on oscillators

[Ian Fritz]

Martin --

As JH pointed out, my experimental triangle VCA core based on a differential
integrator works with an npn converter (he accidentally typed pnp, though).
It's just a two-transistor diff amp with the usual diff op-amp readout
replaced by a diff integrator and with hysteretic feedback. Again, a sketch
is in EN #111.

  Ian

----- Original Message -----
From: "Martin Czech" <martin.czech at intermetall.de>
To: <synth-diy at mailhost.bpa.nl>
Sent: Monday, February 21, 2000 6:25 AM
Subject: on oscillators


> The last thread about current mirrors, hf oscillations etc. etc. made me
think...
> the fewer components an oscillator core has the less problems one may get.
>
> Triangle oscillators are nice, because they can invert direction (FM),
> OTOH you need a bipolar current source, or a mirror, this is
> the CA3080 app. note topology, dozens of designs come from there.
>
> Is there any way arround the current mirror and the pnp-expo converter?
>
> 1.:
> One possibility is to use two sawtooth cores in a alternating fashion,
> thus hiding the discharge time. One is an inverting, the other a not
inverting
> integrator, so the npn current source has to be switched to one of both
> integrators, but not mirrored.
>
> -capacitor matching (waveform symmetry), could be "trimmed" with
additional
>  small caps
> -delay time for non inverting and inverting integrator could be different
> -switching and discharge logic is more "expensive"
> -sawtooth like sync need logic
> -frequency wheel reversal means not to discharge, but to track, again
>  extra logic
> -charge injection due to switching (is there no charge injection using an
ota??)
>
> This dual integrator concept is used in integrated CMOS precision clocks,
> so it seems to work, the complexity of the circuit and capacitor matching
> is no problem there.
>
> 2.:
> The next one is even more complicated, it has only one timing cap,
> and 4 switches determine which side of the cap is loaded, and which is
> the reference.
>
> -charge injection
> -complicated switching
> -even more complicated sync and frequency wheel reversal
>
>
> 3.:
> The next is simpler, we know it all, it is the generic sawtooth.
> What could be improoved: MOSFET source follower/MOSFET current source
buffer.
> Simple, very fast comparator, PMOS discharge switch.
> This should insure complete and very fast discharge to the reference.
>
> -min/max levels could vary with frequency and temperature
>
> There are comparators with good drift specs (bipolar input stage), so
> this shouldn't be too bad. It may happen that the comparator switches too
> late, i.e. the lowest point gets deeper as frequency increases.
> Say the saw has a 5V jump, the current drain would then happen at 50000V/s
> @ 10kHz, or 50mV/us,... this is very slow even with old comparator design
like
> LM106 or so...
>
> A peak-hold device could be used to determine the optimum slicing level
> for the triangle and square wave converter, I've seen such a circuit.
>
> But sawtooth oscillators can not reverse frequency wheel...
> no they can. If you compare the saw wave with the resulting triangle wave
> (aktive rectifier, or switching rectifier using the square output), you'll
see that
> a wheel reverse really means a jump to the inverse sawtooth level.
> This means modifying the discharge reference level, but no extra component
> in the oscillator core.
>
> Some weeks ago there was a thread about triangle frequency wheel reversal
> and it turned out to be difficult when the point of normal operation
> reversal was near. The basic problem will not go away using the saw/set
> feature, but maybe there is a clever way to fix it.
>
> It seems that the good old relaxation saw oscillator can do anything
> the triangle can do, including phase reversal and phase reverse sync
> by simply switching the discharge reference, i.e. the simple vco
> core hasn't to be changed.
>
> Before that I was in favour of triangle osc. but after the reported
problems,
> and problems in finding a good pnp current source or avoiding ota/mirror
problems
> I'm not so sure about it any more. Especially not if the saw is needed
> for timing reference (sequencers, time discrete level network).
>
> m.c.
>
>
>