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

Sun Apr 10 08:23:44 UTC 2016

Just drive the mixer LO at 8MHz and replace the 14MHZ bandpass filter with a 2MHz bandpass filter.
Bruce 

    On Sunday, 10 April 2016 7:23 PM, Bruce Griffiths <bruce.griffiths at xtra..co.nz> wrote:

 No, you can just drive the mixer LO with 8MHz and use a 2MHz bandpass filte=
r (plus the 10MHz bandpass filter) on the mixer RF output rather than 14MHz 
 bandpass filter.
Bruce

      From: Herbert Poetzl <herbert at 13thfloor.at>
 To: Bruce Griffiths <bruce.griffiths at xtra.co.nz>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Sunday, 10 April 2016 5:24 PM
 Subject: Re: [time-nuts] Advise on building a DIY GPSDO?

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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