Dual coil for mpp

[green]

This formula,
http://www.freepatentsonline.com/7994789.pdf

Thanks, looks a lot better. Rd=sqrt(L/(pi*C))=sqrt(136uH/(pi*3.9pf))=3332R, close to 2948R (Rd=pi*L*SRF)

[green]

Hi godigit,
A test looking at MPP amplifier out TP3. dbanner suggested damping the circuit Rd first, then the small coil. Tried, damping small coil first worked better for me. Maybe you could try. When I did the test yesterday I wasn't paying attention to TP3 trace, can see oscillation so don't need to look at coil with a loop. Might be cheating, I see a small oscillation that goes away when I touch my finger to +batt(ground). Coil is shielded but connection wires are exposed so I'm thinking it's ok to touch ground for the test. Doesn't effect coil oscillation. You are getting a lot lower Rd than I do, maybe you could try a similar test and see where we are different. Our coils test similar for inductance and SRF, I'm lost why we ae so much different. I'm ready to send the coils and tester to see what you get with my coil if we can't think of anything else to try first. Played with adjusting a pot when selecting Rd, used fixed resistors for the scope traces. Fixed resistors or pot look the same.

[Carl-NC]

4.7k for the inner damping may very well be correct. The coils are decently coupled so the main Rd does most of the work, the inner Rd does residual work.

Ideally PRF has no effect on Rd. Except that things heat up with high PRF and could have an effect.

Don't get too wrapped around the axle trying to get everything to come out mathematically perfect. The equation for the damping resistor is based on a parallel RLC circuit. What we have is a very messy distributed circuit that we simplify and lump into an RLC circuit. The solution for this is just a starting point for Rd. It is expected that you will need to hand-adjust things to get it right, where 'right' is looking at the preamp output and dialing in to a critically damped (or slightly overdamped) waveform. With the DF coil, this means some back & forth with two Rd's.

[dbanner]

1 I'm really at a loss to account for the high Rd on whites inner coil. Green is getting good results with much lower Rd.

2 All the various formulae for Rd seem to be mere approximations, the math is more convoluted and involves a lot more variables. Such as internal R and Q factor at resonance. (Not to mention the in circuit parasitics and loads)

3 Zeta is the damping factor. when Z=1, the circuit is said to be critically damped, and will just be short of oscillating.
Zeta is defined as how fast the transient response (ringing) will fade, divided by the angular resonant frequency.

Now what actually happens at resonance in an LC circuit( I'm referring to the ringing oscillations)?
This is where the impedence is at maximum relative to frequency!!!!( referred to as peak resonance).

The Value of Rd for critical damping (Zeta=1) is simply taking the same value maximum impedance at peak resonance and shunting the LC circuit.
That's it!!!! Nothing hard about that.

The problem is in trying to calculate this value by using only L and C and forgetting about the small R and other variables such as Q.(think about high frequencies) These formulas are oversimplified and varying according to the practical damping application being sought, in our case, a PI coil.

So I stick with Carl's recommended formula, and Green's adaptation for dealing with uH and mHz. Cause my head is not good for complicated Math.

[dbanner]

4.7k for the inner damping may very well be correct. The coils are decently coupled so the main Rd does most of the work, the inner Rd does residual work.

That's what I've been proposing all along. Hence the high value for inner coil Rd on whites coil.
I proposed damping with the external Rd as primary damping and the inner Rd for any residual damping requirements.

[godigit1]

post 118 what a properly damped coil looks like.


Good luck to you both!!!

[green]

post 118 what a properly damped coil looks like.


Good luck to you both!!!

I agree your trace is better than mine. I can get mine to look like yours with lower damping resistance but it flattens out near 10us not 5us like yours. Only way I can get near 5us is to underdamp. My question, what am I doing wrong?

[dbanner]

post 118 what a properly damped coil looks like.


Good luck to you both!!!

Yes of course it looks fantastic.
But isn't anyone curious as to why the HUGE disparity between 528 Ohms and 4.7K ohms? Surely the whites coil can't be that much different from the self made coils you guys made.

Why don't somebody just put a 4.7K on their inner coil and adjust External Rd while looking at the output of preamp on scope. Just to see what happens?

[Qiaozhi]

Here's my twopenn'orth contribution to this thread:

According to the White's patent, with multiple coils in series with the pulse source, the inner coil is isolated from the pulse source by the inductance of the outer coil. Clearly this is incorrect, as the same current flows in both coils since they are in series. Also, because of the coil configuration, there is an interaction between the coils due to their mutual inductance. It would probably be more correct to say that the inner coil is "partially" isolated by the outer coil, such that it requires a separate damping resistor to prevent ringing caused by its own inductance and self-capacitance.

In addition, the patent states that the total inductance of the double coil is equal to the summation of inductances of the two coils. This is definitely incorrect, as it does not take into account the mutual inductance resulting from electromagnetic coupling between the two coils. The total inductance in practice will be a value that is higher than simply adding together the inductances of the two coils, as they are cumulatively coupled. The correct calculation is:

where M is the value of the mutual inductance.

If we take green's measured values as an example, where , and ,

then you can see that , which is less than the measured value.

In this case, calculating the mutual inductance value:



We can now calculate the coupling coefficient between the two coils from:

Rearranging for k:



As k < 0.5, the two coils are therefore loosely coupled.

The patent also states that the damping resistor can be determined from:



where this comes from, I've no idea, as the correct equation is:



However, it is worth pointing out that , so there is a small(ish) discrepancy between the two equations. I doubt the difference has anything to do with making an allowance for the mutual inductance, as it predicts a higher value of damping resistor than the correct equation.

As a sanity check, let's take the TDI external damping resistor value of 680R (as stated by Carl) and work backwards. Let's also assume that the total coil inductance is ... (a reasonable assumption):

The total coil capacitance is then given by:



and the self-resonant frequency is:



OK - let's now look at the values supplied by godigit1:

He used two 135uH coils with a total inductance of 309uH.
Inner coil: 135uH, 108pF, 2R9, 1.316MHz
Outer coil: 135uH, 83.8pF, 3R3, 1.496MHz
Total coil: 309uH, 30.516pF, 5R6, 1.639MHz

Note that the total coil resistance appears to be less than the sum of the individual coil resistances. Don't know why that is, as the total should be 6R2. This is probably a measurement error. Also, 180pF in parallel with 83.8pF is 47.2pF, and not 30.516pF. However, neither of these are important, as an LTSpice simulation will only need to know the individual coil values.

Repeating the above calculations using godigit1's values results in M = 19.5uH and k = 0.144.
Also note that godigit1's dual field coil is a spider format, and not jumble wound, hence the much higher self-resonant frequencies.

Now, let's calculate suitable values for the inner and overall damping resistor values:

Inner damping:

Outer damping:

Interesting ...
Unless I've done something horribly wrong, it looks like the outer (external) damping resistor needs to be a higher value than the inner one, and with a calculated value of 559R, this looks awfully close to 470R.

Next we need to do an LTSpice simulation to see if this makes any sense:
Firstly I ran a simulation without any damping resistors and measured the frequency of the oscillations, which was found to be 1.49MHz.

From:

So far we're not a million miles out from godigit1's measured values.

Then:

Setting the external damping resistor to 1.447k and the inner damping resistor to 559R, results in a perfectly damped signal at the preamp output.

What can we conclude from this? It appears that the overall (external) damping resistor needs to be a higher value than the inner one. Both calculations and simulations indicate that this is true. Although the photo of the insides of the dual field coil seem to show a 4.7k resistor, this is in some doubt. To me, it looks like it must actually be 470R.

[green]

Here's my twopenn'orth contribution to this thread:

According to the White's patent, with multiple coils in series with the pulse source, the inner coil is isolated from the pulse source by the inductance of the outer coil. Clearly this is incorrect, as the same current flows in both coils since they are in series. Also, because of the coil configuration, there is an interaction between the coils due to their mutual inductance. It would probably be more correct to say that the inner coil is "partially" isolated by the outer coil, such that it requires a separate damping resistor to prevent ringing caused by its own inductance and self-capacitance.

In addition, the patent states that the total inductance of the double coil is equal to the summation of inductances of the two coils. This is definitely incorrect, as it does not take into account the mutual inductance resulting from electromagnetic coupling between the two coils. The total inductance in practice will be a value that is higher than simply adding together the inductances of the two coils, as they are cumulatively coupled. The correct calculation is:

where M is the value of the mutual inductance.

If we take green's measured values as an example, where , and ,

then you can see that , which is less than the measured value.

In this case, calculating the mutual inductance value:



We can now calculate the coupling coefficient between the two coils from:

Rearranging for k:



As k < 0.5, the two coils are therefore loosely coupled.

The patent also states that the damping resistor can be determined from:



where this comes from, I've no idea, as the correct equation is:



However, it is worth pointing out that , so there is a small(ish) discrepancy between the two equations. I doubt the difference has anything to do with making an allowance for the mutual inductance, as it predicts a higher value of damping resistor than the correct equation.

As a sanity check, let's take the TDI external damping resistor value of 680R (as stated by Carl) and work backwards. Let's also assume that the total coil inductance is ... (a reasonable assumption):

The total coil capacitance is then given by:



and the self-resonant frequency is:



OK - let's now look at the values supplied by godigit1:

He used two 135uH coils with a total inductance of 309uH.
Inner coil: 135uH, 108pF, 2R9, 1.316MHz
Outer coil: 135uH, 83.8pF, 3R3, 1.496MHz
Total coil: 309uH, 30.516pF, 5R6, 1.639MHz

Note that the total coil resistance appears to be less than the sum of the individual coil resistances. Don't know why that is, as the total should be 6R2. This is probably a measurement error. Also, 180pF in parallel with 83.8pF is 47.2pF, and not 30.516pF. However, neither of these are important, as an LTSpice simulation will only need to know the individual coil values.

Repeating the above calculations using godigit1's values results in M = 19.5uH and k = 0.144.
Also note that godigit1's dual field coil is a spider format, and not jumble wound, hence the much higher self-resonant frequencies.

Now, let's calculate suitable values for the inner and overall damping resistor values:

Inner damping:

Outer damping:

Interesting ...
Unless I've done something horribly wrong, it looks like the outer (external) damping resistor needs to be a higher value than the inner one, and with a calculated value of 559R, this looks awfully close to 470R.

Next we need to do an LTSpice simulation to see if this makes any sense:
Firstly I ran a simulation without any damping resistors and measured the frequency of the oscillations, which was found to be 1.49MHz.

From:

So far we're not a million miles out from godigit1's measured values.

Then:

Setting the external damping resistor to 1.447k and the inner damping resistor to 559R, results in a perfectly damped signal at the preamp output.

What can we conclude from this? It appears that the overall (external) damping resistor needs to be a higher value than the inner one. Both calculations and simulations indicate that this is true. Although the photo of the insides of the dual field coil seem to show a 4.7k resistor, this is in some doubt. To me, it looks like it must actually be 470R.

Thanks for the interesting reply.

Why does your simulation take over 50us to decay to zero volts when godigit's takes 5us? gain X1000, change gain to 30 takes maybe 10us?

SRF numbers godigit posted reply#192

In greens post #105 ill use as a example and my post #136 we come up with very similar #s for the different methods of loop pick up.
SRF
6.7MHz____PI coil exciting small coil, loop pickup
3,87MHz___small coil connected to Tx with diode, loop pickup
2.82MHz___small coil connected to Tx with diode, x10 probe across coil
550kHz____small coil connected to Tx no diode, x10 probe across coil
1.3MHz____PI coil exciting small coil, x1 probe across small coil

Why did you use SRF for the small coil measured with a X1 probe across the coil?

3MHz_____ PI coil exciting small coil, x10 probe across small coil

[dbanner]

What can we conclude from this? It appears that the overall (external) damping resistor needs to be a higher value than the inner one. Both calculations and simulations indicate that this is true. Although the photo of the insides of the dual field coil seem to show a 4.7k resistor, this is in some doubt. To me, it looks like it must actually be 470R.

Yes, I agree. IT MUST BE A 470 Ohm RESISTOR.
No way it can be 4.7K.

Everything I said before was premised on it being a 4.7K, which baffled me, and as you have shown, it is an impossible value. 4.7K does not make sense. Maybe someone at Whites put in wrong resistor by mistake, not unheard of.
Jose claimed it was a 4.7K resistor, but when I went back and read the thread, he said it "appeared" to be 4.7K
So it looks like the resistor was never actually measured with an ohmmeter
Sorry for all the muddling.

[Qiaozhi]

Why does your simulation take over 50us to decay to zero volts when godigit's takes 5us?

The time between when OSC turns off, and the preamp comes out of saturation, is 9us.
If you do the same measurement for TX, then it takes less than 8us to drop to 700mV.

Why did you use SRF for the small coil measured with a X1 probe across the coi

I used the values that godigit1 posted in a different post. It's the one where the values are listed in a table.

No attempt has been made to "tune" the values in the simulation. I just used the ones from the calculations.

[green]

I agree your trace is better than mine. I can get mine to look like yours with lower damping resistance but it flattens out near 10us not 5us like yours. Only way I can get near 5us is to underdamp. My question, what am I doing wrong?

Still trying to figure out what I'm doing wrong. Another test looking at MMP amplifier out TP3.
from godigit's reply #118 https://www.geotech1.com/forums/atta...9&d=1570936662

Don't see much if any effect at TP3 with small coil Rd open or 500R. Increasing external Rd causes overshoot to increase. My damped signal almost as good as godigit's except my takes about 10us, his 5us to come out of saturation. Any suggestions why or what I might try?

[Qiaozhi]

gain X1000, change gain to 30 takes maybe 10us?

It takes 13us to decay to 0V from switch-off if the preamp feedback resistor is reduced to 30k.

By the way, I'll have to let you all mull over this, as I'm going to be away for several days from tomorrow with intermittent internet connection.

[green]

The time between when OSC turns off, and the preamp comes out of saturation, is 9us.
If you do the same measurement for TX, then it takes less than 8us to drop to 700mV.


I used the values that godigit1 posted in a different post. It's the one where the values are listed in a table.

No attempt has been made to "tune" the values in the simulation. I just used the ones from the calculations.

How low does Tx have to drop to for the MPP amplifier to come out of saturation?