[time-nuts] Z3805 two frequency maxima

Volker Esper ailer2 at t-online.de
Tue Nov 27 19:33:35 UTC 2012


I followed your argument and tried to synthesize such a signal. I built 
a simple power combiner (3 times 18 ohms resistors) and combined the 10 
MHz reference output of my signal generator with a 5 MHz signal from the 
same generators regular output at the same amplitude. My oscilloscope 
showed the locked phase of the two signals.

I applied this combo signal to the SR620 and observed a wonderful 
two-maxima histogram. When reducing the amplitude of the 5 MHz signal 
(while keeping the 10 MHz amplitude) the peaks distance decreased 
linearly with the voltage reduction, until the peaks melted togehter at 
about -20dBc.

On the one hand it was a success, but why only 20dBc? My experiences 
with the Z3805 showed a 5 MHz subharmonic at 62dBc and the peaks spaced 
at 60 ps.

So I startet to add phase delay to the 5 MHz signal by looping-in some 
meters of coax cable.

When coming to a delay time of 66 ns I could distinguish the two peaks 
at a spacing of 240 ps down to an amplitude ratio of about 1000, that 
is, 60dB.

Volker





Am 18.11.2012 03:36, schrieb Bob Camp:
> 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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>>>>>
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