Persistent bug in PI circuit design.

[Teleno]

An aspect of PI design has been stuck in the same place since 70's. I'm talking about the series R-D combination for probing the decay signal at the coil.

Examples:










This design suffers from many drawbacks:

- the R-D acts as a delay line because of the diode's capacitance, this adds several usec. to the earliest possible sampling time.
- the damping resistor is lower than it should be because it's in parallel with R during most of the transient (when D is conducting). This adds further usec. to the decay (and sampling) time.

As a result, sensitivity and signal/noise is horrible.

The use of this deficient solution to this day shows the lack of commitment by competent designers to PI technology. Where are all the brains?

[mickstv]

Here's the frontend I've been using for a while. Need to redraw it at some stage.

[Teleno]

You replace R by a controlled switch SFT1440. The gate-source capacitance is large, the rising edge of the switching pulse will inject a sizeable charge into the source, which charges the capacitance of diode 1N4148. This charge will slowly decay. In fact you're adding a second transient to the coil's own transient and won't be able to sample a meaningful signal until this added transient decays. Whether you're better off in the end with this design remains to be seen.

See "Clock Feedthrough and Channel Charge Injection" here.

[mickstv]

Works fine for me, easily able to sample at 8us and has less noise than the normal RxD method.

[Teleno]

Works fine for me, easily able to sample at 8us and has less noise than the normal RxD method.

I believe 8us is achievable with your solution. That leaves a lot of room for improvement though.

[mickstv]

Here's a pic at the output of the amp.



TX is powered with 8 volts and is 120us wide into a 0.5ohm ML coil. The first sample can be taken at 8us.

[golfnut]

i have been using some devices used for esd protection of exposed pins etc. the capacitance is approx 1pF unclear if good or bad for this

[kt315]

Here's the frontend I've been using for a while. Need to redraw it at some stage.

your diagram has (HAS!!!) THE DIODE on input of opamp. why you tell your way is BETTER????????????????????????????

[ivconic]

An aspect of PI design has been stuck in the same place since 70's. I'm talking about the series R-D combination for probing the decay signal at the coil.

Examples:










This design suffers from many drawbacks:

- the R-D acts as a delay line because of the diode's capacitance, this adds several usec. to the earliest possible sampling time.
- the damping resistor is lower than it should be because it's in parallel with R during most of the transient (when D is conducting). This adds further usec. to the decay (and sampling) time.

As a result, sensitivity and signal/noise is horrible.

The use of this deficient solution to this day shows the lack of commitment by competent designers to PI technology. Where are all the brains?

Yes that's old traditional approach. Simple way to limit and protect front end.
In meantime different ideas appeared too.
Just one example; PI detector with two separate coils, separate coil for TX and separate coil for RX. Now, the RX part is done in different manner.
No need for resistance and also no need for clamping diodes if RX coil is projected in a way to attenuate induced current by it's morphology (less induction). Or balanced with RX coil in a way to achieve less induction.
Now, such two-coils approach is open for more ideas, special ultra fast clamping diode still can be presented there since it will not produce delay as in traditional approach.
In the old days PI detector supposed to work with longer delays and extra 8-10uS doesn't meant a thing. Irrelevant.
Now in these days majority of diyers tend to short the delay as shorter as possible to be able to catch bits of signal under 10uS. Somebody says; "there is the gold"! So that's what provoked quite new fashion of "under 10uS" run.

[mickstv]

your diagram has (HAS!!!) THE DIODE on input of opamp. why you tell your way is BETTER????????????????????????????

Well if I change out my circuit for the traditional resistor + diodes frontend I can't sample any sooner than 12-13us.


If you think you have built something better, post it here so we can see.

[kt315]

something better was (HAD) released by Eric Foster in his '10 Ohm' coil projects. with high resistance coil possible to sample earlier on decay.

seems you do not manage the matter.

The first sample can be taken at 8us. -- I can take sample on 1us and WHAT? we say and MEAN THE stability -!
with your 8us you have to demonstrate us the stability in A FIELD.

[mickstv]

something better was (HAD) released by Eric Foster in his '10 Ohm' coil projects. with high resistance coil possible to sample earlier on decay.

Yeah and if it's so great, everyone would be using it to detect in highly mineralized ground.

The first sample can be taken at 8us. -- I can take sample on 1us and WHAT? we say and MEAN THE stability -!
with your 8us you have to demonstrate us the stability in A FIELD.

That makes no sense at all.

BTW my detector can detect targets in highly mineralized ground, have found gold as small as .2g

BTW I don't have to demonstrate anything to you, because you already know it all.


Once again if you have something better why not post it here for people to see.

[kt315]

FOR PEOPLE TO SEE - SIC!

are you able read my posts??????????????? goldquest and beachscan ARE ERIC'S DESIGNS!!!!!!!!!!!!!!!!!

The major changes since the CS6PI is the ability to sample at 10uS, the addition of a variable SAT control, and a switchable two stage autotune filter, plus you can use DD coils for all but highly mineralised ground.

[mickstv]

are you able read my posts??????????????? goldquest and beachscan ARE ERIC'S DESIGNS!!!!!!!!!!!!!!!!!

The major changes since the CS6PI is the ability to sample at 10uS, the addition of a variable SAT control, and a switchable two stage autotune filter, plus you can use DD coils for all but highly mineralised ground.

If you think it's so good, bring one out to Australia and see how it goes.

If that those detectors are so good, why are people using ground balance designs to cancel mineralized ground then having to put up with loss of depth due to electronic detection holes ?



BTW stop trolling threads with your, "I'm better than everyone else" crap.

[kt315]

stop trolling YOU! AT FIRST. not me is better, better is THE EXPERT IN PULSE INDUCTION. ERIC!