[?] Audio VCO design

[David Chapman]

As part of a larger system, I need to design and build an audio VCO
which will produce tones ranging from around 50Hz to 4KHz or so when the
applied DC input voltage changes from 0.5volts to 2.5volts.
This will be used simply as a tuning aid for visually impaired users
so the actual voltage/frequency relationship, and waveform produced, is
not at all critical. Supply voltage of the VCO can be in the range 3.3v
- 5v DC.

I'm considering using the CMOS 4046 PLL IC, but wonder if anyone in
this NG has any better suggestions to offer. FWIW, price is not the most
important consideration.

Any suggestions will be much appreciated.

TIA - Dave

 

[Holloway,Graham \(UK\)]

As part of a larger system, I need to design and build an audio VCO
which will produce tones ranging from around 50Hz to 4KHz or so when the
applied DC input voltage changes from 0.5volts to 2.5volts.
This will be used simply as a tuning aid for visually impaired users so
the actual voltage/frequency relationship, and waveform produced, is not
at all critical. Supply voltage of the VCO can be in the range 3.3v - 5v
DC.

I'm considering using the CMOS 4046 PLL IC, but wonder if anyone in this
NG has any better suggestions to offer. FWIW, price is not the most
important consideration.

Any suggestions will be much appreciated.

TIA - Dave

David

Don't use the 4046, unles you are prepared to calibrate each one. I found
3:1 variations between parts from different suppliers, i.e. frequency vs.
volts.

regards
Graham H

 

[Rich Grise]

As part of a larger system, I need to design and build an audio VCO
which will produce tones ranging from around 50Hz to 4KHz or so when the
applied DC input voltage changes from 0.5volts to 2.5volts.
This will be used simply as a tuning aid for visually impaired users
so the actual voltage/frequency relationship, and waveform produced, is
not at all critical. Supply voltage of the VCO can be in the range 3.3v -
5v DC.

I'm considering using the CMOS 4046 PLL IC, but wonder if anyone in
this NG has any better suggestions to offer. FWIW, price is not the most
important consideration.

Any suggestions will be much appreciated.

I'll probably get pilloried for this, but it sounds like the app cries
out for a 555 or 7555.

You might have to scale the tuning voltage a bit.

Good Luck!
Rich

 

[Joerg]

As part of a larger system, I need to design and build an audio VCO
which will produce tones ranging from around 50Hz to 4KHz or so when the
applied DC input voltage changes from 0.5volts to 2.5volts.
This will be used simply as a tuning aid for visually impaired users
so the actual voltage/frequency relationship, and waveform produced, is
not at all critical. Supply voltage of the VCO can be in the range 3.3v
- 5v DC.

I'm considering using the CMOS 4046 PLL IC, but wonder if anyone in
this NG has any better suggestions to offer. FWIW, price is not the most
important consideration.

Any suggestions will be much appreciated.

The LM331 can do that, is nicely linear but you'd have to add the proper
control voltage offset so you get the range you want:

http://www.national.com/ds/LM/LM231.pdf

However, check whether supply voltage and power consumption fit your
requirements. Also, only available in DIP package.

A really slick and precise solution is a micro controller with AD
converter in there. The MSP430F2003 would be a good candidate and for
the basic function you'd need almost no external parts other than a
decoupling capacitor. Feed you control voltage to its built-in AD
converter, digitally offset and scale its output to whatever you need
and let the output set the overflow register of the internal timer. The
timer output gets piped to a port pin and, voila, there is your output
frequency. Even without any external crystal and just using the internal
digitally controlled oscillator (DCO) the precision would be in the
single digit percent. TI has super-cheap development kits for that series.

 

[Spehro Pefhany]

As part of a larger system, I need to design and build an audio VCO
which will produce tones ranging from around 50Hz to 4KHz or so when the
applied DC input voltage changes from 0.5volts to 2.5volts.
This will be used simply as a tuning aid for visually impaired users
so the actual voltage/frequency relationship, and waveform produced, is
not at all critical. Supply voltage of the VCO can be in the range 3.3v
- 5v DC.

I'm considering using the CMOS 4046 PLL IC, but wonder if anyone in
this NG has any better suggestions to offer. FWIW, price is not the most
important consideration.

Any suggestions will be much appreciated.

TIA - Dave

If you don't mind a linear 50mV-4V for 50Hz-4kHz, it's pretty easy to
do this with a couple of opamps (or an op-amp and a comparator) and a
transistor (BJT, MOSFET or analog switch). You set one amplifier up as
a +/- integrator, output goes to a comparator with hysteresis which
controls the +/- switching. You get a triangle wave and a square wave
output.



Best regards,
Spehro Pefhany

 

[Phil Allison]

"John Fields"

( snip ASCII art monstrosity )

The LED/LDR combo or an incandescent/LDR combo is available as a single
device called a "Vactrol":

** The OP is in the UK.

Very likely, the RoHS directive will prohibit the use of a Vactrol.

Cos the LDRs ( ie CdS cells) inside are made with evil and forbidden
Cadmium.

Various folk have been trying to get the insane bureaucrats in Brussels to
grant a general exemption for CdS cells on the grounds there is no similar
replacement device available - but with no success.

BTW

I know about the one Marshall Amplification got for professional audio, but
it expires in a few weeks.


.... Phil

 

[Tim Williams]

The curse of a quasi photographic memory, I can remember nearly every
web page I've ever been to via context clues:

Here is a recent memory of where a vco was using a few transistors.
I'm sure Its only good at room temp, but the price is right:

http://www.4qdtec.com/mvbz/vco2.gif

I liked this thing,
http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Triangle.gif
of course it can be simplified a bit if you don't need it to be as fancy.
And if you're not running up to 40MHz, you can ease off on the bias current,
too. ;-)

The nice thing about this circuit is its wire range. 50Hz to 4kHz is a very
small part of the total range.

Tim

 

[David Chapman]

Many thanks to all those who have taken the time to contribute to
this thread.

I'm overwhelmed by the number of different and interesting suggestions
that have been offered. As I said in my original posting, I'm not
looking for an ultra-linear or accurately repeatable solution so the
simpler the better for this application. The cross-coupled
multivibrator arrangement looks very simple and effective so I'll
probably be trying that to start with and then checking out the other
design suggestions.

Thanks once again for all of your suggestions. MUCH appreciated.

- Dave

 

[Frank Buss]

As part of a larger system, I need to design and build an audio VCO
which will produce tones ranging from around 50Hz to 4KHz or so when the
applied DC input voltage changes from 0.5volts to 2.5volts.
This will be used simply as a tuning aid for visually impaired users
so the actual voltage/frequency relationship, and waveform produced, is
not at all critical. Supply voltage of the VCO can be in the range 3.3v
- 5v DC.

I would use a microcontroller, as Joerg suggested, because then you'll need
just one chip and one capacitor and you're done. I've tried it with a PIC:

http://www.frank-buss.de/vco/

The lite edition of the HI-TECH C compiler has even floating point support,
so it is very easy to implement it. You can get the PIC for less than $ 1
for 10 parts:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PIC12F675-I/SN-ND

Works from 2V to 5V. And you'll need a programmer. I have this one:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PG164120-ND

Don't use the PICkit3, until they have fixed the firmware

The MPLAB IDE and the compilers (the non-optimizing versions, which works
good for most projects) are free.

 

[David Chapman]

I would use a microcontroller, as Joerg suggested, because then you'll need
just one chip and one capacitor and you're done. I've tried it with a PIC:

What an elegant solution! Many thanks Frank.

Your proposed way of solving the problem really highlights the
difference between my ancient way of thinking which usually involves
discrete components and some digital or analogue ICs, and the modern
'micro fits all' approach.

I do feel old :-( ....but then I am, so many thanks for bringing
me up to date. I'll certainly give the PIC approach a go now.

Thanks again - Dave

 

[Frank Buss]

What an elegant solution! Many thanks Frank.

My pleasure!

Your proposed way of solving the problem really highlights the
difference between my ancient way of thinking which usually involves
discrete components and some digital or analogue ICs, and the modern
'micro fits all' approach.

I do feel old :-( ....but then I am, so many thanks for bringing
me up to date. I'll certainly give the PIC approach a go now.

It is never too late to learn new things, e.g. I'm 36 and trying to learn
piano playing

BTW: the PIC is not the best solution if you want high precision, because
of the missing capture compare timer, which would be possible with some AVR
microcontrollers or better PICs. The best would be a hardware PWM module,
which are integrated in bigger microcontrollers, but you wrote that it is
not important, so my very simple solution might work for you.

 

[Rich Grise]

The thresholds in a '4046 (CMOS) are variable unit-to-unit, some better
precision can be had with a linear design; I'm thinking of XR2206, LM331,
and similar chips. The basic scheme is to use the voltage input to
program tracking current source + sink elements, onto an
integrator-connected amplifier into a comparator with suitable hysteresis.

In other words, a 555. ;-)

Analog switches, diode switches, current mirrors, or transconductance
amplifiers are the key components.

Cheers!
Rich