A closer look at the AF108 bi-polar drive circuit

[dbanner]

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.

[dbanner]

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.

[Andi68]

That confuses me

[dbanner]

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.

[dbanner]

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.

[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.

[Andi68]

And what is the advantage of this circuit?

[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.

[waltr]

And what is the advantage of this circuit?

EFE naturally cancels.

What Carl said

[Andi68]

EFE- what does this abbreviation mean?

[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.

[dbanner]

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

[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.

[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.

[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.