DA2002
Theory of Operation
11
Jitter Removal
Ordinary phase lock loops circuits (PLL's) do a reasonable job at removing high frequency jitter
from the incoming clock. The same circuits perform very poorly in the removal of low frequency
jitter from the clock signal.
The need to keep enough bandwidth for locking to and tracking the incoming data translates to
zero rejection of low frequency jitter content (typically hundreds of Hz of zero rejection
bandwidth). While some of the jitter content is random, much is due to coupling of the data itself
into the receiver circuitry.
The DA2002 uses a two stage PLL circuit; the first one is based on a tuned circuit approach, and
the second utilizes a narrow lock-range. The combination of the two provides enhanced jitter
removal.
In addition, for 44.1 and 48 kHz inputs the CrystalLock™ circuit provides a non-standard
approach for removing jitter. The deglitcher circuit is clocked by a pullable crystal oscillator but
the control signal for the crystal is freed from having to track down incoming clock variations.
The clock oscillator is controlled by a processor driven DAC, and oscillator frequency is changed
by tiny amounts (.1ppm) and not very often (15 seconds or more) in a manner allowing it to track
only very long term average drift. Using such an approach with ordinary PLL will cause loss of
lock because the slight variations in incoming data rate cause loss of correspondence between the
input and the too steady of a clock circuit. The DA2002 CrystalLock™ approach, stores enough
data in a dedicated memory to guarantee that each clock cycle can find its data. Moving the clock
slowly to track the long-term average drift is done just fast enough to make sure that the buffer
memory does not overfill or becomes empty.
Output Filter and Drivers
The DA2002 operates in low oversampling to allow for maximum settling time of the DAC
circuits and to further reduce sensitivity to jitter. The upsampling filter is calculated by the DSP.
The tradeoff in favor of low oversampling operation poses an increased requirement for analog
anti imaging filtering. The DA2002 incorporates a seven pole analog filter.
The transistor based output drivers are short circuit protected and are capable of driving balanced
300 Ohm loads. As always, for best results it is recommended to use high quality cables. When
running a cable through an electrically noisy environment, a termination impedance of 600 Ohms
(at the destination, not at the DA2002 side) may prove useful.