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I studied the LM3900 app notes a year or two ago, and came to the same conclusion about the Serge pulse divider.  I'm as sure as someone who has never seen the insides of a Serge module can be that it's using a purely analog LM3900 staircase generator to do the dividing.  And part of me respects the pure-analog-ness of that solution. ;-)

- hdan

-----Original Message-----
From: jhaible@... [mailto:jhaible@...]
Sent: Sunday, October 15, 2000 4:23 AM
To: motm@egroups.com
Subject: Re: [motm] Is this useful?

> On the VC Pulse Divider from Serge, there is an output called 'STEPPED'.
I'm
> am pretty sure
> how they did it (not seeing the circuit).

I thought I was pretty sure how they did it, too, but I may me wrong:
I thought they used a charge pump like that NS app note for the
LM3900. I mean they might have used a counter, but seeing Serge's
general fondness of the 3900, I'd be surprised if he would not have
done it with that single chip. (Anybody knows for sure ?)

And yes, bringing out the ramp would be useful. (I have not done it
on my version, but I wished I had.)

JH.

>
> The output is a stair-cased, positive-going sawtooth. The sawtooth is
> generated by a 5-bit
> counter, that is ∗clocked∗ by the input pulses and ∗reset∗ by the
> divide-by-N pulse.
> The output counter bits feed a 5-bit DAC.
>
> So, here are some interesting things about this sawtooth:
>
> a) for a fixed frequency in, the ∗amplitude∗ is inversly proportional to
the
> divide ratio. Think
> about it: the longer you count, the higher the amplitude.
>
> b) of course, for a fixed input frequency, the ∗output∗ frequency is
> proportional to the divide
> ratio. If you input 4Khz and divide by 8, you get out a 500Hz sawtooth
with
> 8 steps. Assuming
> a -5V to +5V output scaling (like all the other signal generators), we
have
> 10V/32 steps = 312mv/step. So for 8 steps = -5V to -2.5V sawtooth.
>
> For straining the brain more: this acts as a low-pass filter! Since the
> divide ratio is a control
> voltage, then if the input is fixed (say 2Khz) then as the divisor is
swept
> from 32 to 1 the
> amplitude AND frequency change. But, the amplitude drops off at higher
> frequency outputs
> since the counter is reset faster and faster.
>
> Of course, if you fix the divide ratio and sweep the input, the amplitude
of
> the sawtooth is ∗constant∗
> at the divide ratio X 312mv.
>
> Will add $15 to the cost of the module. Discuss!
>
> Paul S.
>
>
>
>
>
>

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