SV: Re: [sdiy] Digital VCO

[Eric Brombaugh]

karl dalen wrote:
> Wery simple, most of the so called DCO based units used 8253 counters
> (each unit contains 3 16 bit counters) the output square are shaped into a 
> short pulse that *pings* a normal VCO integrator and there you have your
> ramp! (sawtooth). Then the rest is just a matter of waveshaping into whatever
> wave you want. Roland did some special ASIC waveshapers+counter combination 
> chip for the Juno series, the 106 might have it to.
>   
Interesting. So did they wrap a servo loop around from the comparator to 
the charging current input to keep the amplitude stable? I've done that 
in the past for an adaptive oscilloscope sweep and I found that at low 
frequencies the loop gets pretty hard to keep stable. Maybe if you put a 
current DAC driven from the frequency setting to control the coarse 
charging current / amplitude and then a fine control servo that levels 
it out based on a comparator. Still sounds messy.

Also, if they were using counters for the reset pulse, how did they 
compensate for the reciprocal relationship between the count value and 
the frequency? Plus the fact that the frequency resolution decreases as 
the count value decreases? Did they just use very fast clocks to drive 
the counters?

So many questions...

Eric