500 nano second sampling

[Tinkerer]

So it is in the end simply a protoboarding problem.

Yes, so I learned something about the importance of proper layout, when working with pulses of 10Amp current.

Many thanks to all for the good advise.

The foil is several pieces of about 2" square, to obtain a target TC of somewhere 1us to 2us. The Opamp gain is about 8.

[Davor]

...Added a 0.8uH ferrite, but this increased the oscillation. Then added a AVX 10uF decoupling very near the Mosfet and then a 0.47uF tantalum next to it and that did it.

So it is in the end simply a protoboarding problem.

[Chet]

Hi Tinkerer

That is a good response for a 1 meter coil on foil. How big is the foil?

Thank you,
Chet

[Tinkerer]

2.5us sampling

thanks for the help everybody.
Making progress.

Yellow trace end of TX, 9 Amps peak coil current, took air signal, so it was moving a bit, but you can see the effect of HF damping.
Blue-white trace, no target
Red trace, foil target.

Now I need to draw the schematic and see what I actually have here. Added a 0.8uH ferrite, but this increased the oscillation. Then added a AVX 10uF decoupling very near the Mosfet and then a 0.47uF tantalum next to it and that did it. Still got the first snubber in place, so need to sort out what the real snubber is.

[Chet]

Hi Tinkerer

Here is an interesting paper on printed circuit board layout.

Have a good day,
Chet

[Davor]

...The real problem that still exists, is the Mosfet oscillation at the end of the TX pulse. Adding a ferrite close to the Mosfet lead might help. I will try.

I have added a snubber for that HF oscillation. It has helped some, but it is probably not well calculated. This is where I want to concentrate my next effort.

I managed to simulate oscillations at the end of Tx pulse before, and it goes away with inductances under 1uH in source lead. For that particular reason I mentioned sticking a ferrite bead there. I also played with a simple snubber and achieved a flat top flyback. It prevents avalanching, and simultaneously increases the integral under the flyback curve ... as simple as better.

[Tinkerer]

Do you mind saying what Mosfet your using?, or have you tried a different one?

At present the Mosfet is a IRFP460. Peak Flyback voltage is under 500V. The circuit is soldered on a perf-board. Once I have the PCB ready with a proper high voltage layout I will change the Mosfet to a C2M0160120, 1200v Mosfet and push the flyback to about 1000V.

I know that air wires and bad layout are causing noise and instability, so I am trying to address and understand each source of noise so that I can design a better PCB.

[Thomas]

Do you mind saying what Mosfet your using?, or have you tried a different one?

[Tinkerer]

Now in retrospect, I think my recommendation to use a ferrite bead on source lead was misunderstood. I meant to literally put a ferrite bead on a MOSFET source lead.
The other thing I can think of regarding the supply line is, in case you are using a LDO or 78xx regulator, or any fast regulator, to never use low ESR or low inductance elkos as post-regulator filter, as these tend to cause a regulator to oscillate. It is an odd effect, but well documented at DIYaudio.

Thanks for the help.

The noise on the initial picture had several different causes.
A silly mistake of mine, was to use a 12" ground lead on the scope probe. It made it easy to reach the test points without changing the probe ground clamp, but it is amazing how much noise 12" of straight wire can pick up.
Another problem was the noise picked up by the battery and other wires. The most worrisome noise being on the battery negative terminal - system ground. The ferrites and bypass capacitors did help.

Filtering the signal, LP at 1MHz, helps with the noise, but increases the delay. However, it is not clear yet how much the target signal loss it causes, because the LP filter reduces the very early peak signal of very short TC targets, but also integrates the signal, so that the integrated signal, 3us integration, may show little loss.

The real problem that still exists, is the Mosfet oscillation at the end of the TX pulse. Adding a ferrite close to the Mosfet lead might help. I will try.

I have added a snubber for that HF oscillation. It has helped some, but it is probably not well calculated. This is where I want to concentrate my next effort.

[Davor]

Now in retrospect, I think my recommendation to use a ferrite bead on source lead was misunderstood. I meant to literally put a ferrite bead on a MOSFET source lead.
The other thing I can think of regarding the supply line is, in case you are using a LDO or 78xx regulator, or any fast regulator, to never use low ESR or low inductance elkos as post-regulator filter, as these tend to cause a regulator to oscillate. It is an odd effect, but well documented at DIYaudio.

[Tinkerer]

2.5us sampling at 9 Amps

Cleaned up a bit.
Added some ferrites and AVX capacitors to power supply line.
Added HF snubber to TX, but it is not perfect yet as you can see by the Mosfet oscillations at the end of the TX pulse.

Yellow trace, end of TX
Red trace - foil target with a TC of about 2us. Trace at input to ADC
blue-white trace no target

This is with the 1meter diameter TX coil and a coil current of 9 Amps.

[Tinkerer]

Hi Tinkerer, Haven't had time to work on my noise problem. Hope to soon. Wondering how you are coming with yours.

Hi Green,
making progress, just using standard procedures, but still working on it. Will try to show scope picture later today.

[green]

Hi Tinkerer, Haven't had time to work on my noise problem. Hope to soon. Wondering how you are coming with yours.

[Tinkerer]

I'm no hardware guru. but we had a similar problem on out medical device. The flyback from the transformer created enough noise that coupled to an i/o line that it was triggering and simulating a push button. I.E. it was enough that the processor would see it as a button press. The solution was a cap and resistor to ground to make a filter. Perhaps a small inductor and cap and resistor in the power supply line, creating a low pass filter with a cut off in the khz range would block the noise?

I have managed to get the STM32F411 development platform running, and have compiled and debugged a test program. Currently I am working on the ADC and will be building the LCD display as well. I've been going through a personal crisis the last two weeks. Looks like I will be permanently batching it, the wife left two weeks ago. Right now I am scrambling to get everything done. Way more work than normal there....

I admire your persistence. This STM32F411 is giving you a lot of work. I hope to have my circuit clean and optimized by the time you are ready to sample it with your 2MHz sampling ADC.

All the best

[Tinkerer]

...and maybe a ferrite bead on source lead?

Yes, about the ferrites. On the PCB, I added a few, but actually I just don't know enough about the use of ferrites. I need a good primer to study.
Today I re-routed some leads on the perf-board, added a ferrite on the source lead and some ceramic capacitors where they might help.
There is a lot of improvement in the noise level, but I think it can be improved a lot more.
Thank you all for the help.