My Surf PI Null Offset on Pin 6 of 5534 is different when measured with multimeter & oscilloscope

[Polymer]

I am getting my Surf PI ready for testing with oscilloscope in the sea to see what happens when dunked.

I had adjusted the null a while back with a multimeter when I had no oscilloscope.

Now looking at the trace of Pin 6 I saw that the null is below zero by a whopping 320mV.

It also works the other way round. Adjust null with oscilloscope and have round 320mV on multimeter.

What am I not getting here? Does anyone have an idea what may be causing this?

I am really puzzled.

[kt315]

you measure impulse signal on DC range of your tester. yup? that is nonsense.
you also can not measure impulse signal nor in DC range neither AC range of your tester.

[green]

I am getting my Surf PI ready for testing with oscilloscope in the sea to see what happens when dunked.

I had adjusted the null a while back with a multimeter when I had no oscilloscope.

Now looking at the trace of Pin 6 I saw that the null is below zero by a whopping 320mV.

It also works the other way round. Adjust null with oscilloscope and have round 320mV on multimeter.

What am I not getting here? Does anyone have an idea what may be causing this?

I am really puzzled.

The voltmeter reads the average signal which includes the offset when the coil is on and decay after coil turn off. You are reading the null after the decay with the scope.

[kt315]

first question to you. who from the geotechers did give you the suggestion to use voltmeter for null tuning?
mine? George? Carl? Eric Foster???
WHO?

[Polymer]

The voltmeter reads the average signal which includes the offset when the coil is on and decay after coil turn off. You are reading the null after the decay with the scope.

Thank you green!

Of course ... how could I have not seen this

[green]

first question to you. who from the geotechers did give you the suggestion to use voltmeter for null tuning?
mine? George? Carl? Eric Foster???
WHO?

Don't know about the Surf PI but step 5 of the Geotech Baracuda (RevA) build document suggests measuring the DC volts at amplifier out when adjusting null.

[Polymer]

first question to you. who from the geotechers did give you the suggestion to use voltmeter for null tuning?
mine? George? Carl? Eric Foster???
WHO?

Oh dear, kt315. You seem to be in crappy mood right now or what?

I have read a helluva lot on this great website and really appreciate it, warts and all.

From my point of view the information I find here is sometimes conflicting or half right or wrong or
muddled or sloppily expressed or the question interpreted differently etc. ... such is life.

Here's one of many examples I found (I will get to the DMM after):

"Measure the DC voltage on U2 pin 6, and adjust R6 to give 0V"

Who wrote this doesn't matter.

For me this is ill-defined in the face of a rookie detector builder. It is not defined as what to use for measuring.
An expert would know how to measure this properly. It presumes a certain experiential/knowledge base.

Since it's DC voltage I somehow automatically thought I'll use the DMM. I had nothing better at the time anyway.
This has most likely happened to others too.

So, what can we do with this?

On this forum I have read that you CAN adjust the offset with only a multimeter.
Certain things have to be done first apparently. I believe this to be true. Have not tried it myself.

Since you are more experienced with electronics, may I suggest that you put a clear reply here
which leaves no amateur second guessing. Make it easy to find. I hope I did use the right keywords in this thread.

This will save a lot of repeated questions and you getting sick of them ...

for example: "How to measure and adjust DC offset of pin 6 5534 pre-amp to null"

A.) With a Multimeter
B.) With an Oscilloscope

I think you get what I mean.

I'm grabbing a beer now ... Prost!

[kt315]

Don't know about the Surf PI but step 5 of the Geotech Baracuda (RevA) build document suggests measuring the DC volts at amplifier out when adjusting null.

let me look on this document. link. it seems I missed something important in my life.

[Qiaozhi]

For me this is ill-defined in the face of a rookie detector builder. It is not defined as what to use for measuring.
An expert would know how to measure this properly. It presumes a certain experiential/knowledge base.

Since it's DC voltage I somehow automatically thought I'll use the DMM. I had nothing better at the time anyway.
This has most likely happened to others too.

It does actually say in the introduction of the Build Document: "Please note that it is highly recommended you have access to an oscilloscope during the setup procedure. Although it is not impossible to successfully build a working unit without one, you will need to be very careful and methodical during the construction. However, it is mandatory that you possess a multimeter."

As far as measuring the DC offset at the output of the preamp, it should be fairly obvious that you will need an oscilloscope to do this, unless you disable the TX oscillator first. The problem is that the signal at the preamp output varies with time, even if the coil is disconnected.

And "yes", the Build Document does assume a certain level of knowledge, otherwise where do you start? As I've stated before, it is not recommended that anyone should start with a metal detector design as their first electronics project.

You also have to remember that prior to the Geotech Baracuda REV-A, the original Baracuda details were a complete mess, riddled with design errors and reliability issues.

[green]

Does it make a difference if null is adjusted using a voltmeter or a scope? Does it work better if a scope was used? A scope may not be needed to build a good kit project, but probably is necessary to see how it works.

[Qiaozhi]

Does it make a difference if null is adjusted using a voltmeter or a scope? Does it work better if a scope was used? A scope may not be needed to build a good kit project, but probably is necessary to see how it works.

The output of the preamp periodically hits the supply rail (i.e. the output saturates) before settling to zero. When you adjust the null-offset you're trying to get the 'settled' voltage close to 0V. However, if you measure the output with a DMM, the voltage will be skewed by the flyback period. Some designs actually work better if the preamp output does not settle to 0V, but is offset by maybe 1 volt or so. You can only really understand what's going on if you use a scope, otherwise it's more or less trial and error.

Personally I wouldn't even attempt to build a metal detector design without a scope. It's like trying to work with one hand tied behind your back.

[homefire]

O Scopes NEVER LIE ! LOL, Well almost never.

[kt315]

On this forum I have read that you CAN adjust the offset with only a multimeter.

yup. but... do you attentionally read mine?
i write to DISCONNECT mosfet and dumping resistor then do measurement. LOOK DETAILS.

[Polymer]

On this forum I have read that you CAN adjust the offset with only a multimeter.

yup. but... do you attentionally read mine?
i write to DISCONNECT mosfet and dumping resistor then do measurement. LOOK DETAILS.

WHERE can I find yours? I would read read it with much awareness if I could only find it. Is the coil connected or not?

I found this looking:With your multimeter, you need to adjust the offset pot to give 0v DC on pin 6 of the ne5534

http://www.geotech1.com/forums/showt...ter#post147461

This is not about pointing fingers, but hopefully finding a better way to deal with all the different qualities of information here.

[Polymer]

It does actually say in the introduction of the Build Document: "Please note that it is highly recommended you have access to an oscilloscope during the setup procedure. Although it is not impossible to successfully build a working unit without one, you will need to be very careful and methodical during the construction. However, it is mandatory that you possess a multimeter."

As far as measuring the DC offset at the output of the preamp, it should be fairly obvious that you will need an oscilloscope to do this, unless you disable the TX oscillator first. The problem is that the signal at the preamp output varies with time, even if the coil is disconnected.

And "yes", the Build Document does assume a certain level of knowledge, otherwise where do you start? As I've stated before, it is not recommended that anyone should start with a metal detector design as their first electronics project.

You also have to remember that prior to the Geotech Baracuda REV-A, the original Baracuda details were a complete mess, riddled with design errors and reliability issues.

Hello Qiaozhi,

I am not knocking the Geotec Baracuda at all and I appreciate that the mess around the original has been cleared up.

On one side you have electronics proficient people interested in building detectors.
On the other side you have people interested in building detectors with little or no electronics knowledge.
And everything in between of course. I would take a guess that the latter group is bigger.

I see this as being a huge straddle with such a wide group, with you and other Gurus in the middle.
Surely this is not an easy task. As you were saying - where do you start?

When I am pointing something out, please don't see it as criticism.

I am just hoping to maybe find new ways with more clarity or simplicity,
from the viewpoint of a rookie.