A closer look at the AF108 bi-polar drive circuit

[ivconic]

Having done some more digging and research, I realized that there was also a PI 150 pipe locator which is a totally different circuit(although it does have similarities).
This was the circuit which was back engineered by Ivconic, and not the AF-108.
I also found a photo of the PCB of the AF-108 mine detector. Note that the quad AND gate is a 4081 just as waltr had speculated(on the schematic it is mislabeled as CD4069). And also have a look at those lovely BUY92's, with there custom heatsinks. Wow! that's fantastic.

[dbanner]

The issue with adjusting the timing pots is not knowing what the timing should be since there seems to be no documentation on this.

dbanner will need to make some guesses as to timing then a lot of experimenting to determine best timing.

My strategy in guesstimating the timing sequence would be to assume a PRF the same as the Whites PI1000, and the first and second sample delays are likewise the same( and the pulse widths). This would give me a starting baseline from which I can experiment to see what is the optimum performance. The 390+390 Ohm Rd tells me that the inductance of the coil isn't far off from a regular Surf or PI1000/2000. Thus the PRF I would assume to be within typical range of hundreds of PPS.
In other words, I'm going to superimpose the timing charcteristics of the PI1000, and see what happens.

[waltr]

My strategy in guesstimating the timing sequence would be to assume a PRF the same as the Whites PI1000, and the first and second sample delays are likewise the same( and the pulse widths). This would give me a starting baseline from which I can experiment to see what is the optimum performance. The 390+390 Ohm Rd tells me that the inductance of the coil isn't far off from a regular Surf or PI1000/2000. Thus the PRF I would assume to be within typical range of hundreds of PPS.
In other words, I'm going to superimpose the timing charcteristics of the PI1000, and see what happens.

That is a good plan.

[dbanner]

The final piece of the puzzle. Here is the bipolar pulse drive and RX preamp PCB layout.
For the rest of the circuit, I'll copy the SMP, just for testing.

[dbanner]

This was posted by Brian Deese working on an AF-108 back in 2012:
"I will try it this afternoon,and by they way about the sample.I just wanted to let you guys know,In a previous post I said I would check the "secondary sample also" well there is no secondary sample.This machine feeds one sample from positive pulse and another from the consecutive negative pulse to the differential amp.Thats a change of pace right. And the sample can not start as soon as the other PI machines I have,you cant move the sample up to the egde of the saturation plateau.It can just include a few uS of the linear range of the amp and then the rest of the sample is at zero.I works similarly when you are inching it closer,when you go past a certain point it will not auto tune any longer and no output audio.I set the machine up yesterday for the best performance and its running at 4khz,with a 100uS TX width,and it likes a 40uS sample width the best. And another interesting thing about it since I have it running stable"albeit the preamp output" is when you place the coil on the ground and press the auto tune it oscillates at about 1hz,when you lift the coil it loses some ground influence and it goes quiet,sort of a silent search.Pretty neat I think."

[dbanner]

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

[dbanner]

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?

[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

[dbanner]

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.

[dbanner]

U6-b and VR4 is sets the sample delay.

[Andi68]

Push-pull stage

[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?

[dbanner]

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?

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

[Andi68]

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

Why is this done?