[time-nuts] Z3805 two frequency maxima

Volker Esper ailer2 at t-online.de
Sun Nov 18 18:26:34 UTC 2012


@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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