Waveforms + Measurements

[moodz]

Eureka ! I am surprised and delighted at this result.

Hi WM6,
See picture to understand what we want the pulse to look like.

I removed the 220pF bypass cap and all the 1n4148 diodes in preference for a fast switching heaver current type ( 32 amp impulse / 10 pF reverse ). I have been "tuning" for want of a better word the damping response of the circuit and the traces below are the results.

Below is the output from the circuit ... note the "dip" after the flyback.

Now lets expand that a bit below.

Theres kind of some wave form in there ??

Now see below the waveform response for my trusty dollar coin centred in the coil.
The purple reference trace is NO TARGET
The white trace is TARGET.
Over two volts of response !!!! ( amp gain is 100 )
This response is rock solid and repeatable. I can move the coil and the response changes then restores when moved same position again. There is no jitter on the waveform ... it is rock solid with respect of the timing of the tx pulse / flyback. So presumably a detector will only have to listen in this small window. Of course I have not tested for ground effect etc etc however this result is amazing !!




moodz

[moodz]

The request to Moodz. Before the measurement always place the oscilloscope track exactly on the zero. Himself will avoid like this misunderstanding.
I have simple solution of this problem which I use for a long time. This is short DC correction pulse in the end flyback. I will send schematic when I will draw him.
Mrand

Thanks and noted .... I am used to to just positioning it anywhere and taking relative ... however in this case note that the output does not decay to zero volts. It decays to analogue ground which in this circuit is the battery +ve 12 or so volts. I dont have the fancy differential probe so I can reference the signals to ground. However I will try to be more attentive to initiial positioning of the trace.

Regards,

moodz.

[Mrand]

The request to Moodz. Before the measurement always place the oscilloscope track exactly on the zero. Himself will avoid like this misunderstanding.
I have simple solution of this problem which I use for a long time. This is short DC correction pulse in the end flyback. I will send schematic when I will draw him.
Mrand

[J_Player]

Thanks for explanation KingJL.
It would be great if you and others from design group join this great moodz development.
With faster decay you mean something like this?:

Hi WM6,
See picture to understand what we want the pulse to look like.

[KingJL]

Thanks, even clarifying expert J_P cannot give me better guidance. Hope I am back in phase now.

You are welcomed

So possible problem with present status of discussed construction is in saturation of RX amplifier during fly-back which take too long time (we wish to get 10us or less instead of present ~30us).

Problem isn't the saturation. The differential op-amp in question has excellent recovery charactaristics. There is not really a problem (depends on what you goal is). The coil (as described and verified in the actual measured waveforms) has such a large inductance that the decay is extended considerably. Also contributing to extended decay is an abundance of capacitance. Like I said, it is not a problem if your goals are achieved.

What is then term "pulse delay"? I this the same as H=> TX pulse duration, or it is equal to point of sampling (or sampling delay)? Last question, I hope, in field of signal components terms.

Not sure of your context of "pulse delay". Usually in reference to PI designs, the term refers to the delay of the sample pulse after TX turn-off. In some designs (multi-pulse) it refers to the delay between the multiple TX pulses.

[WM6]

The way I see them :
A=> start of flyback (end of TX)
B=> saturation during flyback
C=> coming out of saturation during decay
D=> ending of decay
F=> quiescent level (fully decayed)
G=> start of TX pulse
H=> TX pulse duration

Kind regards,
J. L. King

Thanks, even clarifying expert J_P cannot give me better guidance. Hope I am back in phase now.

So possible problem with present status of discussed construction is in saturation of RX amplifier during fly-back which take too long time (we wish to get 10us or less instead of present ~30us).

What is then term "pulse delay"? I this the same as H=> TX pulse duration, or it is equal to point of sampling (or sampling delay)? Last question, I hope, in field of signal components terms.

[moodz]

The way I see them :
A=> start of flyback (end of TX)
B=> saturation during flyback
C=> coming out of saturation during decay
D=> ending of decay
F=> quiesent level (fully decayed)
G=> start of TX pulse
H=> TX pulse duration

Kind regards,
J. L. King

Thanks for this analysis and your comments JL & WM .... this last sequence is correct as I understand it. The ADC starts a sample run from C to G ideally at 1 sample each usec.

regards, moodz.

[KingJL]

Thank you KingJL.

Evidently I need some term of part signal explanation for my "inglish" English.

Is this correct (regarding by letters marked parts of signal on attached picture)?:

A => rising pulse
B => pulse delay (pulse duration)
C => point of pulse decay (point of decay)
D => pulse decay
F => fly-back pulse duration
G => fly-back nulling
H => waiting time to next pulse (waiting time), waiting Period

I would be very thankful for help in this term clarification.

The way I see them :
A=> start of flyback (end of TX)
B=> saturation during flyback
C=> coming out of saturation during decay
D=> ending of decay
F=> quiesent level (fully decayed)
G=> start of TX pulse
H=> TX pulse duration

Kind regards,
J. L. King

[WM6]

Not really. The base that you are indicating (the negative section of moodz display) is actually the value during TX. The decay goes to the proper base, but takes 25-30usec to get there (the 25-30usec positive section of moodz display). Remember you are seeing the decay amplified by G=100. For the 1st 20-25usec it is limited at the positive rail by the amplifier being in saturation (all this is normal), thus appears as a positive pulse. In a faster decay the apparent pulse would be shorter as in the attached waveform (the attached waveform does not show the TX pulse as it is sync'ed to start at TX turn-off and is not of long enough duration to show the next TX pulse).

Thank you KingJL.

Evidently I need some term of part signal explanation for my "inglish" English.

Is this correct (regarding by letters marked parts of signal on attached picture)?:

A => rising pulse
B => pulse delay (pulse duration)
C => point of pulse decay (point of decay)
D => pulse decay
F => fly-back pulse duration
G => fly-back nulling
H => waiting time to next pulse (waiting time), waiting Period

I would be very thankful for help in this term clarification.

[KingJL]

Thanks for explanation KingJL.
It would be great if you and others from design group join this great moodz development.
With faster decay you mean something like this?:

Not really. The base that you are indicating (the negative section of moodz display) is actually the value during TX. The decay goes to the proper base, but takes 25-30usec to get there (the 25-30usec positive section of moodz display). Remember you are seeing the decay amplified by G=100. For the 1st 20-25usec it is limited at the positive rail by the amplifier being in saturation (all this is normal), thus appears as a positive pulse. In a faster decay the apparent pulse would be shorter as in the attached waveform (the attached waveform does not show the TX pulse as it is sync'ed to start at TX turn-off and is not of long enough duration to show the next TX pulse).

[WM6]

One thing that the waveforms show is that the under-damped RX wants to resonate at ~110kHz. That somewhat explains the slow decay of ~25usec. The 22 cm diameter coil with 34 turns of twisted pair connected as described results in a total RX inductance of ~2.2mH (68 turns). This combined with a combined circuit capacitance of ~650pf-750pf (MOSFET 330pf, C1 220p, 5 41418 diodes total 20pf, coil 80pf (?), cable 50pf(?) calculates to ~120kHz which corresponds with what the waveforms tell us.

I am excited to explore the possibilities, but I think we need to get a faster decay to be a viable PI design.

Thanks for explanation KingJL.
It would be great if you and others from design group join this great moodz development.
With faster decay you mean something like this?:

[KingJL]

Decay

One thing that the waveforms show is that the under-damped RX wants to resonate at ~110kHz. That somewhat explains the slow decay of ~25usec. The 22 cm diameter coil with 34 turns of twisted pair connected as described results in a total RX inductance of ~2.2mH (68 turns). This combined with a combined circuit capacitance of ~650pf-750pf (MOSFET 330pf, C1 220p, 5 41418 diodes total 20pf, coil 80pf (?), cable 50pf(?) calculates to ~120kHz which corresponds with what the waveforms tell us. I think moodz has done some great work and has advanced the PI technology discussion. I do like the concept of the differential front-end and I am excited to explore the possibilities, but I think we need to get a faster decay to be a viable PI design. I have been toying with a negative supply variant that exhibits ~175pf total capacitance and 800uH RX (effective 212uH TX) inductance that results in 12.5us-13us total decay (10mv @ 12us after amplification G=100). As I get the opportunity and time, I am planning to rig this and test it.
Great work, moodz!

[WM6]

Suppose that different fly-back shape signals on last 4 scope picture you get from different damping R value but need some explanation?

[moodz]

variable damping

Substitute this circuit for the main 1K damping resistor. ( ends of 330 ohm resistors )
I used 3 x 1k resitors in parallel.
VPH is the +VE bias voltage input to 78L05 reg on the diff amp supply.
Note this does add some parasitic cap which will add a few usec to flyback.
The pot is 10K however not that critical.



the installation .... neat huh ?

various settings of the pot are obviously changing the damping value.

[Carl-NC]

This could be an overdrive glitch from the amp; current-feedback amps often do this. As long as you're inside the range of the ADC at first sample, I wouldn't worry about it right now.