bipolar pulsing to remove Earth Field

[Carl-NC]

Mick,

Your last schematic has disappeared, some problem with ImageShack. Keep in mind that you can directly upload pics to the forum, and they will stay around a whole lot longer that pics that are linked in from a temporary host service.

Anyway, was wondering why you used a PMOS switch vs a second NMOS switch, everything else equal.

- Carl

[mickstv]

Mick,

Your last schematic has disappeared, some problem with ImageShack. Keep in mind that you can directly upload pics to the forum, and they will stay around a whole lot longer that pics that are linked in from a temporary host service.

Anyway, was wondering why you used a PMOS switch vs a second NMOS switch, everything else equal.

- Carl

Hi Carl, every time I tried Moodz diff coil version (center tap) the time taken till I could do first sample was extended beyond 10us unamplified. But when I used the P/N version as in my now missing circuit I could always get the sample time well under 10us unamplified.


My view was that it was important to be able to sample early, also when monitoring either the n or p side after separate sampling (for testing) the dc voltage level difference for various targets was small enough not to worry about.



Mick

[moodz]

Hi Moodz, I actually did try that method using the standard config and using two nch mosfets, but like always I couldn't get the circuit to sample in less than 10-15us (with no amp direct off the diodes).

With the configuration shown in the hand drawn circuit earlier I can get the sampling to start in less than 8us from TX off (seen directly of the diodes with no amp).


So this circuit isn't really a moodz coil configuration



Mick

Mick ... I was not in any way suggesting your good work was some sort of rip off ....I was only following Altra's lead on suggesting a look at using the two NFETS.

Your method achieves faster damping ... so does the differential coil if you only connect the damping resistor across one of the coils ( see the mosfet damping thread)

http://www.geotech1.com/forums/showp...2&postcount=51

by adjusting the damping the input to the amp can be zeroed precisely.

you also get descrimination ( for larger targets ) maybe you could see if this effect is shown in your circuit ??

cheers ... moodz

[mickstv]

Mick ... I was not in any way suggesting your good work was some sort of rip off ....I was only following Altra's lead on suggesting a look at using the two NFETS.

Your method achieves faster damping ... so does the differential coil if you only connect the damping resistor across one of the coils ( see the mosfet damping thread)

http://www.geotech1.com/forums/showp...2&postcount=51

by adjusting the damping the input to the amp can be zeroed precisely.

you also get descrimination ( for larger targets ) maybe you could see if this effect is shown in your circuit ??

cheers ... moodz

Hi Paul, thanks for the reply. I think i've been going in circles with this, each time I've tried the nchannel version i've done it with a THAT1510 and an input resistor on each side with protection diodes.

I tried the circuit again in your diff coil config but this time I used a NE5534 and only one input resistor and diode protection on one side, the result was the decay was approx 2us faster than with the THAT amp. It turns out that adding the second resistor and protection diodes to the other side of the diff circuit is what was causing the slow down.

I'll keep experimenting and see what else I can come up with.


Cheers
Mick

[6666]

Mick your schematic has disappeared

[mickstv]

Here's a picture of the frontend I've just completed using the the Moodz Diff Coil, Thanks Moodz. I'm running it as a standard frontend and using the other channel only to generate the bipolar pulse. For me I've found that I can sample 1-2us sooner doing it this way. It seems to work ok, EF is competely cancelled at the output of each of the integrators (using a cro to monitor the output as I haven't finished the output stages).


The test settings are 70us TX with a sample start at 10us and second sample starting at 50us, the sample spacing is 15us. This is the same for both TX cycles.


Does Mods think I should start a new thread for this ?



I'll do the schematic for the Processor and Drive circuits later and post up.

[mickstv]

Here's the micro section. I'm using a PIC16F88-i/p at the moment because it's what I had avaiable, plus its good enough for timing.

I used pretty much the same driver design as what Carl used in the HH2, Thanks Carl.

[Rumcajs]

Hi all,

here is an field shape from Vallon WMH metal detector

[Jeddas]

I wonder if changing rx sample timing with each tx pulse is of use. It could close any gaps in detecting targets with different time constants. Less real time load on micro.
tx pulse - fast rx sample - tx pulse - bit slower rx sample - etc then repeat cycle.

[Davor]

It could do, but in that case (considering bipolar pulsing) you'll need to introduce EF samples to compensate for the difference.

[6666]

H-Bridge PI transmitter

Taking the schematic from patent 6130489. I came up with this interpretation. I left out the zenor diodes, which as far as I could figure, had the purpose of snubbing the flyback.
Common n and p channel fets were used in place of bipolar transistor. Supply voltage 12v, 5v switching logic to the 2N7000s. I used a pulse frequency around 2.3khz and pulse width 60us. These could be adjusted up or down. I had timming program already done and just adapted it to this circuit.

The circuit operates by alternating the polarity of the coil using an h-bridge. The
h-bridge is then pulsed on and off with a p-ch fet. One side of the coil is always connected to B-. So this operates like the p-ch Surf-pi with a coil that reverses after each pulse. A differential amplifier is connected accross the coil. The output is true bipolar and centered about +V/2

I need to play with the preamp some more. Lots of parts, but it works on a single supply.
Some people may ask why go to all the trouble of making a bipolar pulse when a mono polar works fine? If you want to play with sampling during "on" times for experiments in PI discrimination. (see Tinkerers PI-IB) A symetrical bipolar receive signal is best. The differential sampling removes DC offsets and synchronous noise. Any comments or improvements welcome, Thanks

Hi Altra, did you ever proceed with this ?