[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):
_http://www.leapsecond.com/logs/said/4.htm_
(http://www.leapsecond.com/logs/said/4.htm)
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 :)
bye,
Said
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