# converting square waves to sine waves?

J

#### Joe

Jan 1, 1970
0
I have a home built square wave generator, and I would like to be able to
build a circuit that will convert a square wave to a sine wave of
approximately the same frequency. I have googled for square to sine wave
generator schematics, and found a lot of circuits that do the opposite (ie
convert sine to square waves), and I have looked at several wein bridge type
sine wave generators, but would like to use the square wave generator I
already have to generate a sine wave (of about 25Khz). If I need to, I can
build a wein bridge type sine wave oscillator for the frequency I need but I
am hoping someone out there knows of a fairly simple circuit that will take
a square wave and produce a sine wave from it. That way, I could just feed
the signal from the square wave generator into the 'converter' and get a
sine wave out.

Any suggestions appreciated.

TIA,
Joe

J

#### John Larkin

Jan 1, 1970
0
I have a home built square wave generator, and I would like to be able to
build a circuit that will convert a square wave to a sine wave of
approximately the same frequency. I have googled for square to sine wave
generator schematics, and found a lot of circuits that do the opposite (ie
convert sine to square waves), and I have looked at several wein bridge type
sine wave generators, but would like to use the square wave generator I
already have to generate a sine wave (of about 25Khz). If I need to, I can
build a wein bridge type sine wave oscillator for the frequency I need but I
am hoping someone out there knows of a fairly simple circuit that will take
a square wave and produce a sine wave from it. That way, I could just feed
the signal from the square wave generator into the 'converter' and get a
sine wave out.

Any suggestions appreciated.

TIA,
Joe

Just a lowpass filter will work, but the filter cutoff frequency will
have to change if the square wave frequency changes. A square wave
includes the fundamental frequency (the one you want) and odd
harmonics, so you need a filter that passes 25 KHz and attenuates 75K
and up. A simple R-C is the simplest lowpass filter, and more complex
filters (L-C or active) will zap those high harmonics better.

A tuned LC (a bandpass filter) will also work well at a single
frequency.

John

R

#### Robert C Monsen

Jan 1, 1970
0
Joe said:
I have a home built square wave generator, and I would like to be able to
build a circuit that will convert a square wave to a sine wave of
approximately the same frequency. I have googled for square to sine wave
generator schematics, and found a lot of circuits that do the opposite (ie
convert sine to square waves), and I have looked at several wein bridge type
sine wave generators, but would like to use the square wave generator I
already have to generate a sine wave (of about 25Khz). If I need to, I can
build a wein bridge type sine wave oscillator for the frequency I need but I
am hoping someone out there knows of a fairly simple circuit that will take
a square wave and produce a sine wave from it. That way, I could just feed
the signal from the square wave generator into the 'converter' and get a
sine wave out.

Any suggestions appreciated.

TIA,
Joe

For fun, I used a freeware filter builder to design an active filter for
you:

http://home.comcast.net/~rcmonsen/misc/filter.zip

There are jpg files of screen shots of simulations built with CircuitMaker,
and a gif file of the output of the filter program.

The filter designer is at http://www.nuhertz.com/

It won't work for anything except 25kHz. It'll give you great results at
that frequency. The component tolerances are pretty tight, I think.

Regards,
Bob Monsen

B

#### Bob Masta

Jan 1, 1970
0
I have a home built square wave generator, and I would like to be able to
build a circuit that will convert a square wave to a sine wave of
approximately the same frequency. I have googled for square to sine wave
generator schematics, and found a lot of circuits that do the opposite (ie
convert sine to square waves), and I have looked at several wein bridge type
sine wave generators, but would like to use the square wave generator I
already have to generate a sine wave (of about 25Khz). If I need to, I can
build a wein bridge type sine wave oscillator for the frequency I need but I
am hoping someone out there knows of a fairly simple circuit that will take
a square wave and produce a sine wave from it. That way, I could just feed
the signal from the square wave generator into the 'converter' and get a
sine wave out.

Any suggestions appreciated.

Besides the filtering approaches already mentioned,
another approach for fixed-frequency work is to
integrate the square wave to a triangle, then pass
that through a nonlinear shaping stage. Typical
shapers are made from overdriven differential
amplifiers. It is fairly easy to get down to 1% THD
this way, even lower if you want to get extreme.

Similar sine shapers can be made from overdriven
JFETs, or even back-to-back diodes if you are

Note that this approach has problems if the triangle amplitude
isn't constant, which means that the frequency to the
integrator has to be constant.

Most "function generator" circuits actually generate
a constant-amplitude triangle along with a square,
because they are integrating an adjustable current
up to a threshold, then reversing the polarity to
the integrator. So they have a nice triangle to feed
to the sine shaper.

Hope this helps!

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

J

#### Joe

Jan 1, 1970
0
John Larkin said:
Just a lowpass filter will work, but the filter cutoff frequency will
have to change if the square wave frequency changes. A square wave
includes the fundamental frequency (the one you want) and odd
harmonics, so you need a filter that passes 25 KHz and attenuates 75K
and up. A simple R-C is the simplest lowpass filter, and more complex
filters (L-C or active) will zap those high harmonics better.

A tuned LC (a bandpass filter) will also work well at a single
frequency.

John

Thanks John, I didn't think of a filter. I will have to breadboard one and
play around and see how it works.

Joe

J

#### Joe

Jan 1, 1970
0
Robert C Monsen said:
but

For fun, I used a freeware filter builder to design an active filter for
you:

http://home.comcast.net/~rcmonsen/misc/filter.zip

There are jpg files of screen shots of simulations built with CircuitMaker,
and a gif file of the output of the filter program.

The filter designer is at http://www.nuhertz.com/

It won't work for anything except 25kHz. It'll give you great results at
that frequency. The component tolerances are pretty tight, I think.

Regards,
Bob Monsen

Hi Bob,

Thanks for the file and the link!! The zip file contained 2 filters. One was
a sallen key filter which I will have to do some more research on because I
am not familiar with it. That one was not included in the choices for the
freeware filter designer at nuhertz. I downloaded it and have been playing
with it. I haven't had a chance to breadboard anything yet cause I am having
so much fun with this program. The tolerances are tight on your design for
just 1Khz either side of 25Khz. I can probly live with 5Khz either side of
it, but I am going to breadboard a few different ones including yours and
look at it on my scope to verify.
Thanks again !!

Joe

J

#### Joe

Jan 1, 1970
0
Bob Masta said:
Besides the filtering approaches already mentioned,
another approach for fixed-frequency work is to
integrate the square wave to a triangle, then pass
that through a nonlinear shaping stage. Typical
shapers are made from overdriven differential
amplifiers. It is fairly easy to get down to 1% THD
this way, even lower if you want to get extreme.

Similar sine shapers can be made from overdriven
JFETs, or even back-to-back diodes if you are

Note that this approach has problems if the triangle amplitude
isn't constant, which means that the frequency to the
integrator has to be constant.

Most "function generator" circuits actually generate
a constant-amplitude triangle along with a square,
because they are integrating an adjustable current
up to a threshold, then reversing the polarity to
the integrator. So they have a nice triangle to feed
to the sine shaper.

Hope this helps!

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

Hi Bob,

Thanks for the suggestions. I also got some other ideas. I like the filters
because that is what I am going to be experimenting with next as I learn
more about electronics. I will probly tackle a function generator in the
near future because I received some good suggestions for one when I asked
the group for ideas on building my signal generator (square wave only).

Joe

B

#### Bill Vajk

Jan 1, 1970
0
Joe said:
but I

Thanks John, I didn't think of a filter. I will have to breadboard one and
play around and see how it works.

Joe

Also consider a transformer. Common hysteresis taking place in
the core is probably the reason you couldn't find electronic
circuits to do te job.

J

#### John Larkin

Jan 1, 1970
0
Hi Bob,

Thanks for the suggestions. I also got some other ideas. I like the filters
because that is what I am going to be experimenting with next as I learn
more about electronics. I will probly tackle a function generator in the
near future because I received some good suggestions for one when I asked
the group for ideas on building my signal generator (square wave only).

Joe

Get Don Lancaster's excellent paperback "Active Filter Cookbook". It
cuts right to the chase.

John

W

#### Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Robert C Monsen said:
but

For fun, I used a freeware filter builder to design an active filter for
you:

http://home.comcast.net/~rcmonsen/misc/filter.zip

There are jpg files of screen shots of simulations built with CircuitMaker,
and a gif file of the output of the filter program.

The filter designer is at http://www.nuhertz.com/

It won't work for anything except 25kHz. It'll give you great results at
that frequency. The component tolerances are pretty tight, I think.

That's the problem: it's a whole lot easier to convert sine waves to square
waves thant the other way around.

The nice thing about a Wien Bridge (not Wein Bridge) Oscillator is that it
produces an excellent low distortion sine wave with just two variable
resistors, and two capacitors that can be switched to give the different
ranges. After the Wien Bridge, you can use a Schmitt Trigger to give nice
square waves. But since you already have the sq wave generator, that's not
needed.

I would just build a Wien Bridge oscillator to make the sine waves. The
filter limits you to just one frequency.

R

#### R. Steve Walz

Jan 1, 1970
0
Joe said:
I have a home built square wave generator, and I would like to be able to
build a circuit that will convert a square wave to a sine wave of
approximately the same frequency. I have googled for square to sine wave
generator schematics, and found a lot of circuits that do the opposite (ie
convert sine to square waves), and I have looked at several wein bridge type
sine wave generators, but would like to use the square wave generator I
already have to generate a sine wave (of about 25Khz). If I need to, I can
build a wein bridge type sine wave oscillator for the frequency I need but I
am hoping someone out there knows of a fairly simple circuit that will take
a square wave and produce a sine wave from it. That way, I could just feed
the signal from the square wave generator into the 'converter' and get a
sine wave out.

Any suggestions appreciated.

TIA,
Joe
---------------
You need a primer on diode/trasnsistor wave-shaping, and a good
example. Go look at how the ICL8038 IC does this in its inner
schematic, found in its PDF databook excerpt.

-Steve

J

#### Joe

Jan 1, 1970
0
Thanks to all who replied. I have lots of good ideas to try now.

Joe

J

#### Joe

Jan 1, 1970
0
R. Steve Walz said:
---------------
You need a primer on diode/trasnsistor wave-shaping, and a good
example. Go look at how the ICL8038 IC does this in its inner
schematic, found in its PDF databook excerpt.

-Steve

Hi Steve,

I googled for it and could only find an NTE part number, when I looked up
the data sheet, I see that it is a precision waveform generator for
frequencies up to 200Khz. But no transistor level schematic for it, if
that's what you meant. Do you have a link for the data sheet you referenced
above?

Joe

M

#### Michael A. Terrell

Jan 1, 1970
0
Joe said:
Hi Steve,

I googled for it and could only find an NTE part number, when I looked up
the data sheet, I see that it is a precision waveform generator for
frequencies up to 200Khz. But no transistor level schematic for it, if
that's what you meant. Do you have a link for the data sheet you referenced
above?

Joe

The ICL8038 was made by Intersil, then by GE, then Harris, and now
its a Intersil part again. Check the Intersil site to see if they still
have a data sheet. http://www.intersil.com/cda/home/

Maxim makes an updated version, the MAX038. http://www.maximic.com/

B

#### Balaji

Jan 1, 1970
0
Joe said:
I have a home built square wave generator, and I would like to be able to
build a circuit that will convert a square wave to a sine wave of
approximately the same frequency. I have googled for square to sine wave
generator schematics, and found a lot of circuits that do the opposite (ie
convert sine to square waves), and I have looked at several wein bridge type
sine wave generators, but would like to use the square wave generator I
already have to generate a sine wave (of about 25Khz). If I need to, I can
build a wein bridge type sine wave oscillator for the frequency I need but I
am hoping someone out there knows of a fairly simple circuit that will take
a square wave and produce a sine wave from it. That way, I could just feed
the signal from the square wave generator into the 'converter' and get a
sine wave out.

Any suggestions appreciated.

TIA,
Joe

Joe,

It appears that you desire, to make a variable frequency converter.
The problem, with using a low pass analog filter, is that it has also
got, what is called phase distortion. That means that different
frequency signals will have a response that would be follow a little
behind the input. That should not be a problem, if for all
frequencies, the shift is the same, but this difference in time will
change with frequnecy. Also if, you note, at higher frequencies, you
will get attenuated output. That means the amplitude of the output
would be lesser. However, its surely a very low cost design for the
kind of a problem.

Unless, you need very stringent requirement of phase follow up, or
output amplitude, you need not use what is called a digital filter.
You could build it using some sort of a DSP (An expensive solution
when compared to the analog one, and if you have low frequencies, you
could probably do with just any normal processor, instead of a
full-fledged high end DSP) You will need lots of mixed signal
components in order to get a proper output though, but there is
nothing better than that if you want the cleanest possible signal. In
fact, the reason, many people prefer using digital filters to analog
ones is that spikes and sudden glitches in the signal can be cleaned
using a very simple adaptive algorithm.

visit the websites of companies like "Texas Instruments", "Analog
Devices" etc.

But for your problem, it appears that a normal analog filter would
suit the best if you don't really care about phase distortion and the
other bullshit. Try it out. Its really good. All the best.

Balaji

B

#### Bob Masta

Jan 1, 1970
0
As I recall, the 8038 used a whole forest of breakpoints. They
didn't get any better results than the overdriven differential
pair, which only requires a couple of adjustments. Personally,
I'd reserve the whole breakpoint approach for functions that
are really hard to approximate by continuous means.

Just my US\$0.02 worth.

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

B

#### Baphomet

Jan 1, 1970
0
I think diode wave shaping is definately the way to go because it's cheaper,
smaller, and relatively frequency independant. You could do it with only 4
diodes and a handful of resistors if say 5% THD could be tolerated (more
diodes and resistors if a lower distortion level was required . You would
first have to convert the square wave to a triangle wave and then use that
to drive a non linear network. This is how it's generally been done in
function generators since the late 60's.

http://www.ozitronics.com/docs/k23.pdf

J

#### John Fields

Jan 1, 1970
0
I have a home built square wave generator, and I would like to be able to
build a circuit that will convert a square wave to a sine wave of
approximately the same frequency. I have googled for square to sine wave
generator schematics, and found a lot of circuits that do the opposite (ie
convert sine to square waves), and I have looked at several wein bridge type
sine wave generators, but would like to use the square wave generator I
already have to generate a sine wave (of about 25Khz). If I need to, I can
build a wein bridge type sine wave oscillator for the frequency I need but I
am hoping someone out there knows of a fairly simple circuit that will take
a square wave and produce a sine wave from it. That way, I could just feed
the signal from the square wave generator into the 'converter' and get a
sine wave out.

---
Kinda late, but I'd use a PLL with the VCO running at 32 * 25kHz driving
a 5 bit up/down counter with the counter's MSB used to toggle the
counter's UP-DOWN input and as the input to the phase comparator. The
25kHz square wave would go into the other input of the phase comparator
and the counter's numerical outputs would go to a lookup ROM with a sine
table burned into it, and the ROM's outputs would go to an 8 bit DAC
with a lowpass on its output. Voila! 25kHz square wave in, 25kHz sine
wave out. Plus, change the 25kHz input and the output will follow it.
For a while, anyway...

J

#### Joe

Jan 1, 1970
0
Baphomet said:
bridge

I think diode wave shaping is definately the way to go because it's cheaper,
smaller, and relatively frequency independant. You could do it with only 4
diodes and a handful of resistors if say 5% THD could be tolerated (more
diodes and resistors if a lower distortion level was required . You would
first have to convert the square wave to a triangle wave and then use that
to drive a non linear network. This is how it's generally been done in
function generators since the late 60's.

http://www.ozitronics.com/docs/k23.pdf

Thanks Baph, but every time I go to that link, my system seems to lock up. I
have to use ctrl alt del to get out of it. Cannot save the document or even
scroll it. Is it me?

Everything else on my computer seems to be working ok.
I am running windows xp.

Joe

R

#### Robert C Monsen

Jan 1, 1970
0
Joe said:
generator get

Thanks Baph, but every time I go to that link, my system seems to lock up. I
have to use ctrl alt del to get out of it. Cannot save the document or even
scroll it. Is it me?

Everything else on my computer seems to be working ok.
I am running windows xp.

Joe

Works for me. I'll mail it to you.

Regards,
Bob Monsen