spiral pi coil

[green]

Hi godigit, Interesting tests. I haven't tried a figure 8 mono, just the figure 8 Rx with oval Tx. Very small nuggets are one of your desired targets? I'm interested in how short a delay you can get the MPP. I've been thinking amplifier TC should be close to coil TC. MPP has a 1us TC?

Think I calculated MPP TC wrong. Remembered I started a thread on amplifier response awhile back https://www.geotech1.com/forums/show...542#post216542 Trying to remember what I learned from that thread.

[godigit1]

Think I calculated MPP TC wrong. Remembered I started a thread on amplifier response awhile back https://www.geotech1.com/forums/show...542#post216542 Trying to remember what I learned from that thread.

Best Ive been able to get the Mpp to do is 5us and its a bit chatty at that . It goes blank at 3 us if I remember right.
The stock desigh 5534 op amp has a bandwidth of 10 Mhz. Ive been using Max 412 for my pre-amp it has a bandwidth of 28 Mhz.
I remember a thread that stated PPs as twice the Frequency meaning at 2500 KHz you are actually at 5,000pps. If that is the case Ive had the Mpp at 5000Khz so 10'000pps. Biggest problem is the sample window becomes so small its hard to get good samples.
this is my ground balance test board so I have 4 samples to squeez in and still try to get a meaningful EFE sample .

Another reason I had to step away from the Op amp upgrade is I needed to learn more as well. Once again its hard to find MD Specific tutorials. This gentelman has many interesting vidieos and this one on op amp bandwidth I think is good information .
https://www.youtube.com/watch?v=UooUGC7tNRg .

Ill have to check out your thread I think I missed that one.

I have a question about Srf Mainly the Wave form.
Can we tell how fast the coil is by how fast the SRF wave form returns to Zero? What information can we derive from the wave form itself about the coil?
Ive noticed a variation in time to zero as much as three times between some of the coil types ive tested . Both the Figure 8 coils Ive tested have the longest time to Zero. And seem to equalize and stretch out the wave form.
What Is better a short fast resonence or longer smoothed out one?

Curious how the time to zero will differ on your larger deep coil or if it will be faster. And I also wonder what your SRf wave form on your Rx section looks like compared to the mono eights I have? The current I think circulates a different way for your two coils. Do you have any old shots by chance or a thread maybe where you allready posted some shots?
Thanks Again.

[Qiaozhi]

Best Ive been able to get the Mpp to do is 5us and its a bit chatty at that . It goes blank at 3 us if I remember right.
The stock desigh 5534 op amp has a bandwidth of 10 Mhz. Ive been using Max 412 for my pre-amp it has a bandwidth of 28 Mhz.
I remember a thread that stated PPs as twice the Frequency meaning at 2500 KHz you are actually at 5,000pps. If that is the case Ive had the Mpp at 5000Khz so 10'000pps. Biggest problem is the sample window becomes so small its hard to get good samples.

Frequency is the same as pulse rate.
Also, I doubt you're running your MPP at 5000kHz (i.e. 5MHz). You probably mean 5000Hz (or 5000 pps).

[green]

I have a question about Srf Mainly the Wave form.
Can we tell how fast the coil is by how fast the SRF wave form returns to Zero? What information can we derive from the wave form itself about the coil?
Ive noticed a variation in time to zero as much as three times between some of the coil types ive tested . Both the Figure 8 coils Ive tested have the longest time to Zero. And seem to equalize and stretch out the wave form.
What Is better a short fast resonence or longer smoothed out one?

Was hoping someone that knows would answer the question. Think you should think decay cycles not decay time. I think the formula for cycles, for amplitude to decay to 37% of start is(cycles=pi*coil Q). Think SRF determines coil speed. We critical damp the coil, don't think how many cycles it takes to decay when measuring SRF matters unless it effects the SRF reading. Probably still wouldn't matter because we select Rd value for critical damping for circuit resonance not coil resonance.

I'll see what I have for decay with my coil. Think decay rate is controlled by coil Q.

[green]

It's late. replied twice

[green]

I have a question about Srf Mainly the Wave form.
Can we tell how fast the coil is by how fast the SRF wave form returns to Zero? What information can we derive from the wave form itself about the coil?
Ive noticed a variation in time to zero as much as three times between some of the coil types ive tested . Both the Figure 8 coils Ive tested have the longest time to Zero. And seem to equalize and stretch out the wave form.
What Is better a short fast resonence or longer smoothed out one?

Was hoping someone that knows would answer the question. Think you should think decay cycles not decay time. I think the formula for cycles, for amplitude to decay to 37% of start is(cycles=pi*coil Q). Think SRF determines coil speed. We critical damp the coil, don't think how many cycles it takes to decay when measuring SRF matters unless it effects the SRF reading. Probably still wouldn't matter because we select Rd value for critical damping for circuit resonance not coil resonance.

I'll see what I have for decay with my coil. Think decay rate is controlled by coil Q.

It was late, cycles=Q/pi

[green]

I also wonder what your SRf wave form on your Rx section looks like compared to the mono eights I have?

[godigit1]

Hi Green,

Thanks for the wave forms.
Pretty much what I was thinking I would see. Your Rx coils coil behave the same way as the Eights I have.
That is Im seeeing a longer more seems more even voltage drop. Thinking this is due to the way A eight cancels if I could compare it to a twisted pair in that it evens out the end signal.
I have to ask Are the two channels both sides of your Rx Coil or is it Tx ch1 and Rx ch2?

I was thinking there would be a slight time difference between the two RX coils hitting SRF together. Your wave form is stepped between Channel 1 and 2. Rise and fall times are same but one waveform lags. Not sure if im looking at tx or both Rx coils though. If its both Rx coils then I think thats what im seeing.

Im wondering if that extra time is there if it helps to stimulate the target longer?

Here is a picture of the wave form for the eight planar I did.
Long decay time


The other picture below is the TDi folded mono.
Ive hooked it both shield and non shield connected and no matter the way Srf seems to be locked in Im wondering if there is a cap inside Maybe?

[sinclairuser]

hi guys, thanks for all the info and testing it makes interesting reading iv'e built and tested some more coils, i will post some results and pics tomorrow.
aly

[green]

I have to ask Are the two channels both sides of your Rx Coil or is it Tx ch1 and Rx ch2. Both sides of the Rx coil, they are opposite phase. If I flip the CH2 pickup loop over they are in phase. The Rx coils are the same(balanced) so the frequency is the same. With Tx leads open it also oscillates but at a different frequency causing a beat signal(right picture).


I see you are connecting the test coil to your Tx circuit. Have you tried connecting the scope probe to it's ground lead making a loop, laying the loop on the test coil and position the test coil near a operating PI coil to excite the test coil? If you try, the test coil ground lead should to be connected to one of the test coil leads since the test coil is floating. I have forgotten more than once and have connected it to circuit common(wrong).

[green]

A 5 ohm, 300uH coil at 1MHz should have a Q of about 375. When measuring SRF, cycles(for amplitude to decay to .37 of start)=Q/pi. (375/pi=119). The measured decay is a lot less cycles. Is it due to measurement method or Q at SRF isn't a simple calculation? What should Q be at SRF?

g3rbj.co.uk/wp-content/uploads/2014/07/Self-Resonance-in-Coils. suggests Q at SRF is a lot less than at 1/2 of SRF.

[green]

Hi godigit, have you tried storing the scope screen in bitmap, paste in Paint and save as a jpeg file?

[godigit1]

I have to ask Are the two channels both sides of your Rx Coil or is it Tx ch1 and Rx ch2. Both sides of the Rx coil, they are opposite phase. If I flip the CH2 pickup loop over they are in phase. The Rx coils are the same(balanced) so the frequency is the same. With Tx leads open it also oscillates but at a different frequency causing a beat signal(right picture).


I see you are connecting the test coil to your Tx circuit. Have you tried connecting the scope probe to it's ground lead making a loop, laying the loop on the test coil and position the test coil near a operating PI coil to excite the test coil? If you try, the test coil ground lead should to be connected to one of the test coil leads since the test coil is floating. I have forgotten more than once and have connected it to circuit common(wrong).

Ive been connecting the test cois directly to my scope Test lead to one side ground to the other.
Then Ive been setting the test coil on a operating Pi.
Thats what Ive been doing so far. Ive got a frequency generator but its for audio and square wave only. Ive not been able to get it to work well.
Is the way Ive been doing it wrong?
Ill give using a loop a try when I get back. just got back home for a couple days for the first time in 7 weeks.

On the third scope shot with beat signal.
Im wondering that if we think of the coil as a non terminated transmission line "it is not connected to ground". then would the beat oscillation be what is called mirroring?
That is the signal gets to the end of the line and bounces back at half amplitude. I watched a vidio a while back on this but a square wave was used.
Im not sure if thats it just thinking maybe.

"A 5 ohm, 300uH coil at 1MHz should have a Q of about 375. When measuring SRF, cycles(for amplitude to decay to .37 of start)=Q/pi. (375/pi=119). The measured decay is a lot less cycles. Is it due to measurement method or Q at SRF isn't a simple calculation? What should Q be at SRF?"
Good question.
In the report he is saying a coil useful at 1mhz should have a srf of around 20mhz. I just checked what a 300uh coil at 20 mhz would have a Q of 7536.
Surly this cant apple to pi detector can it?
Once again Im not sure if im overcomplicating things a bit. Most of this applies to very high frequencys. Im not even sure what a normal pi circut is capable of.
Does it just come down to op amp bandwidth and circut impediance?
I thought i had a chart of where Q related to usable frequency but i cannot find it at the moment.

As far as saving my scope shots to paint. I have not tried that yet truthfully i bought the scope set aside a old cro I had and kinda just started using it i havent even watched the intro CD.
Bad Diggit.


.

[green]

Ive been connecting the test cois directly to my scope Test lead to one side ground to the other.
Then Ive been setting the test coil on a operating Pi.
Thats what Ive been doing so far. Ive got a frequency generator but its for audio and square wave only. Ive not been able to get it to work well.
Is the way Ive been doing it wrong?
Ill give using a loop a try when I get back. just got back home for a couple days for the first time in 7 weeks.

On the third scope shot with beat signal.
Im wondering that if we think of the coil as a non terminated transmission line "it is not connected to ground". then would the beat oscillation be what is called mirroring?
That is the signal gets to the end of the line and bounces back at half amplitude. I watched a vidio a while back on this but a square wave was used.
Im not sure if thats it just thinking maybe.

"A 5 ohm, 300uH coil at 1MHz should have a Q of about 375. When measuring SRF, cycles(for amplitude to decay to .37 of start)=Q/pi. (375/pi=119). The measured decay is a lot less cycles. Is it due to measurement method or Q at SRF isn't a simple calculation? What should Q be at SRF?"
Good question.
In the report he is saying a coil useful at 1mhz should have a srf of around 20mhz. I just checked what a 300uh coil at 20 mhz would have a Q of 7536.
Surly this cant apple to pi detector can it?
Once again Im not sure if im overcomplicating things a bit. Most of this applies to very high frequencys. Im not even sure what a normal pi circut is capable of.
Does it just come down to op amp bandwidth and circut impediance?
I thought i had a chart of where Q related to usable frequency but i cannot find it at the moment.

As far as saving my scope shots to paint. I have not tried that yet truthfully i bought the scope set aside a old cro I had and kinda just started using it i havent even watched the intro CD.
Bad Diggit.


.

Not saying what is right or wrong. Looking at your scope traces, I assumed your test coil was connected to your PI because the peak voltage is over 200V. I get mV the way I do it. I get a lower SRF if I connect the scope to the coil vs using a loop to pick up decay.

Think the beat frequency is two coils oscillating at different frequencies, amplitude adds when in phase subtracts when out of phase.

I'm thinking coil Q at SRF isn't important with a PI but not sure. Mostly a learning experience, read a formula for Q for decay in a filter book written by Don Lancaster. Couldn't find my book and posted the formula wrong the first time, correct(Q=pi*cycles to decay to 37% of start amplitude). When I try a spice simulation the formula(Q=pi*cycles) matches(Q=X/R)spice doesn't know it's SRF. Q=X/R, R might not be DC resistance due to skin effect, don't know what all effects Q at SRF of the coil.

Pictures are fine if scope is CRO. I have found it easier to store and paste to paint with a digital scope. Less attachment memory and can have a few scope pictures on one page.

[dbanner]

I can't see any obvious relevance of the Q @ SRF to PI. Except it can be charted in some way to show comparison between different coils, perhaps.
What is relevant, I think, is the Q at PI operating frequency. This for sure is very relevant.