[time-nuts] Phase jumps of HP3325A synthesizer ???

Ulrich Bangert df6jb at ulrich-bangert.de
Sat Jul 7 09:55:05 EDT 2007


perhaps you know that Bruce and I are working on an linear phase
comparator similar to the K34-5991A from HP (but with some features of
today) which should enable us to characterize even "better" oscillators
at an very small price. The first expriments (running since some weeks)
are very encouraging but now an problem has arised that I would like to
hear your comments to:

For linearity tests of the circuit I need two signals that are

a) low phase noise


b) have an very small stable frequency offset, say 0.001 to 0.005 Hz.

The two signals that I use are

a) 10 MHz coming from my well hung FTS-1200 + frequency doubler

b) The SQUARE output of my HP3325A set in amplitude and dc offset so
that effectively an cmos like digital signal is generated. The HP3325
uses the FTS-1200 as external reference. I am using this arrangement
because of the ease of offset generation as well because the linear
phase comparator currently features an well designed sine to ttl
converter only on one of its input channels, so the second channel needs
to be fed digital.

The slow beat frequency slowly shifts the phase delay between the phase
comparator input clocks and one should get an linear output of the lpc
over time. Looked at the big scale (200 ns measurement range) the
voltage recorded at the lpc's output looks indeed picture-perfect. 

If however looked at an scale of an few ns it becomes obvious that there
is an regular sudden jump in phase of abt. 320 ps (always same
direction) every 60 seconds or so. Bruce and I have tried to remove
zillions of possible sources for that without any result. As an last
resort I made an measurment that recorded the phase comparator's output
as well as an direct time interval measurement between the positive
slopes of the input clocks and much to my surprise the jump was already
to be seen there although buried in a bit more noise due to 20 ps tic

I am almost sure the FTS-1200 can NOT be the source of these jumps
because it is at the same time the LO of my GPSDO system. If it were to
produce regular phase jumps (even that small) they would accumulate to
an value that would result in an noticeable reaction of the closed loop.

Therefore the qustion: Has anyone of you an theoretical knowledge about
the HP3325 that would explain this behaviour or has anyone of you made
similar findings? In the above example the frequency had been set to
10.000000005 Mhz, square output, 5V amplitude, 2.5V dc offset which
gives an 0 to 5 V when terminated into 50 Ohms. Can it be that the
effect is due to the sqare output? I made some tests whith seeting the
phase of the output signal which led to no noticable changes so there
could be an signifant difference of signal phase behavoiur between sine
and square generstion.

Ulrich Bangert  

Brooke, I hope you find my signature ok now?

Ulrich Bangert
Ortholzer Weg 1
27243 Gross Ippener 

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