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ADC1 Instruction Manual Page 18
How does conversion clock jitter
degrade converter performance?
Problem #1
Jitter phase modulates the audio signal. This
modulation creates sidebands (unwanted
tones) above and below every tone in the
audio signal. Worse yet, these sidebands are
often widely separated from the tones in the
original signal.
Jitter-induced sidebands are not musical in
nature because they are not harmonically
related to the original audio. Furthermore,
these sidebands are poorly masked (easy to
hear) because they can be widely separated
above and below the frequencies of the
original audio tones. In many ways, jitter
induced distortion resembles intermodulation
distortion (IMD). Like IMD, jitter induced
distortion is much more audible than
harmonic distortion, and more audible than
THD measurements would suggest.
Jitter creates new audio that is not
harmonically related to the original audio
signal. This new audio is unexpected and
unwanted. It can cause a loss of imaging, and
can add a low and mid frequency “muddiness”
that was not in the original audio.
Jitter induced sidebands can be measured
using an FFT analyzer.
Problem #2
Jitter can severely degrade the anti-alias
filters in an oversampling converter. This is a
little known but easily measurable effect.
Most audio converters operate at high
oversampling ratios. This allows the use of
high-performance digital anti-alias filters in
place of the relatively poor performing analog
anti-alias filters. In theory, digital anti-alias
filters can have extremely sharp cutoff
characteristics, and very few negative effects
on the in-band audio signal. Digital anti-alias
filters are usually designed to achieve at least
100 dB of stop-band attenuation. But, digital
filters are designed using the mathematical
assumption that the time interval between
samples is a constant. Unfortunately, sample
clock jitter in an ADC or DAC varies the
effective time interval between samples. This
variation alters the performance of these
carefully designed filters. Small amounts of
jitter can severely degrade stop-band
performance, and can render these filters
useless for preventing aliasing.
The obvious function of a digital anti-alias
filter is the removal of audio tones that are
too high in frequency to be represented at the
selected sample rate. The not-so-obvious
function is the removal of high-frequency
signals that originate inside the converter
box, or even originate inside the converter IC.
These high-frequency signals are a result of
crosstalk between digital and analog signals,
and may have high amplitudes in a poorly
designed system. Under ideal (low jitter)
conditions, a digital anti-alias filter may
remove most of this unwanted noise before it
can alias down into lower (audio) frequencies.
These crosstalk problems may not become
obvious until jitter is present.
Stop-band attenuation can be measured very
easily by sweeping a test tone between 24
kHz and at least 200 kHz while monitoring the
output of the converter.
Put UltraLock converters to the
test
We encourage our customers to perform the
above tests on UltraLock converters (or let
your ears be the judge). There will be
absolutely no change in performance as jitter
is added to any digital input on an UltraLock
converter.
Try the same tests on any converter using
conventional single or two-stage PLL circuits.
Tests should be performed with varying levels
of jitter and with varying jitter frequencies.
The results will be very enlightening. Jitter
related problems have audible (and
measurable) effects on ADC and DAC devices.
Practitioners of Digital Audio need to
understand these effects.