[time-nuts] An (unknown?) nasty feature of the DDS principlefortime nuts applications

Bob Camp lists at rtty.us
Tue Jan 25 17:07:03 UTC 2011


Hi

Jitter from the sawtooth phase modulation on a DDS output is indeed a very
real thing. I have an Agilent 33250A generator sitting here on the bench
that makes a very nice variable jitter signal source.

Bob 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Ulrich Bangert
Sent: Tuesday, January 25, 2011 9:59 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] An (unknown?) nasty feature of the DDS
principlefortime nuts applications

Ahhh,

I see what you mean because a few pages later there is word of a "phase
accumulator". But this phase accumulator is a part of the electronics that
is responsible for generating the control voltage of a VCO that enables the
device to produce odd frequencies despite the integer dividers in the main
loop. That IS different from what a phase accumulator's task is in a DDS.

73s de Ulrich, DF6JB 

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Ulrich Bangert
> Gesendet: Dienstag, 25. Januar 2011 15:35
> An: 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] An (unknown?) nasty feature of the 
> DDS principlefor time nuts applications
> 
> 
> Gerhard,
> 
> see page 8.6ff of the service manual to see that the HP3325 
> is not DDS based but uses a "Fractional N Synthesizer scheme" 
> which is something completely different and is not prone to 
> the described effects. Nevertheless it is true that the 
> HP3325 contibutes phase noise to the measurement but I had 
> anticipated that this was clear to everybode in the auditorium.
> 
> 73s de Ulrich, DF&JB
> 
> > -----Ursprungliche Nachricht-----
> > Von: time-nuts-bounces at febo.com
> > [mailto:time-nuts-bounces at febo.com] Im Auftrag von dk4xp at arcor.de
> > Gesendet: Dienstag, 25. Januar 2011 15:14
> > An: time-nuts at febo.com
> > Betreff: Re: [time-nuts] An (unknown?) nasty feature of the 
> > DDS principle for time nuts applications
> > 
> > 
> >  
> > Don't forget that your HP3325 is DDS-based, too, so it adds
> > its own phase error sawtooth.
> > 
> > 73, Gerhard dk4xp
> > 
> > 
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