[time-nuts] Advise on building a DIY GPSDO?
Herbert Poetzl
herbert at 13thfloor.at
Sun Apr 10 01:24:22 EDT 2016
On Sun, Apr 10, 2016 at 01:16:11AM +0000, Bruce Griffiths wrote:
> It can easily be done using a variant of the dual conjugate
> regenerative divider.
> Feed the 24 MHz signal into the LO port of a mixer.Use a dual
> bandpass filters centred on 14MHz and 10MHz to filter the IF
> port amplify the outputs of the bandpass filters and drive the
> mixer RF port with the combined 10MHz and 14MHz signals.
> The 10MHz signal can be extracted from the amplified 10MHz
> output via a splitter.
> When the loop gain and phasing is correct for both the 10MHz
> and 14MHz signals the circuit will produce the required output.
> Excess gain is eliminated by the mixers compression of the IF
> signal.
> The circuitry is all analog with no digital components whatsoever.
Thanks Bruce!
That is something I do understand, although the original
idea was to go from 8MHz to 10MHz, but I presume the 24MHz
can be filtered out from the overtones after running the
8MHz signal through a comparator.
Cheers,
Herbert
> Bruce
> On Sunday, 10 April 2016 12:10 PM, Will <zl1tao at gmx.com> wrote:
> 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 Poetzl 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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>>>> --
>>>> Best regards,
>>>> Timenut mailto:timenut at metachaos.net
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