-or style phase detector and close the
loop with proper filter. Depend on the quality of your crystal
oscillator you will have a very low phase noise 10MHz source.
And you could make a few other variant by mixing the idea of the two
previous and others .
It is important to use the components at the frequency at which they
well perform, but keep the phase comparation at as high frequency as you
could [side bands are easier to filer out if they are fare away from the
carrier ], also use proper shielding and power-supply filtering
73
KJ6UHN
Alex

On 4/8/2016 3:13 PM, Bob Camp 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.
> 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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