Doepfer A-196 PLL - Manual (Page 2)

A-196 PLL System A - 100 doepfer
2
2. Basic principles
The three units VCO, phase comparator (PC) and low
pass filter (LP) form a standard closed-loop frequency-
feedback system: The VCO output (linear response,
rectangle output) is compared with an external signal
(e.g. A-110 VCO) in the PC. The PC output is a digital
high/low signal that indicates if the frequency resp.
phase difference of the two input signals is negative,
zero or positive. This signal is processed by the LP to
generate a smooth control voltage that controls the
frequency of the VCO. The units VCO, PC and LPF
form a frequency feedback loop that works like this:
The CV (LP output) increases (decreases) as long as
the external frequency is higher (lower) than the fre-
quency of the internal VCO and stops increasing as
soon as both frequencies become identical.
But there are some stumbling blocks: Different types
of phase comparators with advantages and disad-
vantages can be made. Some phase comparators e.g.
even lock at harmonics, i.e. if the two frequencies to be
compared are integer multiples. For some applications
this "fault" may be used to create interesting effects.
The A-196 contains 3 different types of phase compa-
rators: PC1 is a simple exclusive OR, that even locks
at harmonics. PC2 is a so-called RS flipflop and PC3 a
more complex digital memory network. The user can
select one of the three phase comparators with a
3-position switch. When PC2 is used a LED displays
the "locked" state, i.e. when the frequency of the
internal VCO is identical to the external frequency.
Special attention has also to be directed to the fre-
quency of the LP. To obtain a smooth control voltage
for the VCO the frequency of the LP has to be much
smaller than the lowest frequency of the internal or
external audio signal. Otherwise the frequency of the
internal VCO will jitter or wobble around the correct
frequency. But for special effects this frequency jitter
can be used intentionally.
Example: frequencies in the range 50Hz...1kHz have
to be processed with the PLL. Therefore the frequency
of the LP has to be about 10Hz or even less. Such a
low frequency of the LPF causes a noticeable slew of
the internal VCO. When the frequency of the external
signal jumps e.g. between 50Hz and 1kHz it takes
about 0.1 second until the internal VCO reaches the
new frequency (like portamento). Consequently one
has to find a compromise between the frequency jitter
and portamento. But these remarks are valid only for
the "ideal" working PLL. As the A-196 is used in a
musical enviroment these "problems" and "dis-
advantages" with jitter and slew time lead to additional
musical applications like portamento effects, wobbling
frequencies or harmonic locking according to the type
of frequency comparator and time constant of the PLL
low pass filter.