Electronics Forum / Basics / August 2006

Hi there,
there are complete function generators in one chip, maxim do one,
they are certainly easy to get going, and produce reasonably good waveforms,
with adjustable this that and the other,
they arnt as cheap as chips though, but work fine upto 1mhz and more.

you can convert a triangle wave to a sinewave with a fairly simple slope
converter,
this is how the function generators produce sinewaves.
2 stages of long tailed pairs will acehive a very good triangle to sine
aproximation.

DDS is a more expensive aproach but very versatile and produces excelent
sinewaves at moderate frequencies and still good sinewaves at quite high
frequencies.

another posibility you might want to look at is to use a micro,
using a PWM output and low pass filter you can program any desired waveform.
quite cheap and simple except maybe for the programming, but I bet someone
has done it before and theres probably some software around.

you might want to use an LC oscillator to produce good sinewaves if you want
to go a lot higher in frequency.

Colin =^.^=

Hi, Mark.  You could put together a limited range function generator
with a few op amps.  The problems come when you want to get a very wide
range (you're talking at least 7 orders of magnitude in your post), and
also are looking for constant amplitude.

The "single IC" solutions are both good.  The less expensive one is
based on the XR2206 IC, but will only give you output to 100KHz.  The
MAX038 one gives you a much wider frequency range, and also gives you
other bells and whistles you'll find you do need.

The issue, of course, is the expense.  If you can spring for $88.44
USD, you can get a complete kit from DIY Electronics based on the
MAX038 and the David Jones Electronics Australia design that will give
you 0.1Hz to 20MHz sine, square wave and triangle wave outputs, with
TTL output, a dedicated buffer, enclosure, connectors, switches and
other stuff.  I helped a friend get one working several years ago, and
I was impressed by the kit.    AFAIK, the only other thing you'll have
to purchase for a complete working function generator is a 9 VAC wall
wart.

I'd try the kit.  As you gain more competence in basic electronics, it
would be a shame to have to ditch your project because it only did half
the job.

Good luck
Chris

The Exar  XR2206  meets all requirements except Fmax=200kHz

Eduardo.

Some are. The Exar XR2206 has a good reputation. While it will cover a
large frequency range, I wonder whether you have thought that through.
For different frequency ranges, different circuit techniques and
standard signal interface values mean that your signal source should
have different output voltage ranges and different connectors. You may
be able to keep the output source impedance at 50 ohms for all ranges.

Full marks for getting that verb right, minus one for not starting with
'I'. (;-)

You can buy on line, although I wouldn't want to persuade you to forgo
healthy exercise.

On 8/19/06 10:37 AM, in article 44e73eac$0$2552$fa0fcedb@news.zen.co.uk,

The 8038 will do everything you want, with few external parts.  Here's a
link to the Intersil datasheet:

http://www.intersil.com/data/FN/FN2864.pdf

They're relatively easy to build from bit parts. 99% of all the commercial
"function generators" (google) are based on generating a triangle and square
and then passing the triangle to  a couple of diodes or transistors where
the diode action 'bends' or distorts the triangle more and more as the
triangle rises and falls. Output looks sinelike and distortion down to 1% is
quite easy.
Simplest way forward is to look at the circuitry for (say) a B+K 3010 0-1MHz
function generator.   http://bama.sbc.edu/ . This is a super classic design
in it's simplicity and comes with an extremely  well written handbook. If
you're handy with opamps, transistors etc then  the sine shaper section
could be straightforward to play with on it's own.
(The output power amp is simple and could also be regenerated as a separate
design.)
For much higher performance, check out how the Wavetek designs do it. Same
technical basis but faster)
john

 There was an IC that did all this, although I don't know about how high
in frequency upper limit but definitely ultrasonic.  I hope I remember the
part number correctly - XR2206.

 It has a built-in triangle-to-sine converter, which gets fine tuned by
varying an external resistor.

 I suspect this IC is discontinued.

 How I suspect they converted triangle to sine:  Use a diode clipper.  
Have an adjustable triangle voltage, driving a variable resistor in series
with a back-t0-back parallel pair of diodes.  I think 1N4007's should do
well.  You could try 1N4148/1N914 to make the clipping harsher or
1 amp 30V Schottky diodes to make the clipping gentler as appropriate,
depending on what results you actually get - but first try various
combinations of both adjustments.

 If you need a symmetry adjustment, then how I figure to do this is
to before the diode clipper to split the signal path into two and feed one
path through a positive precision halfwave rectifier and the other through
a negative precision halfwave rectifier and have a gain adjustment in one
of these paths and combine the outputs of the precision halfwave
rectifiers.
 An example of a precision halfwave rectifier is an op-amp circuit where
the output drives three diodes, with #1 and #2 diodes being a "matched
pair", should be good enough to use two 1N4148 units from the same lot.

#1 - feeds the halfwave rectified output of the "precision halfwave
rectifier".

#2 - feeds the inverting input of the op-amp.  The inverting input should
have a resistor to ground of value as close as possible to the impedance
of the load that the first diode is feeding.  If you have a need to get an
extra adjustment to tweak, then make this resistor a rheostat with maybe a
2.2K or 1K or so in series with it.

 Diodes #1 and #2 have for positive halfwave rectification their anodes
driven by the op-amp output.  For negative halfwave rectification the
op-amp output drives their cathodes.

#3 - in parallel with #2 in opposite direction of #2, to limit opposite
swings of output voltage of the op-amp.  This often makes the op-amp
behave better, especially at higher frequencies.

===========================================

 Alternative:  Learn how to make a microprocessor, such as a Microchip
"PIC" one, in combination with a DAC, to do these.  You will probably need
to add some filtering.

- Don Klipstein (don@misty.com)

Perhaps a little more complicated than absolutely necessary.  But it gets
the job done, as such.
http://webpages.charter.net/dawill/Images/Function%20Generator.gif
A master current mirror sets current (Freq resistor), then another sets
discharge current.  Pull-down current is double, so the pull-up subtracts,
causing equal magnitude charge and discharge currents.

Such a geometrical relation is obviously useless for duty cycle control, so
instead you might could use something like this:
http://webpages.charter.net/dawill/Images/FET_Splitter.gif
But erm... replace the FET with another resistor, and pull the voltage of
the middle node to change relative up/down current.  Then cascode I_sink to
toggle it on/off.

Ok so back to...
http://webpages.charter.net/dawill/Images/Function%20Generator.gif
FET follower, little discrete comparator, you could use an LM311 or half a
393 instead.  If you go discrete, I'm not sure the phase is right on that..
you may have to swap which differential base (or collector) goes where.

Anyways, labeled output is square wave.  How in the hell does this help you,
you ask?  Pick up the signal off the FET source- triangle wave.  Then run
this through a beefier follower, and hit a diode/resistor network to trim it
into a fake sine wave.  Bingo, three waveforms, adjustable duty cycle.  ;-)

Or you could grab an XR2206 or whatever the hell other chips there are that
don't come to mind.

Tim

<snip>

Not what you asked for, but you might want to take a look at
my freeware DaqGen signal generator for Windows.  It uses
any Windows sound card and offers many waveforms and
modulation options, with very precise and repeatable control.

The downside is that sound cards are limited to (typically)
20 kHz on the top end and a few Hz on the bottom.  (DaqGen
goes down to sub-milliHertz, but you won't see it on the outputs,
only in modulations.)

Best regards,

Bob Masta
dqatechATdaqartaDOTcom

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