Surf PI Pro LTSpice Sim Decay Curve

[rickodetrader]

Hi Guys,

I am currently in the process of building a Surf PI Pro and waiting on a few components to show up from eBay so in the mean time I decided to run a simulation to see what is going on in the Rx section. A lot of you will recognize the front end schematic (hopefully I have drawn it correctly). So to help us noobs out I am looking for confirmation of the simulation oscilloscope trace. It is a zoomed in version of the decay curve. The green is the raw curve which spikes to -400V taken on the top connection of the coil (after mosfet). The red is output from opamp.

Is this what the real thing looks like or something like it?

From what I can tell it is in 1uS increments. Could someone please point out where a first sample would usually take place and perhaps for how long?

I am having a go at building an arduino PI system on the side using an IRF740 to make a kind of open source easy to build/program detector. Using an old dodgy oscilloscope so it's not the best at getting up close to verify any results I get. DSO is in the pipeline to cure this but for now it's a matter of getting something simple to work (surf pi).

Any help interpreting this would be excellent

A link to some other good posts with oscilloscope decay curves would be great too if it's not pushing the friendship

Thanks guys,
rickodetrader

[Davor]

It would be a tad easier to say if you attached your project files here and spare us the trouble. Just zip them so that the forum accepts them as attachment.

The Rx decay would be much more meaningful if some target is simulated as well, so just connect a coil and a resistor in parallel and one side to the ground so that LTspice does not complain. For values see some topics with simulations. Then just give your target some coupling with PI coil and you are there. Alternatively you may .step param resistance in a target to see what happens with different metals.

[rickodetrader]

Hi Davor,

Here you go. It's the LTSpice .asc and the others. Not sure what you mean by coupling (1pF capacitor?)

Really just looking for an idea of what a proper decay curve should look like after it's been clamped, so if one were to get experimental in the future they would know they're on the right track... Have seen the hammerhead project but just wanted more references.

Thank for the reply

[Davor]

I'll have a look. On the first glance, there is too much time given for the simulation, but it is of no importance here. I'll make some additions and post it here soon.

[rickodetrader]

Davor -> Thank you. I am getting a grounding of what to expect to see, so if strange things pop up on the real oscilloscope, I can assume something is malfunctioning. LTSpice has a bit of a learning curve but is an awesome program!

[Davor]

So there are a few minor mistakes, and one that must have caused lots of confusion: 1m = 1 miliohm but 1meg = 1 megaohm. You must be extra careful with gain here because it is directly related to the delay of feasible samples.

Try the corrected circuit

[rickodetrader]

Thank you Davor,

That is a big help. I have checked it out and have to learn what's going on. Will attempt to complete the rest of the schematic. I just completed the Surf PI Pro for real and have a few questions about how it runs so this will be good to verify it's output.

Really thank you for the help Davor, good on you

rickodetrader

[Davor]

You beat me to building a unit, so two points go to you.
When in doubt - ask!