Hi
Phase does indeed matter, it just messes up the math. Most multiplier /
filter combinations have significant phase shift between the sub-harmonic
and the carrier. You rarely know what the phase shift is, but you can read
the sub-harmonic. The simple db to jitter ratio gets you close enough to
make rational decisions on how much filtering you need. You could play with
filter phase but I've never seen that done in practice.
Bob
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Volker Esper
Sent: Tuesday, November 27, 2012 2:34 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Z3805 two frequency maxima
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<[email protected]> 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<[email protected]>
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<[email protected]>
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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