[sdiy] Ultrasonic VCO question for Ian Fritz (or whoever)

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: Re: [sdiy] Ultrasonic VCO question for Ian Fritz (or whoever)
Date: Mon, 28 Oct 2002 10:19:57 -0700

> Magnus --

Ian,

> >The compensation will distort the waveform for higher frequencies but trade
> >from waveform accurateness (which is bad to start with due to the reset slope,
> >I've seen it and it's not excessively nice) for better tracking abilities.
> >What can be a problem is that the compensation leads to a slightly lower AC
> >amplitude and raises the DC offset as the frequency raises and compensation
> >starts to kick in.
> 
> 
> How can you say you say you have "seen it"?  Have you built this circuit? 
> Where did you get the schematic if you don't have the EN issues?

If you would have bothered to read the ASM-1 webpage (the one which I happends
to maintain), it says in clear-text (Gene's original text I might add) that the
VCO is "substantially borrowed from Electronotes". I have build the ASM-1 and
followed Genes schematics and description like many others. I've then tossed a
sampling scope at it, which is not very strange either. I actually have stored
away the sampling of the sawtooth at different frequencies.

I don't even know which of the Electronotes Gene is refering to. It is probably
not the same, but the compensation form seems to be the same. I trigged on the
description of it, not the specific EN in question.

> Why so you think the DC offset would change?  This is a tri oscillator, so 
> I see no reason for the pos and neg half cycles to be affected differently.

It is actually quite obvious. The compensation robs the lower end of the
waveform by the resistor offset, but the waveform stops at 5 V regardless due
to the comparator curcuit. This is clearly visible (on a scope) when driving
the oscillator up to its far upper end.

> >IMHO, this is a fair trade anyway, since few depends on the
> >accurate waveform and spectrum distribution about 50 kHz or so.
> 
> Plus that, the module only uses the square output (to drive a digital 
> waveshaper).

Well, I assume this is a result of it's use in your Electronote, but if you
look at the same compensation scheme for direct use of the sawtooth, your in a
different situation.

Cheers,
Magnus