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

SAIDJACK at aol.com SAIDJACK at aol.com
Sat Jul 7 17:30:45 EDT 2007

In a message dated 7/7/2007 06:56:54 Pacific Daylight Time,  
df6jb at ulrich-bangert.de writes:

>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.

Hi Ulrich,
Magnus explained nicely how the 3325 phase jumps may happen.
For an interesting in-debth analysis on a competitive approach please  see 
Tom's excellent investigation of the FireFox Signal generator  on his website 
(thanks again Tom):
Our FireFox DDS Signal Generator is another approach to  generating signals 
using a DDS with very high resolution. Due to the finite  DDS resolution 
(either 32 or 48 bits) there is a small phase  creep on FireFox, it's about 4.8E-013 
at 10MHz, but in our case it's  pretty constant - no sudden phase jumps. Tom 
measured the jitter between the  10MHz reference and the DDS outputs to be 
around +-0.2ps (at 1s ADEV  intervalls). The jitter between the reference and the 
DDS  outputs can go below 1E-016 above 2Ks measurement  intervalls.
BTW: the FireFox would probably work quite well for your  particular test 
requirements: it has a 3.3V/5V CMOS output and a  Sine Wave output. The phase of 
the outputs can be shifted +-180  degrees in 0.1 degree steps, that may help 
to find interpolator  discontinuities/non-linearities. If necessary, the 
hardware could  generate 16-bit phase offset resolution.
One comment: as Tom explains the FireFox has a native DDS mode with 32 bits  
resolution, this is probably comparable to the HP3325. It also has a  
fractional-N mode that expands this native resolution to 48+ bits.
We are working on a reduced-bandwidth (400MHz instead of 1640MHz) version  of 
FireFox to be introduced next year, maybe even battery-operated and  portable 
- that unit will have a reduced cost point that's more attractive  to HAM's 
etc. That unit will have the 4.8E-013 phase creep mitigated as well  :)

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