PLL Frequency multiplier module (prelim, FatMan design)
Scott Gravenhorst, Synthaholic
chordman at ix.netcom.com
Thu Aug 24 23:10:51 CEST 1995
PLL FREQUENCY MULTIPLIER MODULE Scott R. Gravenhorst
chordman at ix.netcom.com
This circuit is a multi purpose phase locked loop designed for
frequency multiplication. Although it was designed around a PAiA
FatMan, it should be adaptable to almost any analog system. The
circuit is based on a 4046 type micropower phase locked loop IC
and uses a divide by N counter inserted in the loop to force the
PLL's VCO to run at N times the input frequency. It's output is
provided as a TTL square wave. You must design an appropriate
attenuator or amplifier, whichever is necessary for your system.
You may also wish to add a low pass filter at it's output to
reduce the odd harmonics present in square waveforms.
This is a sound generating module, intended to enhance a synth by
tracking a VCO to produce it's own waveform at a given frequency.
Because it is a phase locked loop, it has characteristics of it's
own, such as frequency multiplication and a controllable lag when
the input frequency changes.
Note that only two multiplication factors are provided by this
design: 2x and 3x. Although (IMHO) These are the most useful,
other factors could possibly work by changing the number of
flipflops used by the walking ring counter (74164). If this is
done, other circuit parameters will likely have to change. In
particular, the design of the low pass filter, the size of the
PLL VCO MIN/MAX pots and the PLO timing capacitor.
PAiA FatMan INPUT INTERFACE:
The 1458 op amp (which can really be almost any dual supply type
op amp) is nothing more than a level converter. The FatMan uses
a 555 timer IC, powered below ground, for it's VCO threshold
detector. The 4046, which (here) runs on +5 Volts (Vcc),
requires a positive signal with respect to ground. The 1N914
diode clamp prevents the signal fed to the 4046 from going below
ground. Pin 3 of the FatMan VCO 555 IC provides a pulse every
time the oscillator cycles. This pulse is level converted to the
required range and presented to the 4046. In this design, the
4046 needs a rather healthy input signal to stay in solid lock.
If you are using a synth other than the FatMan, make sure you
provide the 4046 a square/pulse wave that toggles between Vcc and
ground. (Not just TTL, which can be .3v to 2.6v or thereabouts)
3F / 2F
In my version, this switch allows 2 different multiplication
factors: 2 times or 3 times. 2F provides an octave above the
input and 3F provides a perfect fifth plus an octave above the
input frequency (19 semitones above the input pitch). My version
also includes a switchable div by 2 counter at the frequency
multiplied output to allow for a 1.5x multiplication, i.e. a
simple perfect fifth above the input frequency.
VCO FREQ CAP
This switch cuts in or out an extra capacitor to allow adjustment
to different note ranges. It can also be used to create bizzaro
This switch cuts in or out an extra capacitor in the low pass
filter circuit. Again, it's use is not strictly defined and has
an effect on how quickly (or if) the loop settles down after a
pitch change. It has a dramatic effect on the playing range and
the sound produced.
This switch forces the divide by N counter to reset to zero.
This counter may have random bit settings when powered up and can
give wierd (you might like them) results if not reset. Press and
release to reset the counter. Note that if this switch is
pressed while a note is playing, the counter (as well as the
output) stops while it is held depressed. Since while depressed,
the phase comparator input is receiving 0 hertz, when the RESET
switch is released, the loop must re-lock. When this happens,
you can get an interesting 'sproing' sound, depending on the
settings of the other controls. Kind of metallic. Use caution
here, you might get a loud 'pop'.
PLO F MAX
Sets the maximum frequency the PLL's VCO can produce. To raise
the maximum frequency, turn this pot to a lower resistance value.
PLO F MIN
Sets the minimum frequency the PLL's VCO can produce. The MIN
and MAX pots will dramatically effect the operation of the loop.
The MAX control should be set to a lower resistance value than
the MIN pot to allow proper PLL operation. To lower the minimum
frequency, turn this pot to a higher resistance value.
This pot controls the damping factor of the PLL. It can be set
so that the tracking lag is short, long or even to allow
significant over/undershoot. Over/undershoot causes a 'sproing'
sound when the input pitch changes. This will be more pronounced
as the setting of this pot approaches zero ohms. When set to
higher resistance values, various damping effects occur. You can
get an interesting 'squink' sound at the attack of any note that
causes relatively large input pitch changes.
I have had alot of fun designing and using this circuit. It
allows creation of yet more color and texture for electronic
sound and music synthesis.
I would like to thank Don Tillman for his input.
This is a preliminary document. Please send comments by email to
me (chordman at ix.netcom.com). The final text and GIF documents
will be posted when this project jumps onto a piece of perfboard.
There is no 12 step program for synthaholics. Thank your Superior Being.
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