Raptor RX coil and phase shift

[dbanner]

An algebraic construct can be derived from the relationship between the RCL of the TX and RX, proportional representation to 360°, voila! phase displacement.

[rickb]

dbanner!

The level of ignorance and just plain BS you have displayed over the last 48 hrs is beyond belief. Are you drinking and smoking dope 24 hrs a day? Most of what you have written above is utter nonsense! Your credibility is now non-existent. Most of your technical knowledge about the inner workings of metal detector circuitry can be written on the head of a pin in 12 point type. Most of the misinformation in this thread and the 5534 thread is coming from you.

[Qiaozhi]

The questions posed by Richisdorfite are interesting ones.

With reference to Gwil's reply:

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

In the past, many members have asked how to determine the correct resonant frequency for the RX coil, given a specific TX frequency. The usual responses have been to either use an RX resonant frequency that is a certain percentage of the TX, or simply to make it a fixed number of kHz lower than the TX. In both cases, these are simply guesstimates. If you run a small-signal ac analysis in SPICE using the TX and RX coil setup for Raptor, this indicates an expected phase-shift of ~17°. Using the Raptor TX oscillator and RX preamp, the phase-shift is closer to 30°. The difference is most likely due to the fact that the simulation models are linearized around the operating point prior to running a small-signal analysis. Hence why it's called "small-signal". Therefore it is possible to get an approximation from SPICE for the required phase-shift that you wish to use.

In practice there are other "parasitics" that can affect the final phase-shift. For example, layout and cable capacitance, component tolerances, and variations in coil winding and shape.

I also tried varying the coupling coefficient between the coils to test its effect on the initial phase-shift. Although the phase-shift does vary, it is much less pronounced than occurs in practice. So something else is going there that's not accounted for in the simulation.

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?

I added a target signal to the time domain simulation, and could clearly see the target phase-shift used for discrimination and target ID. The amount of phase-shift varied according to the time-constant of the target. Changing the RX resonant frequency not only affected the initial phase-shift, but also affected the shift presented by the target. Which would explain why target IDs vary between metal detector manufacturers.

As explained in Chapter 10 of ITMD, the physical relationship between the TX and RX coils does affect the initial phase-shift, as well as the residual RX voltage. The discrepancy between theory and practice (with regard to SPICE) appears to have something to do with the idealized inductor models being used in a tranformer application, which is essentially the case here. Simply changing the coupling coefficient does not have the same effect as coupling the two coils with a third coil that represents the target. A typical coupling coefficient value is 1m, which produces a small residual voltage in the RX coil. This matches fairly well with reality. However, varying the coupling coefficient is not the same as mechanical alignment of the TX and RX coils. At the end of the day, the required initial phase-shift can be estimated (either by hand calculation or using SPICE), but mechanical tweaking is required to achieve the final result.

There are known limitations in SPICE when using ideal inductors in transformer applications, especially when non-linear magnetics are involved. Hence why there are whole books written on the subject which use IsSpice, PSpice, HSpice, Spice3, etc., that work around these issues by using non-linear dependent sources.

This is the reason why it's advisable to use a commercial coil for your detector projects, at least in the initial stages of understanding the design. Once you have a good working unit, then by all means experiment with building your own coils. This (of course) applies mainly to VLF designs, whereas PI coils impose much less stringent design requirements.

[Richlsdorfite]

I will make some network analysis to caracterize the real complex impedance of TX and RX and to determine the potential phase shift between TX and RX and, perhaps, IC1.

I already measure the RX individual components (Inductance & Capacitor = LC Tank) and measure TX complex impedance.

I will provide you the final measurements but you will find, attached to this post, the following data:

- TX complex impedance
- RX_Inducatance complex impedance
- RX_Capacitor complex impedance
- RX complex impedance

RX_TX.zip

[Gwil]

A question for the gurus - How would a detector like the Raptor behave if the null signal was reduced to zero?

[Qiaozhi]

A question for the gurus - How would a detector like the Raptor behave if the null signal was reduced to zero?

You will have major trouble trying to ground balance the detector if you reduce the null signal to zero.

[Gwil]

I'm afraid I remain unconvinced by this "null signal phase" idea. It's clear that the tuning and Q of the receive coil affects the phase of all the received signals, and therefore influences the design of the phase shifting circuitry that supplies the reference (switching) signals to the synchronous demodulators. However the null signal is just another signal, and can't affect the phase of any of the others.

It must be significant that published designs for IB detectors using sync. demods never say anything about the phase of the null signal, only that it should be made as small as possible.

Two examples on this website -

(1) Don Bowers' TGSL building instructions:

"Power on the TGSL and adjust coil overlap while monitoring U101a, pin 7 with a DVM in the A.C. setting. The goal is to find a sharp null in the voltage between U101a pin 7 and the PCB ground. You should be able to get the signal down to about 4mV but the actual reading may be subject to the frequency response of the DVM used."

(2) Andy Flind's PE Magnum:

"Coil adjustment is actually not as critical as it is for a normal IB machine [i.e. a detector that simply responds to signal amplitude] but there is a best point and for a GEB machine [i.e. a detector using synchronous demodulation] it is the position where absolute minimum residual amplitude output (and maximum phase shift effect) is obtained from the pickup coil"

As I said, no mention of null signal phase.

[Hyena]

I'm afraid I remain unconvinced by this "null signal phase" idea. It's clear that the tuning and Q of the receive coil affects the phase of all the received signals, and therefore influences the design of the phase shifting circuitry that supplies the reference (switching) signals to the synchronous demodulators. However the null signal is just another signal, and can't affect the phase of any of the others.

It must be significant that published designs for IB detectors using sync. demods never say anything about the phase of the null signal, only that it should be made as small as possible.

Two examples on this website -

(1) Don Bowers' TGSL building instructions:

"Power on the TGSL and adjust coil overlap while monitoring U101a, pin 7 with a DVM in the A.C. setting. The goal is to find a sharp null in the voltage between U101a pin 7 and the PCB ground. You should be able to get the signal down to about 4mV but the actual reading may be subject to the frequency response of the DVM used."

(2) Andy Flind's PE Magnum:

"Coil adjustment is actually not as critical as it is for a normal IB machine [i.e. a detector that simply responds to signal amplitude] but there is a best point and for a GEB machine [i.e. a detector using synchronous demodulation] it is the position where absolute minimum residual amplitude output (and maximum phase shift effect) is obtained from the pickup coil"

As I said, no mention of null signal phase.

Hi Gwill, I bet you never made coil for any of Tesoro detector. Deepest null has VERY poor dept performance

Your examples are meaningless, because Dons note about 4mV is not about DVM, it is real residual voltage. After first opamp voltage will be about 200mV (I have very good results with 120mV - will be 2,4mV on coil). Check this with scope and you will see what is noise and what is signal from Tx.

And Magnum on steroids is completely different case...

I guess that phase shift is result of combination of inductance of coils, shielding material, null voltage... How can Don tell you anything about phase shift when he measure this only with DVM? But from all above, you can not just tell, that phase shift does not affect result. It will move range of GEB and DISC. Is it something you can omit in design?

[Koala]

I seem to remember when making a DD one side of the null point didn't work as well as the other.

To wide 200mv - best null 10mv - 200mv too narrow?

also seem to remember the null altering when applying the shield to the top but came back to normal when I applied the shield to the bottom


It was several years ago and I may be wrong

[Hyena]

I seem to remember when making a DD one side of the null point didn't work as well as the other.

To wide 200mv - best null 10mv - 200mv too narrow?

also seem to remember the null altering when applying the shield to the top but came back to normal when I applied the shield to the bottom


It was several years ago and I may be wrong

Yes, I have similar experience. Underlap make 200° phase shift and it is bit more sensitive to ferous targets. Overlap produce phase shift 20° and is more sensitive to precious metals.

[manhunt47]

Yes, I have similar experience. Underlap make 200° phase shift and it is bit more sensitive to ferous targets. Overlap produce phase shift 20° and is more sensitive to precious metals.

Hello Hyena
What did your metal detector test do?
What was the frequency of the transmitter, as well as the resonant frequency of the receiver portion RX؟؟