[Re: VCDO - asking again + new idea]

[Harry Bissell]

Harry Bissell sez: I posted a solution to the "variable amplitude " problem
before. Every ADC has a reference input (forget that my idea was analog
comparators, same difference). We are interested in the Peak amplitue of the
sawtooh over some short period of time. A peak detector will do this easily.
There will be some slight "ripple" or instability when slewing rapidly in
frequency, but for (reasonably)steady state it would work fine. Maybe I'll try
to build a simulated circiut for EWB and find out for sure.
Don't Don't Don't use an "AGC" (that's a feedBACK circuit that must have
errors to work...) The peak detector feeding the reference is a feed-forward
circuit, much faster. I've done this at low audio frequencies in a color organ
circuit using an LM3914, the peak value of the envelope sets the voltage at
the top of the reference string.
 Of course you could also use an amplitude stable VCO...

Another idea would be to use a cascade of frequency multipliers on the
waveform. this is easily done on triangle waves.

;-)  Harry



"Paul Maddox" <space_banana at hotmail.com> wrote:
JBV,

>
>I didn't get any response, so I'm asking again : did anybody try to
>implement that ?
>

no, but thought long and hard and did some testing..



>Let's say we have an audio range VCO producing a positive sawtooth, 

with
>a freq range from 0 to
>20 KHz. As it ramps up periodically, the sawtooth is sampled through 

a
>fast ADC, the 6 MSB
>from the ADC are used to adress a ROM where various waveforms are 

stored
>(64 bytes for each).
>Of course, the most significant adress bits of the ROM are used for
>waveform selection. Then the data
>output of the ROM are sent to a fast DAC.
>In order to still have 64 samples for the highest freq (20 KHz), the
>sawtooth must be sampled at
>1.28 MHz min. In the Farnell catalog I found a 6 bits ADC (CA3306CE 

by
>Harris) with a typical
>conversion time of 0.1 us. Then we need a fast 8 bits DAC, and there 

are
>plenty of those.
>

nice ideo, one iddy biddy problem.

>Yes, I know it's not the cheapest solution for a VCDO, but my 

estimation
>is that it should come for
>less than $30.00. The other interesting thing is that a regular VCO 

can
>be used.
>

***BUZZZ***

Nope, you can't... 
stick a scope on the output of just about ANY VCO, look carefully at 
the amplitude.... it changes with frequency, so you need some kind of 
scaleing input amplifier that keeps the input to the dac at a constant 
amplitude... BIG problem...

>
>Any thought on the above ? Has anyone already played with smthing
>similar ? Am I re-inventing the
>wheel, or what ?
>

its easier just use a tri/square vco running at 800Khz or more...

>jbv
>

Just many tuppence worth..

Paul Maddox


 
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