A closer look at the AF108 bi-polar drive circuit

[Andi68]

Thank you for your explanations

[dbanner]

I see there is a zener diode(1N5341 - 6.2V) and a schottky diode(11DQ03) connected between the base and VC of each of the two output transistors.
Some sort of clipper or voltage clamp? So 6.2+0.3=6.5V. So it clamps the voltage at 6.5V? But VC is supposed to be app. 3.3V, so what's the point in clamping at 6.5V if the voltage never gets that high? I'm beginning to think that VC is mislabeled here, and should be B+ What the hell do these two diodes actually do?
Does it also help with switch-off time, allowing the stored charge in the transistor to be swept out very quickly?
Very interesting.
Someone please help me to figure this one.

[dbanner]

Here is the front end I refer to in above post.

[dbanner]

I wonder how this transistor setup performs versus the mosfet transistor for driving the coil?

[dbanner]

I see there is a zener diode(1N5341 - 6.2V) and a schottky diode(11DQ03) connected between the base and VC of each of the two output transistors.
Some sort of clipper or voltage clamp? So 6.2+0.3=6.5V. So it clamps the voltage at 6.5V? But VC is supposed to be app. 3.3V, so what's the point in clamping at 6.5V if the voltage never gets that high? I'm beginning to think that VC is mislabeled here, and should be B+ What the hell do these two diodes actually do?
Does it also help with switch-off time, allowing the stored charge in the transistor to be swept out very quickly?
Very interesting.
Someone please help me to figure this one.

Maybe there is 3.3+3.3=6.6V between the two bipolar halves, so clamping at 6.5 makes sense???? Don't know.
The calculated Vc from the power supply is approx. 3.3- 3.4 volts.
It does seem to allow the stored charge in the transistor to dissipate very quickly, this translates to fast switch off.
Using just a single end of this transistor Tx driver on a conventional PI should offer some improvements.

[ivconic]

I was lazy to finish it, only PS stage is missing.
Also one "4069" from "original" schematic is actually a CD4081.

[ivconic]

The '108 is a mine detector. A critical requirement is that it generates ZERO net magnetic field to avoid triggering a magnetic mine. So it pulses a positive field followed by a negative field. A side benefit is the cancelation of EFE.

This answers the question i was about to post.
Apart from what's explained; are there any other benefits from this or not?

[Altra]

This answers the question i was about to post.
Apart from what's explained; are there any other benefits from this or not?

I noticed you can get equal signals on hi and low conductors when not using ground balance. Normal pi's the late ef sample subtracts from the early sample and targets that are high conductors like silver give less response than gold. A bipolar can operate with a single early sample per phase. PI's by nature ignore light mineralization, so in some locations you can increase the range of different targets at equal depths. Ground balance is easier to accomplish, since the earth field is not in the equation.

[Carl-NC]

Apart from what's explained; are there any other benefits from this or not?

Since there is no late EFE sample, you can run a much higher pulse rate.

[Altra]

Carl is there any type of noise reduction with bipolar?

[ivconic]

Since there is no late EFE sample, you can run a much higher pulse rate.

That's good!
Problem is in schematic.
I am not sure is it without further mistakes and errors, the one i am redrawing from.
I only have found that CD4081 was renamed to "4069", so far...

[6666]

I noticed you can get equal signals on hi and low conductors when not using ground balance. Normal pi's the late ef sample subtracts from the early sample and targets that are high conductors like silver give less response than gold. A bipolar can operate with a single early sample per phase. PI's by nature ignore light mineralization, so in some locations you can increase the range of different targets at equal depths. Ground balance is easier to accomplish, since the earth field is not in the equation.

Is there a simple timing / sample drawing that shows how the ground balance is achieved with bipolar pulsing ? thanks

[Altra]

Is there a simple timing / sample drawing that shows how the ground balance is achieved with bipolar pulsing ? thanks

I'll post a simple diagram tomorrow. What I have been doing 100us Tx, Sample delay 10us, sample1 10us, GB Adjust(delay) 0 to 50uS, GB sample 30uS. This repeats for the negative cycle. Signal = (Sample1 + (-sample1)) - (GB + (-GB)). The differential integrator takes care of the sign on the negative samples by inverting them. Not sure if this is the best way, probably better schemes. If you change the sample delay the GB needs re-adjusting.

[6666]

I'll post a simple diagram tomorrow. What I have been doing 100us Tx, Sample delay 10us, sample1 10us, GB Adjust(delay) 0 to 50uS, GB sample 30uS. This repeats for the negative cycle. Signal = (Sample1 + (-sample1)) - (GB + (-GB)). The differential integrator takes care of the sign on the negative samples by inverting them. Not sure if this is the best way, probably better schemes. If you change the sample delay the GB needs re-adjusting.

Thank you greatly appreciated.

[Carl-NC]

Carl is there any type of noise reduction with bipolar?

Yes, subtractive sampling forms a comb filter. But this also happens with the late EFE sample in a monopolar design.