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triggered function generator circuit

I'm trying to figure out a way to create a simple function generator
circuit that produces square and triangle waveforms. These could be
skewed with a pot to produce a variable duty cycle at the square wave
output, and sawtooth waveforms at the triangle output. There are some
excellent chips that can do this, but the output frequencies are
usually controlled by a resistor-capacitor network or an input voltage
(VCO). I would like to control the output frequency of the circuit
with a square wave input so that the frequency of the various output
waveforms is slewed to the frequency of the input square wave. I
suppose I could use a frequency to voltage chip to control a VCO type
function generator but this seems overly complicated. I could
integrate the input square wave to get a triangle, but I want to be
able to vary the duty cycle/skew with a pot (10K - 100K) so I could
morph the triangle wave into a sawtooth. This all has to happen at
audio frequencies. Anybody have any ideas?
 
T

Tim Williams

Jan 1, 1970
0
I'm trying to figure out a way to create a simple function generator
circuit that produces square and triangle waveforms. These could be
skewed with a pot to produce a variable duty cycle at the square wave
output, and sawtooth waveforms at the triangle output.

Simple enough, I could even do it with discrete transistors. The triangle
would be a little more complicated but nothing too hard.
There are some excellent chips that can do this

...Or you can also use a chip, which works basically the same way.
but the output frequencies are
usually controlled by a resistor-capacitor network or an input voltage
(VCO). I would like to control the output frequency of the circuit
with a square wave input so that the frequency of the various output
waveforms is slewed to the frequency of the input square wave.

It would be nicer if you had the instantaneous CV output for the squarewave
generator, or any sort of prior signal source before just a generic
squarewave. Otherwise you have to frequency > voltage convert it, then a
square/triangle VCO or something.
I
suppose I could use a frequency to voltage chip to control a VCO type
function generator but this seems overly complicated. I could
integrate the input square wave to get a triangle, but I want to be
able to vary the duty cycle/skew with a pot (10K - 100K) so I could
morph the triangle wave into a sawtooth. This all has to happen at
audio frequencies. Anybody have any ideas?

Is the output clocked to the input square wave, or what is the function of
input to output frequency?

Tim
 
I

Ignoramus1740

Jan 1, 1970
0
I'm trying to figure out a way to create a simple function generator
circuit that produces square and triangle waveforms. These could be
skewed with a pot to produce a variable duty cycle at the square wave
output, and sawtooth waveforms at the triangle output. There are some
excellent chips that can do this, but the output frequencies are
usually controlled by a resistor-capacitor network or an input voltage
(VCO). I would like to control the output frequency of the circuit
with a square wave input so that the frequency of the various output
waveforms is slewed to the frequency of the input square wave. I
suppose I could use a frequency to voltage chip to control a VCO type
function generator but this seems overly complicated. I could
integrate the input square wave to get a triangle, but I want to be
able to vary the duty cycle/skew with a pot (10K - 100K) so I could
morph the triangle wave into a sawtooth. This all has to happen at
audio frequencies. Anybody have any ideas?

May I ask a stupid question, why do you want to do it. Is that like a
school assignment, that is immutable? If so, then I have no
suggestions, but if not, then maybe there are other approaches.

i
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that [email protected] wrote (in
I'm trying to figure out a way to create a simple function generator
circuit that produces square and triangle waveforms. These could be
skewed with a pot to produce a variable duty cycle at the square wave
output, and sawtooth waveforms at the triangle output. There are some
excellent chips that can do this, but the output frequencies are
usually controlled by a resistor-capacitor network or an input voltage
(VCO). I would like to control the output frequency of the circuit
with a square wave input so that the frequency of the various output
waveforms is slewed to the frequency of the input square wave. I
suppose I could use a frequency to voltage chip to control a VCO type
function generator but this seems overly complicated. I could
integrate the input square wave to get a triangle, but I want to be
able to vary the duty cycle/skew with a pot (10K - 100K) so I could
morph the triangle wave into a sawtooth. This all has to happen at
audio frequencies. Anybody have any ideas?

The way to do a sawtooth with variable tooth slopes is to charge a
capacitor from one constant current source and then discharge it through
another current source. Switch sources with your square wave and two
CMOS analogue switches. Adjust the currents to change the slopes.
 
Tim said:
Simple enough, I could even do it with discrete transistors. The triangle
would be a little more complicated but nothing too hard.


..Or you can also use a chip, which works basically the same way.


It would be nicer if you had the instantaneous CV output for the squarewave
generator, or any sort of prior signal source before just a generic
squarewave. Otherwise you have to frequency > voltage convert it, then a
square/triangle VCO or something.


Is the output clocked to the input square wave, or what is the function of
input to output frequency?

Tim
 
This is a music circuit. The square waves are coming out of a top
octave divider which keeps all the notes in relative tune. I want to
turn these into triangle/sawtooth and variable duty cycle pulses with
the slope/duty cycle being controlled by a pot. The input and output
frequencies have to be identical. I've been looking at the MAX038
function generator which has a phase detector input. I think it can be
configured as a PLL that will synch the chip's oscillator to an input
square wave from the top octave divider. The chip can produce all the
waveforms I want with duty cycle adjustment I need. I don't know much
about PLLs and I wonder if the chip will be able to capture and lock on
to the wide range of frequencies I'm going to send to it. Maybe not.
 
M

Mac

Jan 1, 1970
0
I'm trying to figure out a way to create a simple function generator
circuit that produces square and triangle waveforms. These could be
skewed with a pot to produce a variable duty cycle at the square wave
output, and sawtooth waveforms at the triangle output. There are some
excellent chips that can do this, but the output frequencies are
usually controlled by a resistor-capacitor network or an input voltage
(VCO). I would like to control the output frequency of the circuit
with a square wave input so that the frequency of the various output
waveforms is slewed to the frequency of the input square wave. I
suppose I could use a frequency to voltage chip to control a VCO type
function generator but this seems overly complicated. I could
integrate the input square wave to get a triangle, but I want to be
able to vary the duty cycle/skew with a pot (10K - 100K) so I could
morph the triangle wave into a sawtooth. This all has to happen at
audio frequencies. Anybody have any ideas?

I'm thinking of a fast clock, some kind of programmable logic, and one or
more DAC's (with suitable low-pass filters). I guess you would also need
an ADC to convert the pot control voltages to digital form.

Maybe that is too expensive or too complicated for your application?

Just a thought.

--Mac
 
F

Fred Bloggs

Jan 1, 1970
0
This is a music circuit. The square waves are coming out of a top
octave divider which keeps all the notes in relative tune. I want to
turn these into triangle/sawtooth and variable duty cycle pulses with
the slope/duty cycle being controlled by a pot. The input and output
frequencies have to be identical. I've been looking at the MAX038
function generator which has a phase detector input. I think it can be
configured as a PLL that will synch the chip's oscillator to an input
square wave from the top octave divider. The chip can produce all the
waveforms I want with duty cycle adjustment I need. I don't know much
about PLLs and I wonder if the chip will be able to capture and lock on
to the wide range of frequencies I'm going to send to it. Maybe not.

We will be wondering too if you don't say what the "wide range of
frequencies" is exactly. Following the app note, it can be seen that the
PDO->Fadj PLL allows a lock range of +/-70% about center frequency, or
6:1 range with no other adjustments. The 038 is willing to accept assist
from external components so that nearly anything can be done with it,
the coarse frequency adjust range is 350:1.
 
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