[time-nuts] Z3805 two frequency maxima

Bob Camp lists at rtty.us
Sun Nov 18 21:45:13 UTC 2012


Hi

Most modern counters have an internal oscillator that they lock up to the external reference. The PLL bandwidth is (hopefully) chosen to optimize the noise contribution of the internal oscillator (quiet broad band) and the external reference (quiet close in). It will reject anything MHz away from the reference. Even if the reference signal is used directly, it would only be a problem if divided by an odd number. Even number division will wipe out the jitter. The most likely gate times are all a power of 10, so the division should always be even in a counter. Bottom line - you are unlikely to run into trouble using 10 MHz out of a Z3805 as reference signal.  

Crazy stuff….

Bob

On Nov 18, 2012, at 4:35 PM, Volker Esper <ailer2 at t-online.de> wrote:

> 
> > ...A PLL multiplier will (likely) also ignore the sub-harmonic, the
> > jitter will not show up if you use the 10 MHz as a counter timebase
> > in a HP counter...
> 
> 
> Yes, I wondered what the histogram would show when using the 2-maxima-Z3805 signal as a reference for the SR620 counter. Would the PLL filter it or would I get a mixer-like result with two maxima in the histogram?
> 
> I applied the Z3805 to the reference input and the SMX signal generator to the counter input. The histogram looked equal to that I got when using the internal oscillator of the counter for reference.
> 
> That obviously confirms Bob's statement.
> 
> Volker
> 
> 
> 
> Am 18.11.2012 21:23, schrieb Bob Camp:
>> Hi
>> 
>> This is one of those interesting areas where a broadband measurement (like a counter) and a narrowband measurement (like a DMTD) will give very different results. The counter will see the sub-harmonic, and call it jitter. It can show up in an ADEV calculation. A DMTD will ignore the sub-harmonic, and show no added jitter. With a DMTD, the ADEV will look fine. A PLL multiplier will (likely) also ignore the sub-harmonic, the jitter will not show up if you use the 10 MHz as a counter timebase in a HP counter.
>> 
>> Which one's right / does it matter - as always, that depends on what you are trying to do with the signal.
>> 
>> Bob
>> 
>> On Nov 18, 2012, at 1:26 PM, Volker Esper<ailer2 at t-online.de>  wrote:
>> 
>>> 
>>> @Azelio: Yes, (like Bob told) it's the phase lock, that locks the two maxima. Actually the counter is able to look between the 60ps spaced towers, have a look at the pictures in the first mail of this thread. However, there is a smallest time value the counter can determine, but it is much smaller than 60ps.
>>> 
>>> Volker
>>> 
>>> 
>>> 
>>> Am 18.11.2012 17:11, schrieb Bob Camp:
>>>> Hi
>>>> 
>>>> The 5 MHz and 10 MHz are "locked together" by the multiplier. Their relative phase is fixed. The 5 MHz component is not free to wander independently relative to the 10 MHz.  Since it's a doubler, you get a two peak plot. If it was a X3 you would get three peaks.  It really only makes sense if you look at it on a scope. Of course at 60 ps, it would have to be a mighty good scope. Much easier to see when the sub-harmonic is a bit stronger...
>>>> 
>>>> Bob
>>>> 
>>>> On Nov 18, 2012, at 9:53 AM, Azelio Boriani<azelio.boriani at screen.it>   wrote:
>>>> 
>>>>> Interesting this, but why exactly 60pS? I would expect also something
>>>>> in-between. That is, to have the 5MHz energy 64dB below, in my opinion
>>>>> nothing prevents to have the edge to wander not only at 60pS but
>>>>> continuously between 0 and 60pS. Maybe the SR620, being a 25pS resolution
>>>>> counter, simply doesn't see what's going on in-between and the resulting
>>>>> histogram looks that way.
>>>>> 
>>>>> On Sun, Nov 18, 2012 at 3:51 AM, Bob Camp<lists at rtty.us>   wrote:
>>>>> 
>>>>>> Hi
>>>>>> 
>>>>>> A little more detail:
>>>>>> 
>>>>>> What's going on is more clear if you trigger a scope on the positive edge
>>>>>> of the 5 MHz and look at the 10 MHz. With an ideal multiplier, both the
>>>>>> positive and negative edges of the 5 MHz should line up exactly with a
>>>>>> positive edge of the 10 MHz. In reality, negative edge (the one not
>>>>>> triggered) does not quite line up. It's a bit ahead (or behind) the ideal
>>>>>> location. Since it's a small angle, the delta in phase and delta in
>>>>>> amplitude both follow the same basic law.
>>>>>> 
>>>>>> Bob
>>>>>> 
>>>>>> 
>>>>>> On Nov 17, 2012, at 9:36 PM, Bob Camp<lists at rtty.us>   wrote:
>>>>>> 
>>>>>>> Hi
>>>>>>> 
>>>>>>> Just good old Fourier series.
>>>>>>> 
>>>>>>> Bob
>>>>>>> 
>>>>>>> On Nov 17, 2012, at 9:12 PM, Volker Esper<ailer2 at t-online.de>   wrote:
>>>>>>> 
>>>>>>>> 
>>>>>>>> I'm impressed - but what law is behind this?
>>>>>>>> 
>>>>>>>> 
>>>>>>>> Am 17.11.2012 21:26, schrieb Bob Camp:
>>>>>>>>> Hi
>>>>>>>>> 
>>>>>>>>> 60 db isn't to bad a number. More or less:
>>>>>>>>> 
>>>>>>>>> 100 ns ->    100 ps is 1000:1. 20 log of that is 60 db. 100 ps to 60 ps
>>>>>> is about 4.4 db. That would sum up to -64.4 dbc. The main gotcha is that
>>>>>> you *might* also have some 15 MHz (and higher) energy in the signal as
>>>>>> well. Also phase gets into the calculation.  Still, pretty close.
>>>>>>>>> 
>>>>>>>>> Bob
>>>>>>>>> 
>>>>>>>>> On Nov 17, 2012, at 12:50 PM, Volker Esper<ailer2 at t-online.de>    wrote:
>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>> So let's have a look into the machine... and what do we see? There's
>>>>>> a nice little Symmetrcom oven, with the sign reading "5.000 MHz" - bingo!
>>>>>>>>>> 
>>>>>>>>>> May be there's a time saving way to determine the energie of the sub
>>>>>> harmonic: using my spectrum analyzer. It tells me, that there's a 5 MHz
>>>>>> subharmonic at the level of -62dBc.
>>>>>>>>>> 
>>>>>>>>>> How would you have calculated the energy? What would be your ansatz?
>>>>>>>>>> 
>>>>>>>>>> Thanks so far
>>>>>>>>>> 
>>>>>>>>>> Volker
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>> Am 17.11.2012 17:55, schrieb Bob Camp:
>>>>>>>>>>> Hi
>>>>>>>>>>> 
>>>>>>>>>>> That's what you get if you have "sub harmonic" energy in the output
>>>>>> of your OCXO. I'd bet you a warm glass of beer that you have a 5 MHz /
>>>>>> doubled to 10 MHz MTI OCXO in your Z3805.  If you have a lot of time on
>>>>>> your hands, you can calculate the likely level of the energy from the
>>>>>> amount of jitter (spacing between the two peaks) you get.
>>>>>>>>>>> 
>>>>>>>>>>> Bob
>>>>>>>>>>> 
>>>>>>>>>>> On Nov 17, 2012, at 11:41 AM, Volker Esper<ailer2 at t-online.de>
>>>>>> wrote:
>>>>>>>>>>> 
>>>>>>>>>>>> Hi,
>>>>>>>>>>>> 
>>>>>>>>>>>> while playing with my recently aquired TIC (SR620) and measuring
>>>>>> the period time of some oscillators I discovered something I hadn't expect
>>>>>> at all:
>>>>>>>>>>>> 
>>>>>>>>>>>> The output of my GPSDO (Z3805) writes two maxima in the period
>>>>>> histogram (at a spacing of 60ps).
>>>>>>>>>>>> 
>>>>>>>>>>>> I didn't believe that result and assumed an inherent error in my
>>>>>> measuring setup or the counter itself.
>>>>>>>>>>>> 
>>>>>>>>>>>> So I plugged another oscillator, the reference TCXO of my signal
>>>>>> generator (R&S SMX), and that result made me happy and uneasy at once: The
>>>>>> TCXO hat only one maximum.
>>>>>>>>>>>> 
>>>>>>>>>>>> I havn't calculated the ADEV curve, yet.
>>>>>>>>>>>> 
>>>>>>>>>>>> See pictures.
>>>>>>>>>>>> 
>>>>>>>>>>>> Why does my GPSDO produce such a weird result?
>>>>>>>>>>>> 
>>>>>>>>>>>> Cheers
>>>>>>>>>>>> 
>>>>>>>>>>>> Volker - DF9PL
>>>>>>>>>>>> 
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