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RE: DRSSTC-3 VCO drive test (long)

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

Hi Steve, list,

Good to hear you got decent results with a VCO.

>The fact that
>the streamer detunes the coil is causing a bit of an issue.  My idea
>for a solution is a ramp generator synched with the interrupter to
>slowly down-shift the frequency during the burst.  I wish to control
>the amount of frequency shift as well.

I have been playing with a circuit that does this, I'll draw up the
schematic if you want to see it. It uses two analog switches and an op-amp.

Basically you have a pot that gives an adjustable voltage, and one analog
switch connecting it to an op-amp integrator during the burst. The
integrator turns the adjustable voltage into an adjustable ramp. When the
burst is finished, the first switch opens and another analog switch shorts
out the integrator capacitor and resets it to zero for the next burst. I
tested it with a 4066 quad analog switch and one section of a TL074 op-amp.

>(right now the upper >pole seems to be the winner!

I like the upper pole too. The reason is that (I think) it's
self-stabilising when using primary current feedback. When streamers detune
the secondary, it should strengthen the primary current and hence the
feedback signal, so the system stays stuck to the upper pole. With the lower
pole, detuning by streamers weakens the primary current and might make the
system jump to the upper one.

As Terry pointed out, this is maybe not necessarily a bad thing if a self
resonant driver just "rides" whatever happens, or a VCO keeps on driving it
where it "ought" to be. I think it's mainly a concern with a PLL type

Steve C.