A closer look at the AF108 bi-polar drive circuit

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I wonder if the AF-108 can even be considered a "bipolar drive" circuit.
After all, it's the centre tap coil being driven at either end which makes it bipolar, not the drive circuit itself. It is definitely a bipolar transmitter, but only because of the coil configuration.

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Both transistors are driven by consecutive "high" pulses, but their outputs are driving the coil from opposite ends of the centre-tap. The two output transistors are independent of one another and so are not complimentary NPN-PNP as in the classic push-pull amplifier. The Fisher Impulse is more akin to a push-pull design, in fact, that's exactly what it is.

[Andi68]

That confuses me

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Q6 an, Q8 aus, Q6aus, Q8 an.....

Warum wird das so gemacht?

Q6 and Q8 appear to fire consecutively, but NOT in a push-pull arrangement. Each transistor is independently driven. At least it appears so to me.

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Strangely, the sample delay and sample width appear to be "tied" to one another. Varying VR4 not only changes the sample widths, but also the delay as well??????because of the clck input of the 4024.

[Andi68]

Q6 on, Q8 off, Q6 off, Q8 on .....

Why is this done?

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I think Deese was right! The two samples run consecutive to one another, without any delay between them. The 4069 inverter in combination with the two "AND" gates delivers the two samples. You can see from the two AND gates in question, that when one goes high the other goes low. Or am I completely misreading the schematic?

Yes indeed, Brian Deese was correct, the samples do appear to run consecutively, one after the other. Never seen that before.

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Push-pull stage

Yeah, sort of, when one gate is on, the other is off.

[dbanner]

I think Deese was right! The two samples run consecutive to one another, without any delay between them. The 4069 inverter in combination with the two "AND" gates delivers the two samples. You can see from the two AND gates in question, that when one goes high the other goes low. Or am I completely misreading the schematic?

[Andi68]

Push-pull stage

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U6-b and VR4 is sets the sample delay.

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Yes. It shows the scope set to 50uS/div. There are about 5.5 divisions between pulses so 5.5 x 50 = 275us then 1/275us = 3636hz. Pulse width = 50us

Great, thanks for verifying this.

[Altra]

Yes. It shows the scope set to 50uS/div. There are about 5.5 divisions between pulses so 5.5 x 50 = 275us then 1/275us = 3636hz. Pulse width = 50us

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Can the TX pulse rate and pulse width be determined from these scope shots? It looks to be around 3.8kHz and pulse width of about 50uS. What do you guys think?

[dbanner]

So there is no second sample. but a consecutive negative pulse????