Variable Square Wave to Sine Wave

FyberOptic
Guest

Mon Mar 08, 2010 5:29 pm

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

Thanks!

- Jeff

Rich Webb
Guest

Mon Mar 08, 2010 5:29 pm

On Mon, 8 Mar 2010 07:29:40 -0800 (PST), FyberOptic
<fyberoptic_at_gmail.com> wrote:

Quote:

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

You could use a DDS chip. These guys make quite a few:
<http://www.analog.com/en/rfif-components/direct-digital-synthesis-dds/products/index.html>
Or, a lower frequency DDS -- something that can be built with a
microcontroller. Analog has a note on the fundamentals of DDS
http://www.analog.com/static/imported-files/tutorials/MT-085.pdf
(no pun intended) or do a net search on "DDS phase accumulator" or
similar. Pair the lower freq DDS to a phase-locked loop (PLL) and you're
there (modulo tuning the loop and filtering the assorted artifacts).

Here's http://www.myplace.nu/avr/minidds/index.htm an example of using a
microcontroller to make a DDS. I've built this one (back when the
AT90S2313 was new Wink

and it does work as advertised.

--
Rich Webb Norfolk, VA

Jim Thompson
Guest

Mon Mar 08, 2010 6:21 pm

On Mon, 8 Mar 2010 07:29:40 -0800 (PST), FyberOptic
<fyberoptic_at_gmail.com> wrote:

Quote:

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

Thanks!

- Jeff

Jim Thompson
Guest

Mon Mar 08, 2010 7:35 pm

On Mon, 08 Mar 2010 10:21:41 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote:

Quote:

On Mon, 8 Mar 2010 07:29:40 -0800 (PST), FyberOptic
fyberoptic_at_gmail.com> wrote:

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

Thanks!

- Jeff

Back up a wee bit: How is the square wave being generated? Perhaps
from a triangle-wave oscillator?

...Jim Thompson

Jon Slaughter
Guest

Mon Mar 08, 2010 7:50 pm

FyberOptic wrote:

Quote:

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

Thanks!

A square wave is made up of a sum of sinusoids in a special relation. In
fact all periodic functions can be thought of as a simple sum of sinusoids.
The only thing different is the relative amplitudes. A square wave has only
odd harmonics(integral multiples of the fundamental frequency) while a saw
tooth has every harmonic.

The low pass filter reduces the amplitudes of the higher harmonics which
essentially emphasizes the lower harmonics. Note that if you have a
fundamental frequency of f then the next harmonic is at 2f, the 3rd at 3f,
etc... So an ideal low pass filter inbetween f and 2f of *any* periodic
signal will return a sinusoid of frequency f. Between 2f and 3f one gets
the sum of first two harmonics.

If you place a notch filter on any of the harmonics with a high Q then you
can "pick" out that frequency if one exists and it will be
sinusoidal(ideally). So say you have a square wave at 10Mhz. This mean's it
has sinusoids at 10Mhz, 20Mhz, 30Mhz, 40Mhz, etc... If you want to get some
sinusoid at 1000Mhz then you simply create a notch filter at that position.
Unfortunately the harmonics amplitude decause as 1/k which means that
amplitude of that 1000Mhz sinusoid will be 1/100th of the original.

If we used a 1Mhz square wave then it will be not only harder to pick out
the 100th harmonic because the amplitude is 1/1000th of the original but
also because now the harmonics are only 1Mhz apart which means our notch
filter has to have a much higher Q. (and this analysis is not considering
external noise involved)

Your first question is, at least for a rough method, is to simply LP any
signal. Eventually you'll arive at the fundamental which, ideally, will be a
perfect sinusoid. The second question's answer is to notch filter a
"spectrally rich signal. But as I have explained, this gets harder and
harder to do. You won't be able to take a 1V 1Hz square wave and recover a
1Ghz sinusoid as the amplitude is 1pV. Not only that you have other
harmonics right next to it. Of course depending on your application it may
not be all that important. If you used a 100Mhz signal then the amplitude is
only 0.1V and the side harmonics are 100Mhz away.

Tim Wescott
Guest

Mon Mar 08, 2010 8:03 pm

FyberOptic wrote:

Quote:

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

Most modern mixers act as switches driven by the local oscillator
signal. So even if you made a nice sinusoidal signal out of your local
oscillator, it'd be effectively squared up inside the mixer.

So use the chip, and don't worry about the square wave output -- it's
not only not a problem, for some mixers it's desirable.

_Do_ concern yourself with phase noise and frequency stability -- local
oscillators for weak signal work have a lot of requirements imposed on
them that just don't apply to -- or don't matter for -- oscillators for
digital circuits.

Were it me I'd just make sure that the LO is on it's own board, with
connectors to make it reasonably easy to swap out. Then I'd proceed
from there.

Get a copy of the ARRL Handbook -- it addresses a lot of these issues
for you.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

Guest

Mon Mar 08, 2010 9:18 pm

One, google "softrock radio"

Two, with all due respect to my learned colleague, instead the of ARRL
handbook, try

Experimental Methods in Radio Frequency Design, by Campbell, and
Hayward, its more geared to modern, simple, home made gear then the
ARRL handbook is , sadly...

then look at
http://www.kangaus.com/2m_converter.htm

then
http://www.pongrance.com/

the old kit

Steve

Tim Wescott
Guest

Tue Mar 09, 2010 12:53 am

osr_at_uakron.edu wrote:

Quote:

One, google "softrock radio"

Two, with all due respect to my learned colleague, instead the of ARRL
handbook, try

Experimental Methods in Radio Frequency Design, by Campbell, and
Hayward, its more geared to modern, simple, home made gear then the
ARRL handbook is , sadly...

Is it? Sad about the Handbook, but at least there's something out there.

When I'm feeling wordy I recommend the latest Handbook, plus one from
the 50s, 60s, or 70s. If you don't mind toobs, there's a _lot_ of good
stuff in those older book!

I like Hayward's "Radio Frequency Design", but it's a text for a
4th-year college class, and requires that you have strong analog chops
to just pick it up and read.

Quote:

then look at
http://www.kangaus.com/2m_converter.htm

then
http://www.pongrance.com/

the old kit

Steve

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

Bitrex
Guest

Tue Mar 09, 2010 5:47 am

Tim Wescott wrote:

Quote:

osr_at_uakron.edu wrote:
One, google "softrock radio"

Two, with all due respect to my learned colleague, instead the of ARRL
handbook, try

Experimental Methods in Radio Frequency Design, by Campbell, and
Hayward, its more geared to modern, simple, home made gear then the
ARRL handbook is , sadly...

Is it? Sad about the Handbook, but at least there's something out there.

When I'm feeling wordy I recommend the latest Handbook, plus one from
the 50s, 60s, or 70s. If you don't mind toobs, there's a _lot_ of good
stuff in those older book!

I like Hayward's "Radio Frequency Design", but it's a text for a
4th-year college class, and requires that you have strong analog chops
to just pick it up and read.

then look at
http://www.kangaus.com/2m_converter.htm

then
http://www.pongrance.com/

the old kit

Steve

Hayward's "Introduction to Radio Frequency Design" was the first book on
RF design I ever bought a few years back...I bought it because it had
"Introduction" in the title... O_O

oopere
Guest

Tue Mar 09, 2010 8:38 am

FyberOptic wrote:

Quote:

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

Thanks!

- Jeff

To produce "higher frequency sine waves" (RF) you make use of a VCO. You
may design your own (perhaps Colpitts) VCO or you may buy a canned one
frome Minicircuits or similar. To reliably adjust the oscillation
frequency you phase-lock your VCO to a stable crystal reference. This is
achieved by a PLL (Phase-Locked Loop) which is offered by different
vendors. A microcontroller is used to program some registers of the PLL
to achieve the desired frequency. Commercially available PLLs clearly
state the signal level that is required from your (sinusoidal) VCO. So,
the whole thing looks like this:

XTAL--------PLL-->Filter-->VCO-->Power splitter---> synthesized sine
^ ^ |
| | |
| ·-----------------------·
uController-·

Once you become familiar with this you may start looking at spurs,
jitter, etc.

For "lower" frequencies, direct digital synthesis (DDS) may also work.

Pere

Robert Baer
Guest

Tue Mar 09, 2010 10:17 am

FyberOptic wrote:

Quote:

Hiya folks. I've been interested in trying to build myself a radio
receiver for various bands, primarily as a learning experience. The
thing is, though, I've heard that it's kind of hard to build a stable
VFO above 50mhz. One of the bands I'd eventually like to receive is 2
meters (144-148mhz). Might be fun to receive television audio, as
well. Not to mention, standard FM.

Since I'm more familiar with digital electronic components rather than
analog, I've seen that there are programmable frequency oscillator
ICs, which can be set via a microcontroller. This sounds like an easy
solution for tuning, and would open the door to setting station
presets and the like as well. The problem is, I'm fairly sure that
these would output a square wave.

I've read about how you can basically use a low-pass filter to tune
out harmonics of a square wave and get a sine wave output. But from
the way I understand it, an RC filter would just filter out a specific
frequency, defeating the purpose of the variable oscillator.

So that's pretty much my question: How does one turn a variable
square wave into a sine wave?

Or, alternatively, what are more reliable ways to produce higher
frequency sine waves to begin with? Though I'm still interested in
the former question as well, for curiosity's sake.

Thanks!

- Jeff

Plain RC filters have a "rolloff" thus work on a range of frequencies.

A simple low pass filter would have lower output amplitude as the
input frequency goes up.
Now why do you think you need a sine wave?

Nial Stewart
Guest

Tue Mar 09, 2010 2:30 pm

Quote:

Two, with all due respect to my learned colleague, instead the of ARRL
handbook, try

Experimental Methods in Radio Frequency Design, by Campbell, and
Hayward, its more geared to modern, simple, home made gear then the
ARRL handbook is , sadly...

I've just had a look at amazon.co.uk (not available) then amazon.com.

4 Used available from _$498.97_ (up to $937.87).

It would need to be good at that price!

Nial.

Guest

Tue Mar 09, 2010 4:29 pm

Lets see,mine was 29.99 when they switched from the first edition to
the second.

Steve

George Herold
Guest

Tue Mar 09, 2010 5:28 pm

On Mar 9, 8:30 am, "Nial Stewart"
<nial*REMOVE_TH...@nialstewartdevelopments.co.uk> wrote:

Quote:

Two, with all due respect to my learned colleague, instead the of ARRL
handbook, try

Experimental Methods in Radio Frequency Design, by Campbell, and
Hayward, its more geared to modern, simple, home made gear then the
ARRL handbook is , sadly...

I've just had a look at amazon.co.uk (not available) then amazon.com.

4 Used available from _$498.97_ (up to $937.87).

It would need to be good at that price!

Nial.

That's crazy! They had them on Amazon in the US for $49.00 to
$41.00.

George H.

Muzaffer Kal
Guest

Tue Mar 09, 2010 5:55 pm

On Tue, 9 Mar 2010 13:30:21 -0000, "Nial Stewart"
<nial*REMOVE_THIS*@nialstewartdevelopments.co.uk> wrote:

Quote:

Two, with all due respect to my learned colleague, instead the of ARRL
handbook, try

Experimental Methods in Radio Frequency Design, by Campbell, and
Hayward, its more geared to modern, simple, home made gear then the
ARRL handbook is , sadly...

I've just had a look at amazon.co.uk (not available) then amazon.com.

4 Used available from _$498.97_ (up to $937.87).

It would need to be good at that price!

Nial.

It seems to be available at ARRL for $49.99:
http://www.arrl.org/catalog/?category=Circuit+Design#9239
--
Muzaffer Kal

DSPIA INC.
ASIC/FPGA Design Services

http://www.dspia.com