Benchmark ADC16 Instruction Manual download pdf (Page 30)

ADC16 Instruction Manual Page 30

seconds to lock to an incoming signal, and may have a very limited frequency range. Two-stage

PLLs may fail to lock if the reference signal is slightly off frequency. Two-stage PLLs may also fail

to lock when jitter is too high.

UltraLockDDS™ converters exceed the jitter performance of two-stage PLL converters, and are free

from the slow-lock and no-lock problems that can plague two-stage PLL designs. UltraLockDDS™

converters are highly immune to interface jitter under all operating conditions.

UltraLockDDS™ isolates the conversion clock from the digital audio interface clock. Jitter on the

reference input can never have any significant effect on the conversion clock of an UltraLockDDS™

converter. Interface jitter cannot degrade the quality of the audio conversion in an UltraLockDDS™

converter. Specified performance is consistent and repeatable in any installation!

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 over-sampling converter. This is a little known

but easily measurable effect. Most audio converters operate at high over-sampling 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

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Benchmark ADC16 Instruction Manual Page 30