----- Original Message -----
From: "Bob & Linda McGraw K4TAX" <RMcGraw@InfoAve.Net>
Sent: Wednesday, December 13, 2000 10:12 PM
Subject: Re: [TenTec] Poor CW note on Pegasus and Jupiter
> Carl very good points.
>
> In the world of digital audio, we call this effect "clock jitter".
> Nothing new, just fact of life. Hence the reason most digital audio
> system use a switching PSU. Plus, it's easy to lock the digital clock
> and the PSU switching rate together. Hence, no jitter.
>
> 73
> Bob K4TAX
>
Hello Bob and list
I have a little different take on all this.
Clock frequency instability is usually called FM in digital audio since it
is a frequency modulation with a comparatively high modulation index. Clock
jitter is a phase ambiguity ( FM with a very low modulation index) of a
normally stable word-clock signal. That happens when the rise and fall time
of a stable clock is altered by reactive components in cables or connectors
or circuit design after the a stable clock has been created. In Jitter, the
adjacent clock transitions have unequal periods due to the increased
ambiguity of threshold crossing points on the slower state transition. It
can result in an altered phase coherency between channels in a multichannel
system. That is detectable as altered stereo image and upper harmonic phase
and slight amplitude changes in relationship to the fundamental and other
harmonics. It's auditory equivalent in the analog world is TIM distortion.
The case of the master oscillator in the Ten-Tec rigs is a classic one of
microphonic FM, modulation of the physical environment of the reactive
frequency determining components of the oscillator itself. The
instantaneous frequency can be measured anywhere in the latter signal path.
Jitter is induced in different proportions in each later circuit that relies
on the word clock. That is the problem, a good design and a stable
word-clock generator can be compromised by an end user installing poor
cables and each device synchronized by the same word-clock will have a
different sample period. On the other hand, cure the Ten-Tec master
oscillator stability problem and all subsequent stages are improved.
My Paragon also suffers from master clock modulation but I haven't done
anything about it since it works fine for my casual operation. Additionally,
about once a year the PLL unlock "feature" returns. I figure it is just my
rig's attempt to get more attention from me. After a few hours of soldering
and unsoldering she is happy again. I recently added a Titan I to my
station and it is wonderful!
73
Stan KM6XZ
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