uPC Controlled PI

[KingJL]

Checked out the AD797 pre-amp. Only problem was a 17mv output offset. With the offset balance as designed in the schematic, could not completely balance. The offset would vary 17mv-37mv. Ripped out the offset balace and replaced with a traditional offset balance adjustment circuit consisting of a 100K multi-turn trimmer between +5V & -5V, fed through a 2200 ohm to pin2 of the AD797. Ahhhh, that is better!

Next in line is to set up a gain conrol input and test out the AD600. So far, so good!

The AD797 offset balance citcuit changes will be included in the next version of the schematic and PCB.

After I complete testing of the AD797, I will hook up a coil and take some oscilloscope pictures of the signals at different test points and post them in this forum.


Regards,
J. L. King

[KingJL]

Got sidetracked this week. Had to get the camper ready for a trip to the NC coast (Carolina Beach) for Xmas. Will return to the project after I return on the 1rst.
Everyone have a great holiday season.
Regards,
J. L. King

[KingJL]

... RX blocking appears to be now working. I believe I have to find another P-channel MOSFET for the RX block. There appears to be too much capacitive coupling of the gate signal to the output. I get about a 100mv leading and trailing edge spike. I will research this to see if there is a better component for this.

Well I'm slowly getting back on project. Still no pictures or further check out. Have been focusing on the blocking MOSFETs and the switching spike. I think I have identified the main problem. The current design has the gate voltage to the MOSFETs switching between +5v & -5v. This is driving the gates 5v past the switch-off voltage. The PMOS should have the gate voltage switching between 0 & -5v; The NMOS should have the gate voltage switching between 0 & +5v. Verified the problem with simulation models. With the gates switching between -5v & +5v, simulation runs exhibit the same charactaristic as observed in the actual circuit. Changing the gate switching voltages as described above decreased the switching noise spike considerably. Now to change the actual circuit!! Need to study this for a while to find the best way without having to re-make the board until the rest of testing is done.

Regards,
J. L. King

[KingJL]

Well, I think I have designed a fix for the Rx blocking circuitry that works superbly (simulated). Next step is to implement and test in the real world. The simulation suggests that changing the resistor in the collector of the 2N5088 to 220 ohms gives much faster turn-off. Putting an 820 ohm in the base circuit limits the 2N5088 cuurent to 90ma peak during the block pulse. The gnd connection of the TPS2814 is changed from -12V to -5V. This solves the overdrive of the MOSFETS. A 4148 diode between the collector of the 2N5088 and -5V prevents negative overdrive of the TPS2814 inputs. The 2N4403 is added to derive a AD600 Rx gate signal that is "stretched" to 0.7 usec after MOSFET turn-off. I also am removing the reversed parallel diodes after the P-MOSFET (at the input to the AD797). Also changing the resistor between AD797 input and ground to a 8K2.

I believe I can make the above changes with the exixting board (though somewhat hacked up) to enable further testing.

Attached is the schematic that I captured from the LTSPICE simulations of the changes. The LTC1693 is substituted for the TPS2814 in the model.

[KingJL]

I have changed the schematic and am modifying my board to facilitate the changes.

Although the changes provide a more usable front end, the VP3203 is still the wrong MOSFET to use here (too much input capacitance). I am trying to obtain some samples of the Zetex ZVP2106, which should be much better. That seems to be about the best that I can locate with a TO92 package. The best I have found overall is the Fairchild VP304 and the Philips BSS84, but these are only available in SMD packages.

[bugwhiskers]

Hi KingJL,

Considering the series diode in the TX coil circuit to minimise the effect of MOSFET capacitance and the front end blocking FET's in ML PI detectors to speed things up one must ask then why do they sample so late. The SD2000 timing diagrams show a whopping 45 uS sample delay after the 240uS TX pulse. I tried the blocking FETs but the ringing caused the earliest sample time to blow out past the ordinary current limiting resistor and clipping diodes arrangement.

I think the answer lies in the GB. In a late sample the ratio of ground to target will be much greater. If you don't plan to use any form of GB then the standard approach will probably work much better.

regards
bugwhiskers

[bugwhiskers]

If you click on the following link and go to the bottom of the thread then scroll up until you see some schematics. Sheet 2 of 4 shows a very elegant 1 chip solution to drive the P & N channel Fets as well as the MOSFET driver chip with the correct voltages for each.

http://goldprospecting.invisionplus....t=0&#entry4419

regards
bugwhiskers

[KingJL]

I have solved the issue. Changed blocking methodoly slightly. Use the FQN1N60C as in the previous schematics, changed the VP3203 so that during the block period it shunts the signal to ground. It uses the same gate signal as the FQN1N60C. Minimal switch noise (20ns 200mv spike 10ns after unblock as seen at the output of the pre-amp). This small spike is blocked from the AD600 by the AD600 gate. Can sample at 5us after TX with no problem.

Will take pics of o'scope test points after i clean the board up a bit. A schematic of the changes is included here. I kludged the board to test it and it works. No need for different MOSFETS. Now to clean it up a bit.

[Monolith]

Kingfisher project

Are there any updates on the Kingfisher project?

Monolith

[Unregistered]

all project file download link

mainboard + mcu + hex

[Unregistered]

Can somebody post,final sucess of this detector,some air test atc...

Regards too all!

[scuba840]

Deleting AD797 Amp and use only the two AD600's?

I have a question. The current design uses a AD797 and a one half of a AD600. Why not just use BOTH amps contained within the AD600 and delete the AD797?

[scuba840]

I have a question. The current design uses a AD797 and a one half of a AD600. Why not just use BOTH amps contained within the AD600 and delete the AD797?

Answered my own question: The AD600 needs a 100 ohm input resistance. So either a discrete wide band amplifier would have to be used or a AD797 op amp.

[smity]

This project is completely dead?

[der_fisherman]

This project is completely dead?

How sad if true, I read from the beginning, it was really interesting and informative.....
Is Mr King still here?

regards

Andy