[time-nuts] Advise on building a DIY GPSDO?
Bob Camp
kb8tq at n1k.org
Sun Apr 10 00:07:27 UTC 2016
Hi
To filter out the close in noise on the output of the GPS module (regardless of
output frequency) you need a very narrow bandwidth loop. Cross over points in the
0.01 Hz to < 0.001 Hz range are not at all unusual for these loops. Starting off at a high(er)
frequency does not help in this case.
The module puts out information on the serial port that “corrects” the PPS output by
more than an order of magnitude over the correction on the other outputs. That makes
it more attractive than a higher frequency output (it’s more accurate). The only reason
not to use it would be if the PLL is easier. With the very narrow bandwidth involved, there
is no significant advantage in the PLL.
Bob
> On Apr 9, 2016, at 6:20 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 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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>>>>
>>>>
>>>> --
>>>> Best regards,
>>>> Timenut mailto:timenut at metachaos.net
>>>>
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