Half Wave Symmetrical Sawtooth Waveform

Thread Starter

Carlos Lengua

Joined Jun 30, 2018
4
I am new to this forum and I hope the question isn't too broad but I am seeking a circuit or even a device/instrument that can generate a half wave symmetrical sawtooth waveform. I do have a basic idea of how I might try to generate such a waveform but I'm not sure if it would work.

Charles
 

AnalogKid

Joined Aug 1, 2013
8,484
Please post a sketch of the waveform you want to achieve. Based on what I think a normal sawtooth waveform is, "half wave symmetrical sawtooth waveform" is a contradiction. Symmetrical about what?

ak
 

MrChips

Joined Oct 2, 2009
21,275
Half wave
symmetrical
sawtooth

Those terms conflict. We know what a sawtooth waveform looks like.



Edit: ak beats me to it.
 

danadak

Joined Mar 10, 2018
4,057
An analog approach, add a mux to convert triangle to saw using square output.

http://www.learningaboutelectronics.com/Articles/Function-generator-circuit.php


Use a comparator based circuit -



Then a digital approach using wavetable and DAC

http://www.cypress.com/documentation/application-notes/an69133-psoc-3-psoc-5lp-easy-waveform-generation-wavedac8-component

http://www.cypress.com/file/122721/download

With this approach you can control freq and amplitude, and using a small amount of other
onchip resources burst N saw waves with controlled delay between them. Very versatile.
Or trigger it synchronously with external input or generate simultaneously a tri or sine......

This project shows a switchable sesign. sine or saw. two wavedacs could have been used to gen
them simultaneously. See attached.

What freq and amplitude do you need ?



Regards, Dana.
 

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Thread Starter

Carlos Lengua

Joined Jun 30, 2018
4
Please post a sketch of the waveform you want to achieve. Based on what I think a normal sawtooth waveform is, "half wave symmetrical sawtooth waveform" is a contradiction. Symmetrical about what?

ak
I had in mind something pretty much what's posted in this video:

 

danadak

Joined Mar 10, 2018
4,057
Here is the circuit to implement sign change, feed it with
a saw and it creates the waveform crutschow posted. Note
gate of switch feed by saw generator square waveform if
available, otherwise use a comparator.

upload_2018-7-4_9-29-17.png

Choice of MOSFET mainly low RDSon, << 1K ohm

Regards, Dana.
 

Kjeldgaard

Joined Apr 7, 2016
398
I have an idea for the classic 2 operational amplifier Integrator / Schmitt Trigger circuit:
TriSquWav_1.jpg
At the point I have called OUT, there will be a signal that reminds of the TS wishes.

But these are just thoughts, and there is a lot of detail with regard to supply, common point / GND and buffer on OUT.
 

crutschow

Joined Mar 14, 2008
25,095
Here's the LTspice simulation of a circuit to generate the desired waveform.

It uses LM339/339 comparators to generate a unipolar sawtooth, with current-mirror Q2/Q3 providing a constant-current to give a linear sawtooth ramp at C1, adjustable by pot U2 to change the frequency.

The quad LM6484 opamps provide signal buffering, inversion and adjustable gain.

Flip-flop U2 changes state at the end of each sawtooth retrace pulse, which controls switch U7.
This reverses the phase/polarity of U5's output (the circuit posted by danadak) on alternate sawtooth periods to generate the bipolar symmetrical sawtooth.

Pot u9 adjusts the output between 0V and ±5V (maximum with 5V supplies).

The simulation is shown for the two extreme settings of pot U2 which gives frequencies from about 700Hz to 7kHz.

(Note that the circuit in post #10 generates a triangle wave, not a sawtooth.)

upload_2018-7-4_11-59-19.png
 

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Last edited:

MrAl

Joined Jun 17, 2014
7,656
Hello,

It seems to me it would easier to just create a normal sawtooth generator, then invert the output with anther op amp stage, then use an analog switch to 'choose' which output gets linked to the final output using a flip flop triggered by the original sawtooth generator.
I think you can do a normal sawtooth gen with two op amp sections, then one more for the inverted output, then one flip flop and one analog SPDT switch.
So that could happen in as little as one quad op amp package, one flip flop package, and one analog switch package, and a few small components like resistors and caps.
 

MrAl

Joined Jun 17, 2014
7,656
I liked to see that circuit.
I know you can do a triangle-wave with two op amps, but a sawtooth is a little more difficult.
Hello,

I have seen a design using zeners but did not analyze it yet.

But couldnt you just create a standard triangle generator and use diodes to control the duty cycle separately for the rise and fall times? A sawtooth is just a triangle with faster fall time than rise time, or vice versa.
 

crutschow

Joined Mar 14, 2008
25,095
But couldnt you just create a standard triangle generator and use diodes to control the duty cycle separately for the rise and fall times? A sawtooth is just a triangle with faster fall time than rise time, or vice versa.
In theory yes, but typically you want the sawtooth fall time to be very short, and an op amp maximum output current is not usually sufficient for that, so a separate, high-current switch is used to generate the fall-time (rapidly discharge the capacitor).
In the circuit I posted, the LM339 output transistor switch is used for that purpose.
 

crutschow

Joined Mar 14, 2008
25,095
Okay.
MrAl jogged my thinking and while I was doing that, Kjeldgaard came up with a circuit similar to the one I was working on.
Basically it's just adding a period square-wave offset to the triangle-wave.
Requires just one quad op amp.
Here's the LTspice simulation of the result.

upload_2018-7-5_0-14-29.png
 

MrAl

Joined Jun 17, 2014
7,656
In theory yes, but typically you want the sawtooth fall time to be very short, and an op amp maximum output current is not usually sufficient for that, so a separate, high-current switch is used to generate the fall-time (rapidly discharge the capacitor).
In the circuit I posted, the LM339 output transistor switch is used for that purpose.
Hello,

Depends on the requirements of the application.
Requires one dual section op amp. Maybe LM358 which is a 8 pin package.
If you want the inversion then another op amp section is needed.
 
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