coil to plus input

[Qiaozhi]

A 10k or 15k resistor is faster than 100k with spice for me. Got the parts, been busy and haven't had time to try yet. Hope to soon. I'll try larger input resistors also.

That's what I found as well. 10k seems to be the optimum value, but in reality the output starts to go into oscillation. It seems like there's no free lunch there.

[mickstv]

I'm not sure if that's the reason for the instability. I also tried simulating this in LTSPICE, and you can see evidence of the same thing happening there, even though the resistor models are ideal.

The PI I use started life with the typical 1k resistor and 2x 1N4148 frontend, the audio output was unstable so the threshold was always backed off. Later I changed to a single Nch blocking fet frontend, now the audio is stable and the threshold control can be turned up more, plus smaller targets can be detected.

[green]

Tried a couple things today. BSP135 depletion mode N ch, gate to ground, source to 500 ohm to + input with diodes to ground, drain to coil. Coil decay looks the same as coil with R damping only. Get some oscillation at amplifier out. Amp with coil to - input looks better. KR is using a LM4562 so I used one for the amp. Need to wire it for - input to see if the LM4562 looks good to - in. Tried a LND150 depletion mode, didn't work. Need to try some more.

[mickstv]

Hi Green, this is what I see on the output of my preamp, using a blocking fet frontend. TX is powered with 8 volts and is 130us wide into a 0.5ohm ML coil. The first sample can be taken at 8us.

The circuit I use to set the blocking fet is a 74HC221 that connects to a mosfet driver which triggers a SFT1440-D fet. The delay is adjustable using a pot on the 74HC221.

I can post the circuit, if you want.

[green]

Hi Green, this is what I see on the output of my preamp, using a blocking fet frontend. TX is powered with 8 volts and is 130us wide into a 0.5ohm ML coil. The first sample can be taken at 8us.

The circuit I use to set the blocking fet is a 74HC221 that connects to a mosfet driver which triggers a SFT1440-D fet. The delay is adjustable using a pot on the 74HC221.

I can post the circuit, if you want.

Thanks for the reply. Looks a lot cleaner than what I had. I would be interested in seeing the circuit

[greylourie]

Hi Green, this is what I see on the output of my preamp, using a blocking fet frontend. TX is powered with 8 volts and is 130us wide into a 0.5ohm ML coil. The first sample can be taken at 8us.

The circuit I use to set the blocking fet is a 74HC221 that connects to a mosfet driver which triggers a SFT1440-D fet. The delay is adjustable using a pot on the 74HC221.

I can post the circuit, if you want.

Hello mick, could you describe what is happening in your image with the two traces. I see a small brief "blip" in the lower signal trace, almost one division in height.
What time/volts are the divisions set to ?
Thanks for posting the picture.

[mickstv]

Hi Green, here's a quick drawing of the working circuit. So one half of the 74HC221 is used for the flyback adjustment and the other half of the IC is used to control the ground balance point between the end of the first sample and the start of the wider GB sample.

The reason the GB is built this way, is because smaller micros like the 12f1840 can't step any smaller than 125ns which isn't fine enough for manual GB.


Circuit may have a couple of typo's but it was done quickly and will redo at a later stage.

[mickstv]

Hello mick, could you describe what is happening in your image with the two traces. I see a small brief "blip" in the lower signal trace, almost one division in height.
What time/volts are the divisions set to ?
Thanks for posting the picture.

Hi Grey, that small blip is leakage, and is just the point at which the fet turns on to allow the received signal through. The CRO was set to 10us/div for that picture.



Top flyback control on gate of the SFT1440, bottom pix output of preamp at 100x. CRO 5us/div.

[greylourie]

Very respectable times there ! Are there any pitfalls for the uninitiated to be aware of, like switch capacitance ? Everything around the front of P.I seems to revolve around it.

[mickstv]

The blocking FET needs to be low capacitance but also low resistance, but it is hard to find FET's that are lower on both. Normally lower capacitance FET's have a higher Rgs. I tried heaps of FET's before settling on the SFT1440-D.

[greylourie]

Very nice, good inspiration for all.

[mickstv]

Very nice, good inspiration for all.

I might even post up the complete design at some stage. It seems to work ok. I've even found a few small bit of gold with it.

[Qiaozhi]

TX is powered with 8 volts and is 130us wide into a 0.5ohm ML coil. The first sample can be taken at 8us.

You mentioned that you used a 0.5 ohm MineLab coil, but what's the inductance?

[mickstv]

You mentioned that you used a 0.5 ohm MineLab coil, but what's the inductance?

Just a bit over 300uh

[mickstv]

Just noticed a mistake in the circuit.

The 10k resistor on pin 2,4 of the TC4424 driver IC, the other end of the resistor has to go to pin 6 not pin 3. I'll fix that when I redraw the circuit.