Getting ready to build the MPP

[Silver Dollar]

Look here for a lot of scope shots of my receintly completed Hammer Head; http://www.geotech1.com/forums/showt...s-best-Damping

[KRinAZ]

Hey Chet on your TP3 scope shot - it looks like TP3 is on CH2, set to 5V/Div & 10uS/Div - I set my scope the same and my waveform looks pretty much like yours - my scope shot:

So I'm wondering if my MPP is actually working correctly & I should proceed, even though this differs from the build doc, or ?

I'm going to stop here and see if anyone has feedback on this...

[KRinAZ]

OK I'm stuck at step 5, dang it!.

Here's what I'm seeing - with the multimeter:
The build doc says there should be a:
+1VDC offset at TP3, I have -.36VDC
Small (quarter) target close to coil and level should drop maybe 10mV, I have a rise in voltage to around -.42VDC
Large target close to coil and level should drop to around 150mV, I have a rise in voltage to around -.6mVDC

So TP3's offset is negative instead of positive and behaves opposite of how it's supposed to.

On U5 (NE5532) pin1 (opamp A output) signal is identical to Pin7 (opamp B output), I'm thinking there should be signal gain or some change by the second opamp?

I have re-verified all steps up to this point - power supplies good, TX circuit working.
I re-verified that all components in step 5 are the correct ones (and measured the resistors with my meter to make sure) and in the correct place, no solder bridges, I can't see what's wrong.

What I see with the scope is the waveform of the low voltage version of the tx pulse and the flyback - & I can see the change in the flyback due to a target.

If anyone has some checks or ideas I would sure love to hear them!

By the way, I got rid on the noise at TP3 - it was all the lights & stuff around my work area.

[KRinAZ]

Awesome, many thanks Chet - this seems like just what I'm experiencing. That diode apparently makes an enormous difference in the area we all want to achieve - up-ing the circuit resonance freq - seems like this mod should go into the next rev of the MPP - don't you think?

OK, the only electronic components I have are from gear that I have salvaged and robbed components from. I'll have to look and see what I can find in the way of a high speed high voltage diode. This can be a challenge as diodes can be hard to read the part number on - if they have one...

I also noticed green referenced holding the probe close to the diode to see the signal - and I had the probe connected to the circuit (you asked about that) so I will measure again by just holding the probe near the Tx area.

...& no shielding on the coil or wire leads & really don't want to shield - that's why I'm so keen on these basket weave coils - they are supposed to self-shield...we'll see...

[Chet]

Hi KRinAZ, Attached is a file from green which is an example of your problem. I forgot that I had added this diode to my circuit and it works just like green shows. Here are the reference posts for this;

From Davor; post #396 of Replies-to-Minipulse-Plus
“@Qiaozhi, I may suggest a small improvement for a future release of the board. I placed a MUR460 diode in series with R3 resistor (my choice, not a suggestion), but it turned a bit of a struggle, as the diode's pins are very thick. I had to grind one pin so that it may enter a hole. Having larger holes there could be beneficial, as R3 has thick-ish pins as well.”


From green; post #21 of of N-Channel-vs-P-Channel-MOSFET
“Some real scope pictures of coil turn off. With no diode the circuit resonance drops from 1.64 Mhz to 395 Khz. Would need a lower value damping resistor, longer delay before first sample. Depending on the targets maybe not a problem.”

The circuit green used is somewhat different but shows the polarity of the diode. Most any high speed, high voltage diode can be used. Also forgot to ask if you had shielded your coil. Shielding of course will add more capacitance.

[KRinAZ]

Hi KRinAZ, great work, thank you
Hey Chet, welcome!

.

[KRinAZ]

Hi KRinAZ, Attached is a shot of TP3 with the flyback pulse on top. The frequency of your coil should be much higher. That's whaThe capacitance is to high for it to be the coil only. The cable between the circuit board and the coil may be adding to much capacitance. Also your scope test lead may be paralleling it with capacitance. Try the frequency test again by just laying the probe near the coil circuit so there is no direct connection except the ground clip attached to the Minipulse ground. The 400v signal will couple to your probe through the air just fine.

The high frequency noise at TP3 looks like florescent light, TV or some other similar source. Try turning stuff off to see if you can locate the source.
Are you using a bench power supply? If so try batteries instead.
Thx Chet! I'm using a battery pack to power it, but the room is lit by fluorescent's, the soldering station is near, as is my CRT scope (which I would hope contains it's noise but maybe not), I'll try to isolate as much noise as possible. Thx for the scope shots. I made the coil with 4' of the wire as the leads back to the detector to eliminate using coax or a cable, and put the 3 turns per inch in the wires so I would hope the cable is as low capacitance as it can be. I'll do some more testing this eve and see if I can get a clean signal...

.

[Chet]

I made templates for cutting the Lexan forms for the basket weave coils I'm learning to build - made PDF's of them - scalable - so you can reduce or enlarge them as desired for your coil size. They are intended for 8" x 10" x .091" Lexan sheets, but you can scale it to the size of your project - the wire slots should be cut 1/8" wide and rounded at the inner end - see the Coils/Chance PI Coil... thread for much more info - enjoy

[ATTACH]31551[/ATTACH]
8" x 1.1" Ring

[ATTACH]31552[/ATTACH]
8" x 10" x 1.1" Ellipse

Hi KRinAZ, great work, thank you

[Chet]

Hi KRinAZ, Attached is a shot of TP3 with the flyback pulse on top. The frequency of your coil should be much higher. The capacitance is to high for it to be the coil only. The cable between the circuit board and the coil may be adding to much capacitance. Also your scope test lead may be paralleling it with capacitance. Try the frequency test again by just laying the probe near the coil circuit so there is no direct connection except the ground clip attached to the Minipulse ground. The 400v signal will couple to your probe through the air just fine.

The high frequency noise at TP3 looks like florescent light, TV or some other similar source. Try turning stuff off to see if you can locate the source.
Are you using a bench power supply? If so try batteries instead.

[KRinAZ]

I wonder if someone could confirm this is what TP3 should look like after step 5 of the MPP build completed - or if this is not good if I could get a scope photo of your TP3 - thx!

I'm running the 180uH basket weave coil, just using the 470 damping resistor for now, no ringing in the flyback

The manual says should be about 1VDC offset with no target near coil, I'm seeing about (-.6VDC), with lots of noise making it hard to see the actual offset level, and the Rx pulses also.

For these photos my scope is set to 2V/Div with 0V at the center line & DC coupled, .2ms/Div time base:

No intended target near, but this is in a work area with metal objects everywhere, and lots of nearby electrical noise (I don't have a "cleaner" work area at the moment)

Heavy 4" looped rusted iron wire right against coil

[KRinAZ]

Excellent, thx much Qiaozhi! And this is with R1 470R removed I'm thinking? and preamp output being TP3?

[Qiaozhi]

Could you tell me if that is what I do? - just find the highest resistance that damps any ringing? & if you know could you tell me just what "critically damped" means?

Damping resistor adjustment tool:
10k pot + 200R series resistor, both in parallel with 1k2 resistor.
Adjustment range is between 171R and 1k1.

Critical damping occurs when the receive signal (measured at the preamp output) decays smoothly to zero volts without any ringing or overshoot.

[KRinAZ]

With the 180uH 8" basket weave coil - I calc'd the ring period and got the capacitance via LC resonant circuit calculator - here's what I got:

2.15uS ring peak to peak = 465.1KHz on 180uH coil = 650.1pF capacitance - not very good!

It would appear the wire I'm using is no good, or the coil build is no good, or both LOL

I will proceed with the MPP build and use this coil for now...& make a new coil when I get some different wire...

If anyone uses the templates in post #51 - I would like to find out what wire you used and how the coil came out

[KRinAZ]

Welcome!

[6666]

Tnks for pdf's