Just started to build MPP

[egonw3]

Better (pulse widths seems correct), but still secondary sample delay is too short:

[Qiaozhi]

I think I've found a mistake: R11 is 2,2k and should be 22k...

Aha Sherlock! You've beaten me too it.
I was compiling a reply at the same time, with a suspicion that R11 was the culprit.

[Qiaozhi]

Better (pulse widths seems correct), but still secondary sample delay is too short:
[ATTACH]32126[/ATTACH]
[ATTACH]32127[/ATTACH]

The secondary pulse delay looks fine to me. It appears to be ~120us. Don't forget that the secondary delay is triggered on the falling edge of the main sample pulse, so (in your first image) you need to measure the delay time from the rising edge of TP7 (which is the same as measuring from the falling edge of TP5). In theory the delay should actually be 100us, but component tolerances could easily extend this to the measured value of 117us. In fact, I've puposely increased the secondary delay time on my MPP to 200us.

[egonw3]

OK. I've changed C10 from 10nF to 15nF and now I have secondary pulse delay ca 185uS.

How does the secondary sample delay influence behavior of the detector? I gues that longer secondary sample delay will reduce power consumption but what are the other effects of such a change?

[Qiaozhi]

OK. I've changed C10 from 10nF to 15nF and now I have secondary pulse delay ca 185uS.
[ATTACH]32133[/ATTACH]
How does the secondary sample delay influence behavior of the detector? I gues that longer secondary sample delay will reduce power consumption but what are the other effects of such a change?

The idea of the secondary pulse is take a sample a long way from the decay curve, where any eddy currents from a nearby metal target have essentially died away. Since the Earth's magnetic field will be present in both the main and secondary samples, subtracting the secondary from the main will extract the Earth field, leaving only the target information. Obviously you don't want the secondary sample to be too close to the decay curve, as this will weaken the target signal. Likewise, it must not be so far away that it overruns the TX pulse period when the delay (reject) pot is at maximum.

[egonw3]

So main sample pulse width and secondary sample pulse width determine time when the detector listens for target signal that is send to integrator and having too short sample pulse width results in less sensibility and less accuracy?

[Qiaozhi]

So main sample pulse width and secondary sample pulse width determine time when the detector listens for target signal that is send to integrator and having too short sample pulse width results in less sensibility and less accuracy?

A wider sample pulse width will effectively increase the gain, but the signal-to-noise ratio will be degraded. The settings you choose to use on a PI detector are always a compromise. I've successfully used a 100us main and secondary sample pulse width in a Hammerhead variant, but you can end up digging some enormous holes.

[6666]

I've successfully used a 100us main and secondary sample pulse width in a Hammerhead variant, but you can end up digging some enormous holes.

What was at the bottom of the hole, a target ?

[Qiaozhi]

What was at the bottom of the hole, a target ?

Most of the time yes, unless you give up digging.

[turboaleks]

MPP pcb from Silverdog is out of stock. Does anybody know when its available again?

[Qiaozhi]

MPP pcb from Silverdog is out of stock. Does anybody know when its available again?

I've just been in contact with Silverdog, and the boards are out of stock.

So ..... it's time to create a REV-D version.

Please post your requests for any changes. The changes will need to be small, as I'm completely snowed under with other work. Please don't ask for a ground balance version, as no-one has bothered to test Davor's idea, and it would just take too much time to implement and test at the moment.

So far [off the top of my head] I think we have the following requests:

1. Make resistor and capacitor pad sizes larger. If anyone has a particular size in mind, then please say.
2. Add a diode in series with the MOSFET. Do we really need this? Has it been confirmed in practice that this has a positive effect?
3. Add the synchronizing capacitor for the LT1054.

Is there anything I've forgotten? I'll give you a few days to think about it.

[Chet]

I've just been in contact with Silverdog, and the boards are out of stock.

So ..... it's time to create a REV-D version.

Please post your requests for any changes. The changes will need to be small, as I'm completely snowed under with other work. Please don't ask for a ground balance version, as no-one has bothered to test Davor's idea, and it would just take too much time to implement and test at the moment.

So far [off the top of my head] I think we have the following requests:

1. Make resistor and capacitor pad sizes larger. If anyone has a particular size in mind, then please say.
2. Add a diode in series with the MOSFET. Do we really need this? Has it been confirmed in practice that this has a positive effect?
3. Add the synchronizing capacitor for the LT1054.

Is there anything I've forgotten? I'll give you a few days to think about it.

Hi Qiaozhi

I have the series diode installed and it does work the same as demonstrated by green below.

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.

Thank you,
Chet

[green]

Hi Qiaozhi

I have the series diode installed and it does work the same as demonstrated by green below.

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.

Thank you,
Chet

A chart I posted in another thread awhile back. Shows being able to sample sooner, but loose some signal strength do to loss in voltage with the same power supply. Each coil and circuit would have different delay times, but I think the results would be similar. A higher PS voltage would have less effect on signal loss.

[egonw3]

OK - I've just finished Step 7. What i've noticed is that for main sample delay 22us I have ca 250mV at TP4. When I adjusted R6 to have 500mV at TP4, I have main sample delay ca 12us. On this setting detector seems to react better for placing target in coils range (is more sensitive).

[egonw3]

OK. After removing some "jammers" (CFL bulb above table etc) it is closer to 100mV for 22us main sample delay. For 500mV i have ca 10us sample delay.
Which setting is better for now?