Inverted LFO

[moosapotamus]

The question of how to invert an LFO signal came up not too long ago. Apparently, it's not as simple as sending the LFO through an inverting buffer...
Or, is it?

Here's a stereo vibrato project that illustrates how to generate two control signals that have their polarity inverted relative to each other. In this application, phase moves in opposite directions between two parallel phase-shift blocks. True stereo is achieved by sending the output of each block to separate, left & right outputs.

Vibrato-Matic_a
Vibrato-Matic_b
Vibrato-Matic_c[/b]

~ Charlie

[RickL]

I was the one asking about this, for a Small Stone Phaser. The problem for me seems to be that the SS uses a non-standard clock signal so the standard solutions won't work.

[moosapotamus]

Yeah, I remember. It's an idea I've been thinking about for a while, too. Although, not necessarily the exact same application as you tried with the small stones. In fact, I haven't had a chance to actually try it out on any thing at all, yet. But, as I recall, you tried splitting the LFO/clock signal, sending one side to a single inverting buffer, and it didnt work?

Since the phase stages in the stereo vibrato circuit I posted are also OTA-based, I wonder if you could scrap the clock generator (LFO) in the small stone, and use the LFO/inverter section from the stereo vibrato, instead, to drive two small stones in stereo (phase inverted relative to each other).

~ Charlie

[Rob Strand]

Or, is it?

With most modulation waveforms it is.  That circuit looks a bit more complicated than necessary, even with the added job of DC bias adjustment.

The small stone is trickier and cannot be done by flipping the waveform.

The reason is kind of tricky what you want is to shift the waveform in *time* by half a period.  It turns out when you invert a sine, triangle or square wave it looks like a time delayed signal.  The idea doesn't work with the small stone,  the root cause is the presence of even harmonics in the modulation waveform. (Incidently the 3 opamp ckt in that vibrato ckt won't work form small stone either because it's not much more than a simple inverter.)

[R.G.]

The small stone LFO is a current based LFO. Duplicating the voltage on it won't help.

However - I believe you can do the job with current mirrors.

You can have the LFO drive the input to a current mirror with two outputs. One output drives a PNP current mirror on the +9V line, the other drives the collector of a PNP diffamp fed from Imax at the +9V line.

The diffamp bases are biased in the middle, and the non driven PNP drives the inverted LFO current.

It's a theory... 8-)

[R.G.]

On the Vibro-Matic -

the phase shifter section is really odd - they used the voltage-variable-resistor setup on the OTA sections to make a standard opamp phase shift section work. I wonder at why they did this when the OTA section will do the job all by itself - you use half the ICs, and get rid of a bunch of other parts.


Odd engineering.