Raptor RX coil and phase shift

[Richlsdorfite]

Hello,

I have made the Raptor using breadboard. The goal is to practice using different coils and frequencies.
But I have to understand how RX and TX coils are tuning together.

The book " Inside Metal Detector" (thank you Mr OVERTON & Mr MORELAND) is focused on the electronics but lacks precision about coils tuning (my POV).

I cannot purchase a second hand TESORO coil, I have to make my coils myself and as I want to make my coil myself, I want to understand how RX is tuned to TX to achieve the 200° or 20° phase shift between TX and RX signal.

Assume the resonance frequency of TX (FR_TX) to be 9992Hz (1/[2*pi*root[F*L]] with L=5,9mH and C=43nF), using complex impedance I can say that at FR_TX the impedance |Z_TX| is maximal and the phase arg(Z_TX) is equal to 0.

Assume the resonance frequency of RX (FR_RX) to be 10866Hz and RX resistance equal to 22,41Ω (1/[2*pi*root[F*L]] with L=6,5mH and C=33nF), using complex impedance I have for RX at FR_TX (9992Hz) Z_RX= 861+2382i thus |Z_RX|= 2533Ω and arg(Z_RX)= 70°

Ok I have 70° or (90°-20°), it seems to match with the 20° or 6us indicated page 141 of the 2nd Ed book.

First question: I cannot understand why, page 103, a 200° phase shift is specified. Perhaps 180° + 20 ° but as I compute arg(Z_RX) = 70° a rotation of 90° gives me 90°+70° = 160° not 200°. Do I lack something ?

Second question: I have build one TX coil (5,623mH) and one RX coil (6,126mH), first attempt be indulgent ;-), to tune the RX coil for the 20° shift, it would seem to me logical that I have to find the right capacitor (in this case 34nF) value to achieve these 20°. Is it really logical ? Or is it a fine combination between inductance and capacitance to achieve the better power transfert between TX and RX when something is detected ?


But I took into account that none of inductor, capacitor and resistor are perfect theorical components, and I have measured, using a cheap LCRmeter (Hutale LCR-DDS-F 4.2 bridge meter kit), the impedances of my TX & RX coils and capacitors at 100-250-500-1000-2500-5000-10k-25k-50kHz, using Excel I managed to have some nice trendlines for each of these components. Have |Z| and arg(Z) for each, I was able to compute each complex impendance from 100 to 50kHz so I computed the resonance frequency of TX (10035Hz) and RX (10936Hz) and at FR_TX (10035Hz) :
>>>>>The complex impedance of RX is Z_RX=1117 + 1802i thus |Z_RX|= 2120 and arg(Z_RX)= 58,2° which is very different to the theorical 70° !!!!!
So It seems to be mandatory to measure the complex impedance of the RX coil into a frequency range to perform the adequate capacitor matchning !!!!!

Thank you in advance for your answers and comments !

Best regards,

Michel

[dbanner]

When the Rx coil is tuned off resonance in relation to the tx coil, at null the Rx minimal induced signal is "phase shifted", the amount of phase shift being determined by the off resonance spread between the tx and Rx.
I can't find any explanation on forum of the relationship between amount of "off resonance" and how it affects the overall phase shift of the Rx signal. I have come across no calculations/analysis regrading this aspect of setting up coils in tesoro circuit and similar circuits. So this is interesting question to me.
Experts have to help
Maybe the issue is adequately explained in the book which I am yet to read.

[dbanner]

20+180=200, therefore is a phase shift of 20° effectively the same as 200°? Because the leading edges of the Rx waveform will intersect the tx waveform at 20° and 200°.(assuming sinusoidal)

[Qiaozhi]

I am on holiday for the rest of this week, and will reply to your question when I return.

[Richlsdorfite]

Yes ! Of course ! Thank you !

[Qiaozhi]

Hello,

I have made the Raptor using breadboard. The goal is to practice using different coils and frequencies.
But I have to understand how RX and TX coils are tuning together.

The book " Inside Metal Detector" (thank you Mr OVERTON & Mr MORELAND) is focused on the electronics but lacks precision about coils tuning (my POV).

I cannot purchase a second hand TESORO coil, I have to make my coils myself and as I want to make my coil myself, I want to understand how RX is tuned to TX to achieve the 200° or 20° phase shift between TX and RX signal.

Have a look at Chapter 10, pages 137 to 142. Here there is a description of how to build a DD-coil that is suitable for Raptor, and how to adjust it to get the correct phase-shift between TX and RX.

Assume the resonance frequency of TX (FR_TX) to be 9992Hz (1/[2*pi*root[F*L]] with L=5,9mH and C=43nF), using complex impedance I can say that at FR_TX the impedance |Z_TX| is maximal and the phase arg(Z_TX) is equal to 0.

Assume the resonance frequency of RX (FR_RX) to be 10866Hz and RX resistance equal to 22,41Ω (1/[2*pi*root[F*L]] with L=6,5mH and C=33nF), using complex impedance I have for RX at FR_TX (9992Hz) Z_RX= 861+2382i thus |Z_RX|= 2533Ω and arg(Z_RX)= 70°.

Ok I have 70° or (90°-20°), it seems to match with the 20° or 6us indicated page 141 of the 2nd Ed book.

OK - I see that you must have read the pages mentioned above. However, trying to calculate the phase-shift is complicated because it depends on the physical characteristics of the coil. i.e. the coil configuration and the amount of overlap. Many people make the assumption that it is necessary to achieve the lowest possible residual signal when adjusting the overlap, but this is incorrect. The amount of overlap needs to be adjusted to match the circuit.

First question: I cannot understand why, page 103, a 200° phase shift is specified. Perhaps 180° + 20 ° but as I compute arg(Z_RX) = 70° a rotation of 90° gives me 90°+70° = 160° not 200°. Do I lack something ?

The coil that was used to initially test the Raptor design was a 4" diameter concentric. The TX-RX phase relationship in this coil introduces a 200° phase-shift because the RX signal is inverted. For a DD coil, the phase-shift will be 20°.

Second question: I have build one TX coil (5,623mH) and one RX coil (6,126mH), first attempt be indulgent ;-), to tune the RX coil for the 20° shift, it would seem to me logical that I have to find the right capacitor (in this case 34nF) value to achieve these 20°. Is it really logical ? Or is it a fine combination between inductance and capacitance to achieve the better power transfert between TX and RX when something is detected ?

There may be a method of calculating the resultant phase-shift based on the resonant frequencies of the TX and RX coils, and taking into account the mechanics of the coil construction, but I've never investigated that. Raptor was always intended to be used with a commercial Tesoro coil. However, despite the warnings about the difficulties of building both the electronics and the coil, and trying to get this working properly, we knew that many people would still want to do it anyway. Hence the reason for the section entitled "Step-by-Step Instructions for constructing a DD coil" in Chapter10.

But I took into account that none of inductor, capacitor and resistor are perfect theorical components, and I have measured, using a cheap LCRmeter (Hutale LCR-DDS-F 4.2 bridge meter kit), the impedances of my TX & RX coils and capacitors at 100-250-500-1000-2500-5000-10k-25k-50kHz, using Excel I managed to have some nice trendlines for each of these components. Have |Z| and arg(Z) for each, I was able to compute each complex impendance from 100 to 50kHz so I computed the resonance frequency of TX (10035Hz) and RX (10936Hz) and at FR_TX (10035Hz) :
>>>>>The complex impedance of RX is Z_RX=1117 + 1802i thus |Z_RX|= 2120 and arg(Z_RX)= 58,2° which is very different to the theorical 70° !!!!!
So It seems to be mandatory to measure the complex impedance of the RX coil into a frequency range to perform the adequate capacitor matchning !!!!!

Thank you in advance for your answers and comments !

Best regards,

Michel

I think the discrepancies you are finding between theory and practice are associated with the coil mechanics. It is the coil overlap that has most effect on the phase-shift, rather than the resonant frequencies of the coils.

[dbanner]

I will make a comment on this topic, your explanation of the issue is misinformation . phase shift is the difference between the two resonant frequencies which you failed to explain adequately in your book.

[Richlsdorfite]

Thank you Kiaozhi and dbanner.

I read a lot about "coil coupling", to much documentation ;-)

But it seems that we can assume that the entire coil (TX+RX) is acting as an air core transformer with the less possible coupling factor.

Also, it seems that the inverting amplifier with compensating resistor induces is own voltage shift.

So first, I will isolate the LC circuit from the following amplifier to mesure the voltage phase shift between TX and RX.
Second, I will insert the Inverting Amplifier ( IC1a - CA3240) and measure the shift induced.

So, I expect to be able to calculate the voltage phase shifts induced by:
- the difference between TX resonance frequency and RX resonance frequency
- the common RX & TX ground and shield
- by IC1a

Cross my fingers !

[Gwil]

With no target present, the Rx coil output is a "Null" signal which has a certain phase relative to the Tx phase.

With a target present the Rx coil output is the phasor sum of two signals - the null signal, and the target signal which has its own phase relative to the Tx phase.

If the phase of the null signal alone changes, does that affect the phase of the target signal? Or not?

[dbanner]

The phase response of the target signal is of a consequence of the tx frequency, which is then " phase shifted as a result of the atomic structure of the elements involved.

[dbanner]

The molecules vibrate in " resonential response" to the Electro- magnetic radiation of the tx. Inverting or non inverting opamp has no effect on phase of the Rx signal.

[dbanner]

We are dealing with harmonic relationship between and amongst waves of Electro- magnetic radiation. The third harmonic, the fifth harmonic, the seventh, etc,etc.

[dbanner]

I will post a PDF of the mathematics involved, stay tuned.

[dbanner]

Coil overlap as described by Administrator has absolutely no effect on phase relationship or phase " shift". It is strictly a matter of resonance. Overlap can impact the phase shift, but this is only consequential, the coils are balanced such that the residual voltage is minimal. The extracted signal from the tx oscillator is comparative, so that their use is only an apparent phase relationship to provide a reference for discrimination to occur, or be derived.
It is truly profound that he doesn't grasp such fundamentals.
As a general introduction to the topic, one should read this
https://www.electronics-tutorials.ws...harmonics.html
In other words, off resonance means that the Rx coil is tuned to receive a multiple of the fundamental frequency of the tx. Hence the " apparent" phase displacement.

[dbanner]

When comparing two sinusoids, one either leads(0-180°), or lags(180-360°) the other, the degree to which is determined by the comparative resonance between the two waveforms.
A(t) = Am sin(ωt ± Φ)