Just started to build MPP

[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]

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.

[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.

[egonw3]

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

[Qiaozhi]

I've double checked and C11 is 2n2 (marked 222 = 2200pF).

At TP1 pulse width is 58us so TX Pulse width is correct (here channel A is TP1 and channel B is TP5).
[ATTACH]32121[/ATTACH]
At TP5 pulse width is 22us and this pulse starts when TP1 pulse ends ((here channel A is TP1 and channel B is TP5, trigger on TP1s falling edge).
[ATTACH]32122[/ATTACH]
TP6 according to documentation is equal to second sample delay. TP6 pulse width in my MPP is 117us (here channel A is TP7 and channel B is TP6):
[ATTACH]32123[/ATTACH]
So we can see, that second sample starts 117us after main sample (here channel A is TP7 and channel B is TP:
[ATTACH]32124[/ATTACH]

From your final scope picture, both the main and secondary pulse widths are the same (~8us), but they should be ~58us.
How about the value of R11?

Have a look at the attached datasheet for the MC14538B monostable, and you will see that the output pulse width is defined by Rx * Cx.
On the first page it states that R is in kohms, and C is in uF, which is completely misleading, since R is actually in ohms and C is in farads as far as the calculation is concerned. You can readily confirm this by examination of Figure 5, where Rx=100k and C=0.1uF, yielding a nominal output pulse width of 10ms.

[egonw3]

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

[egonw3]

I've double checked and C11 is 2n2 (marked 222 = 2200pF).

At TP1 pulse width is 58us so TX Pulse width is correct (here channel A is TP1 and channel B is TP5).

At TP5 pulse width is 22us and this pulse starts when TP1 pulse ends ((here channel A is TP1 and channel B is TP5, trigger on TP1s falling edge).

TP6 according to documentation is equal to second sample delay. TP6 pulse width in my MPP is 117us (here channel A is TP7 and channel B is TP6):

So we can see, that second sample starts 117us after main sample (here channel A is TP7 and channel B is TP:

[Qiaozhi]

OK. I've replaced this pot with 470R resistor and soldieret Step 6.
I've set main sample delay at 22us. But i have following readings from my scopometer:

secondary sample delay 117us instead of 160us
main and secondary sample pulse width 8us instead of 58us.

Here are screenshots from my oscilloscope: https://drive.google.com/open?id=0B3...ejg&authuser=0

Any ideas what went wrong?

The main and secondary sample pulse widths of 8us are incorrect. The correct value(s) should be 58us, as stated in the Build Document.

The secondary sample delay has a dependence on the main sample delay. The secondary delay is set by R18 and C10. (i.e. 10k * 10n = 100us) But, you need to add the main sample delay to this value. In your case, it appears that you have this set to 17us. Obviously there will be some variation between builds for the value of 100us due to component tolerances, but the actual result should be close. The value of 160us is not correct. That probably came from either the REV-A or REV-B version.

The main and secondary pulse widths are defined by R11 and C11. (i.e. 22k * 2n2 = 48.4ns) However, due to the presence of D4 and D7, this delay will be extended somewhat. In this instance, the delay should 58us. I suspect you have a 220pF capacitor fitted in place of C11, instead of a 2n2 (2.2nF).

[Chet]

I guess that these incorrect values are because of differences between nominal and real capacitance of C10 and C11.
Should I fit some capacitors to achive correct pulse widths and delays or these values are OK?

Hi egonw3

8us sample gates are fine for now. I think step 6 is incorrect for stating the 58us sample pulse widths. Look at figure #10 for a proportional view of the pulse widths. I have adjusted the value of C11 and R11 for 5us sample gates. For the 160us delay check the values of C10 and R10 for the correct values. 117us is also ok for now.

Regards,
Chet

[egonw3]

I guess that these incorrect values are because of differences between nominal and real capacitance of C10 and C11.
Should I fit some capacitors to achive correct pulse widths and delays or these values are OK?

[egonw3]

OK. I've replaced this pot with 470R resistor and soldieret Step 6.
I've set main sample delay at 22us. But i have following readings from my scopometer:

secondary sample delay 117us instead of 160us
main and secondary sample pulse width 8us instead of 58us.

Here are screenshots from my oscilloscope: https://drive.google.com/open?id=0B3...ejg&authuser=0

Any ideas what went wrong?

[Qiaozhi]

This pot is rated 2W. Do you think it may burn down?

The problem is the flyback from the coil. The pot has a wiper that may arc, just like the points in a distributor of a car. Eventually the wiper contacts will become pitted, and it will breakdown.

[egonw3]

This pot is rated 2W. Do you think it may burn down?

[Chet]

Until Step 3 averything was perfect (1,058 kHz on TP1, pulse width ca. 58us).
I've just finished Step 4 (at least soldiering). I've connecter 1k pot in place of R1 to be able to change it's value. I've made a "temporary coil" that is about 350uH 1,8ohm (basket coil 10x8 made of AWG24 enemalled copper winding wire). I don' know much about osciloscopes, but that's what I got on my old scopometer: https://drive.google.com/folderview?...00&usp=sharing
And here I need your kind assistance: I don't know if it is OK.

Hi egonw3

It looks fine as Qiaozhi has stated. Later go to the attached post and put together a tool to help get to the correct value of damping. The 1k pot may burn out if left in the circuit too long.

Attached are some scope shots without the oscillation/ringing with the correct damping resistor.

Regards
Chet


http://www.geotech1.com/forums/showt...957#post197957

[Qiaozhi]

Until Step 3 averything was perfect (1,058 kHz on TP1, pulse width ca. 58us).
I've just finished Step 4 (at least soldiering). I've connecter 1k pot in place of R1 to be able to change it's value. I've made a "temporary coil" that is about 350uH 1,8ohm (basket coil 10x8 made of AWG24 enemalled copper winding wire). I don' know much about osciloscopes, but that's what I got on my old scopometer: https://drive.google.com/folderview?...00&usp=sharing
And here I need your kind assistance: I don't know if it is OK.

At this stage you can simply set R1 to 470R.
Once you have a working unit, then you can tune the damping resistor (R1) to its optimum value.