Phase delay for different targets

[mikebg]

Hi Mikebg,

This is going back to the VLF TX posts above.

I am working on a hybrid VLF-PI detector.

In the simulation attached, I tried to produce a high power sine wave. At a certain point, with the TX ON and OFF times at the proportion shown, the circuit gets resonant and produces an extremely powerful sine wave.

This only happens with IGBT's, every one of the different simulation models of the MULTISIM software.

The same circuit with Mosfets does not get resonant.

So I built a PCB to test the circuit in real life.

The results are totally different.

Interestingly, I do get very good sensitivity with the attached coil of 450mm diameter. I also get FE discrimination.

Could you help me understanding the problem?

Tinkerer

Tinkerer,
The drive frequency in your SPICE simulation is different from resonance frequency of TX tank circuit. When I tuned them to equal frequency 10kHz, the visual distortion disapeared, despite collector voltage enters in saturation region.

[simonbaker]

Simon, with the below circuit I will try to explain why tuned RX coil creates phase problem in conventional metal detectors.
The conventinal blok diagram uses voltage across TX coil to create by phase shifting one or more reference signals for synchronous demodulators and for compensationn (balance) of AIR and GND signal in RX front end as shown. However the AIR and GND signals are generated by TX current because it creates magnetic field. When we use a ferrite core and a piece aluminium foil for balance, the phase problem of tuned RX coil is solved correct and easy. Remains to use the same principle to obtain reference voltages for synchronous demodulation. Vaino Ronka used for this purpose additional RX channel and automatic balance.

The voltage across self-inductance is in exatly 90 deg phase lead to TX current, however we can not connect to it two wires for reference:-(
The voltage across TX coil is different from voltage across its self-inductance L because contains in addition voltage drop across coil resistance r and imported by ground impedance Z. Note that imported impedance is variable because TX coil moves relative to "earth core".
The same happens with RX coil because near it is moves te same earth core. The earth imports an impedance Z which changes when we alter the height of search coil and when soil properties alter.

But let we see how arises the problem of phase instability in RX coil tank.
To detect deep targets, we should amplify TGT (target) signal much more times than in a conventional metal detector. For high gain, in this circuit are used two stages for amplification of received frequency. The first stage is low noise preamp and preselector. The TGT signal exists in too narrow bandwidth - no more than 16Hz. When front end receives narrow band of frequencies, it receives less interferences and generates less thermal noise. To narrow the band we simply tune capacitance of C1 for resonance to radiated by TX frequency. However, the so tuned resonance of tank circuit shifts the phase of induced EMV by a very steep phase characteristic. Let SPICE display the phase and amplitude characteristic of tuned circuit.

Yes, if I understand correctly, I agree. Using an RX coil whose resonant frequency is the same as the TX oscillator frequency makes it very difficult to control the phase shift of targets -- just a little variation in the oscillator frequency can create a significant change in the expected target phase shift.

Of course, some of us consider this a challenge to try anyway! Maybe find other ways to stabilize the operating point, because there might be big benefits of signal-to-noise if you can operate at resonance. Just something to play with, especially because other people have avoided it.

Note that the TGSL operates quite a distance from resonance, so the phase shift from the tuned circuit is only about 5 deg.

Regards,

-SB

[simonbaker]

Simon, subtraction means difference. For example, the difference between 5 and 3 is 2, ie we subtract 3 from 5 to find the result.
I searched web for the demodulating circuit diagram of White's and for educational explanation of device "difference integrator". Google found several books where this term may be is explained comprehensive, however because of copyrights they are not readable online.

Since I'm not familiar with English terminology in electronics, I used the correct translated term "difference" for subtracting procedure. However, as often occurs in English literature, instead "difference" or subtraction is used the incorrect term "differential". So the name of above circuit is not only "difference integrator", but also "differential integrator". Translated in other European languages, the term "differential integrator" means the nonsense "derivating integrator". When we integrate a function and then differentiate the result, the function remains unchanged, ie when "differential integrator" operates, it makes nothing.
I found the circuit diagram shown above in Application note AN1515 of National:
http://www.national.com/an/AN/AN-1515.pdf
Here is the circuit with formula that shows that is calculated integral of the subtraction:

Yes, but the interesting thing about the switched Synchronous Detector circuit you showed earlier is that the switch essentially acts like a full-wave rectifier (to the locked signal), so the net effect is that the two halves of the detected signal are essentially "added" rather than subtracted as they charge the capacitor. At least that is my impression, I haven't played with the circuit.

I'm just making an obvious point really -- in fact it is the reason you showed the circuit, to show a better SD that utilizes the full period rather than a half-period like the TGSL does. I'm just quibbling over terminology to make sure it's clear that you need to invert one of the detected signals so that the two halves add rather than cancel each other out. This is done by integrating on half the cycle to the top of the capacitor, the other to the bottom of the capacitor (I believe).

No big deal.

Regards,

-SB

[mikebg]

To diminish saturation, I decreased the voltage and duty cycle of drive generator. Channel A of oscope is connected to measure the collector voltage. According datasheet of your transistor, the collector voltage should not enter in region below 2.5V because this is saturation threshold. Channel B of oscope is virtual connected across coil resistance to show the shape of tank current (to see if there is distortion).
For real shape watching of tank current, I'm conecting my oscope to a turn of wire placed near to TX coil and dumped by a 51 ohm resistor.

All types transistors BJT, JFET, MOSFET, IGBT are suitable for your TX. Radio amateurs prefer MOSFETs because they are cheapest for given power and very durable for breakdown and overheating. Your circuit is not energy efficient because there is no "antenna matching". To decrease battery drain , you should use a coil with center tap and larger tank capacitance. This decreases the weight of TX coil and increases efficiency. An automatic control should maintain transistor near to threshold of saturation. I have posted several such circuit diagrams in the forum. They use pulse width modulator for driving and P-I controller to prevent saturation and amplitude modulation.
The TX transistor for SI (sine induction) should work near to point of saturation but never to saturate. When transistor is not saturated, it operates as ideal current source.. The current pump charges LC tank with energy. If TX transistor saturates even for a very small part of oscillating period, this is equal a low ohm switch to discharge tank capacitance to supply rails. Despite this returns energy from tank to battery, this is not preferable because creates harmonics and the large reverse current can destroy TX transistor.
For PI principle, all above mentioned is the opposite. It is preferable transistor saturation because there is no tank capacitor, we have only stray capacitance across RX coil. The MOSFET type also is preferable because operates as low ohm switch and has low saturation voltage. In addition should be noted its specific voltage breakdown operating as high voltage Zener diode (no negative resistance). This is ideal for dumping at flyback maximum.
I will start a thread "TX for metal detectors" in section "General electronics" to discuss TX design problems.

[simonbaker]

To diminish saturation, I decreased the voltage and duty cycle of drive generator. Channel A of oscope is connected to measure the collector voltage. According datasheet of your transistor, the collector voltage should not enter in region below 2.5V because this is saturation threshold. Channel B of oscope is virtual connected across coil resistance to show the shape of tank current (to see if there is distortion).
For real shape watching of tank current, I'm conecting my oscope to a turn of wire placed near to TX coil and dumped by a 51 ohm resistor.

All types transistors BJT, JFET, MOSFET, IGBT are suitable for your TX. Radio amateurs prefer MOSFETs because they are cheapest for given power and very durable for breakdown and overheating. Your circuit is not energy efficient because there is no "antenna matching". To decrease battery drain , you should use a coil with center tap and larger tank capacitance. This decreases the weight of TX coil and increases efficiency. An automatic control should maintain transistor near to threshold of saturation. I have posted several such circuit diagrams in the forum. They use pulse width modulator for driving and P-I controller to prevent saturation and amplitude modulation.
The TX transistor for SI (sine induction) should work near to point of saturation but never to saturate. When transistor is not saturated, it operates as ideal current source.. The current pump charges LC tank with energy. If TX transistor saturates even for a very small part of oscillating period, this is equal a low ohm switch to discharge tank capacitance to supply rails. Despite this returns energy from tank to battery, this is not preferable because creates harmonics and the large reverse current can destroy TX transistor.
For PI principle, all above mentioned is the opposite. It is preferable transistor saturation because there is no tank capacitor, we have only stray capacitance across RX coil. The MOSFET type also is preferable because operates as low ohm switch and has low saturation voltage. In addition should be noted its specific voltage breakdown operating as high voltage Zener diode (no negative resistance). This is ideal for dumping at flyback maximum.
I will start a thread "TX for metal detectors" in section "General electronics" to discuss TX design problems.

Center-tapped coil (which I believe similar to using a higher voltage across coil) is definitely on my list for experiments. Probably dfbowers will get to it first!

I see your point about saturation and distortion. Of course, if you're not saturated you need to worry more about stability of oscillator, some kind of regulator to make sure it doesn't become under-stimulated. You can even get "limit cycles", which would not be nice.

I would like to see analysis of efficiency of transistor in oscillator circuit. Operating in linear region is generally not efficient. That's why I'm still interested in more pulse-driven ideas. Is distortion really bad? Maybe, maybe not?

On the list for future experiments...

-SB

[mikebg]

Simon, the other metod is dumping of RX tank.
I simulated RX tank of Mikronta. It is dumped by 3 resistances:
- resistance of coil r
- resistance of R4 and
- input resistance Hib of transistor circuit common base.
Note that phase characteristic is not so steep in resonance region. Amplitude characteristic also is slant. However that means wide received band, amplification of interferences, more generated thermal noise.

[pebe]

You have chosen different values to those of my 4003.

My measured values are :
L=11.5mH. C=47nF. Series R = 63.4Ω

That resonates at 6.85kHz - (I gave before a figure of around 12kHz, because I used the value of the Tx inductance for calculations, in error).

So It's way off resonance.

[simonbaker]

Simon, the other metod is dumping of RX tank.
I simulated RX tank of Mikronta. It is dumped by 3 resistances:
- resistance of coil r
- resistance of R4 and
- input resistance Hib of transistor circuit common base.
Note that phase characteristic is not so steep in resonance region. Amplitude characteristic also is slant. However that means wide received band, amplification of interferences, more generated thermal noise.

Is "dumping" same meaning as "damping"?

Did you want to label coil as TX in the diagram, or did you mean RX? I'm having some trouble understanding.

Regards,

-SB

[Tinkerer]

Mikebg,

thanks for looking at my simulation.

attached is a simulation using the same parameters that you used. As you see, the results are very different.

I suspect that there is a problem with the simulation setup. Where should I look for the problem?

Now, going back to my first simulation picture. There you see a completely different problem. The simulation "run away". The sine wave starts at very low amplitude and steadily increases to 1084V. This would not be possible in real life, since the IGBT is rated at 600V.

The sine wave is distorted in both of my setups. It was not always like that. Something happened and now it comes out like that.

Then there is the other thing. With real components, IGBT, coil and capacitors etc. and a real Oscilloscope, the output looks very different.

The real components give me quite good depth and sensitivity results this is why I need to find out what the real situation is and where the mistakes in the simulation come from.

Thanks for your help with that.

I will look at your other suggestions, thanks for that also.

Tinkerer

[mikebg]

Is "dumping" same meaning as "damping"?

Did you want to label coil as TX in the diagram, or did you mean RX? I'm having some trouble understanding.

Regards,

-SB

Simon, you are correct! There are two errors in my posting #36. Instead "dumping" should be "damping" and instead "TX coil" should be "RX coil". I will revise more carrefully my drafts before posting them.
Thanks.
Mike-BG

[simonbaker]

Simon, you are correct! There are two errors in my posting #36. Instead "dumping" should be "damping" and instead "TX coil" should be "RX coil". I will revise more carrefully my drafts before posting them.
Thanks.
Mike-BG

Of course we read every word you write!

-SB

[Tinkerer]

PHASE SHIFT WITH PI

Large sheets of very thin alu foil behave in a strange way. while trying to understand the reason for this behavior, I happened on this PHASE SHIFT, maybe this helps answering Pebe's question?

The pictures show the phase shift produced by a 3.5x3.5" very thin alu foil. 3" above the coil.

Tinkerer