[time-nuts] Z3805 two frequency maxima

Sun Nov 18 21:35:06 UTC 2012

 > ...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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