A closer look at the AF108 bi-polar drive circuit

[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.

[dbanner]

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

[dbanner]

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

[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.

[Andi68]

Thank you for your explanations

[Carl-NC]

And what is the advantage of this circuit?

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.

[ivconic]

Earth Field Effect ... this causes a signal at the end of each swing with a PI detector due to the Earth's magnetic field.

In a non bipolar circuit there is a second sample pulse taken much later than the main sample. This is subtracted from the main sample to remove the EFE.
In a bipolar circuit there are two transmit pulses of opposite polarity. However, the polarity of the EFE signal remains the same for both cases, and hence cancel each other when the samples are combined.

I made timing schedule to experiment with Barracuda circuit, yet i never finished the code, because circuit also will need some modifications.
But this is illustrative enough, hopefully:

//******************************** MAIN PULSE - TX ******************************
DIGITAL_OUT |= (1<<TX_PIN_A);                   
VoidDelay (63935);  // 65535-63935=1600, 1600/16=100uS
DIGITAL_OUT &= ~(1<<TX_PIN_A);                   
//*******************************************************************************
//******************************** MAIN DELAY ***********************************
VoidDelay (Sample1Delay); // Main Delay pauza
//*******************************************************************************
//****************************** MAIN DELAY PULSE *******************************
DIGITAL_OUT |= (1<<SAMPLE_PIN_B);               
VoidDelay (64850); // 65535-64850=685, 685/16=42,8125 uS
DIGITAL_OUT &= ~(1<<SAMPLE_PIN_B);             
//*******************************************************************************
//******************************** EF DELAY *************************************
VoidDelay (65135); // 65535-65135=400, 400/16=25uS
//*******************************************************************************
//****************************** EF DELAY PULSE *********************************
DIGITAL_OUT |= (1<<SAMPLE_PIN_C);                  
VoidDelay( 64850); // 65535-64850=685, 685/16=42,8125 uS
DIGITAL_OUT &= ~(1<<SAMPLE_PIN_C); 
//******************************************************************************* 
//******************************** EF+TX DELAY **********************************
VoidDelay( 64850); // 65535-64850=685, 685/16=42,8125 uS
//*******************************************************************************

"...In a non bipolar circuit there is a second sample pulse taken much later than the main sample..."

I was also thinking to make it adjustable and not fixed like in example above.

[waltr]

EFE- what does this abbreviation mean?

Earth Field Elimination

This is the effect due to moving a coil through the Earth's Magnetic field which produces a current to flow in the coil.

[dbanner]

Would have been nice to see a scope shot of the sample pulses from a working unit.

[Qiaozhi]

EFE- what does this abbreviation mean?

Earth Field Effect ... this causes a signal at the end of each swing with a PI detector due to the Earth's magnetic field.

In a non bipolar circuit there is a second sample pulse taken much later than the main sample. This is subtracted from the main sample to remove the EFE.
In a bipolar circuit there are two transmit pulses of opposite polarity. However, the polarity of the EFE signal remains the same for both cases, and hence cancel each other when the samples are combined.

[Andi68]

EFE- what does this abbreviation mean?

[waltr]

And what is the advantage of this circuit?

EFE naturally cancels.

What Carl said

[dbanner]

As I said in post 22, the '108 uses bipolar pulsing using a CT coil. It is a "pull-pull" TX, not push-pull. Because it is bipolar, there is no need for a secondary EFE sample, EFE naturally cancels.

So pull from one side of centre-tap, and then pull from other side of centre-tap....Pull-pull. I like it.
The pulses themselves aren't bipolar.(one negative going, followed by another positive going). It's the CT coil arrangement which reverses the current flow about the coil.

[Andi68]

And what is the advantage of this circuit?

[Carl-NC]

As I said in post 22, the '108 uses bipolar pulsing using a CT coil. It is a "pull-pull" TX, not push-pull. Because it is bipolar, there is no need for a secondary EFE sample, EFE naturally cancels.