Mod TX of Coinmaster, TM808, Eagle

[derick]

Anybody tried this modification?It is possible for XLT?

[mikebg]

Another visual explanation:

[mikebg]

A visual explanation:

[mikebg]

Pebe,
(R)EMI group is abbreviation of “Rediscovering, reinventing, revision, redesign and rename of Electromagnetic induction”. Our hobby is design of TXs, RXs and antennas for QRP amateur radio.
We can design very efficient TXs and extremely sensitive RXs, but for ham radio, not for metal detectors. The combination of TX and RX with an EMI sensor for MD is problem. We published a series of postings in Carl’s forum relative to frequency response of targets and results obtained by SPICE simulation of sensing network.
A metal detector consists of TX, RX and an antenna named EMI sensor or “Coil”. Carl Moreland described one difference between our antennas and EMI sensors for MD. The other difference arises when EMI sensor moves near to ground. This changes antenna impedance.
The answer of your question is in the attached figure:

[pebe]

Peter,
Parasitic modulation means that it is caused by a parasite that absorbs energy from your oscillator. In our case, the parasite is ground or salt water. The closer you are to this parasite, the more power it consumes and the amplitude of oscillation decreases.

Caused by a parasite!!!! You call it parasitic modulation? Any engineer would call it 'loading' of the source.

Both side bands from 0.1Hz to 6Hz are formed due to the change in amplitude, without using non-linear process.

You mean you have sidebands 0.2Hz apart at Ftx - 0.1Hz and Ftx + 0.1Hz ?

How do you measure them? On second thoughts, don't bother!

[mikebg]

PARASITIC MODULATOR

Peter,
Parasitic modulation means that it is caused by a parasite that absorbs energy from your oscillator. In our case, the parasite is ground or salt water. The closer you are to this parasite, the more power it consumes and the amplitude of oscillation decreases. Both side bands from 0.1Hz to 6Hz are formed due to the change in amplitude, without using non-linear process.
Here is the circuit of a device to cause parasitic modulation, which we use to measure effect of dismodulation and to adjust PID controller for stability and minimum settling time.

[pebe]

Pebe, I use one term expressing the removal of parasitic modulation. If you do not like "dismodulation", then offer some other.

It's not that I dislike the word 'dismodulation'. I can't understand it because there is no such word in the English language.

When a carrier wave is amplitude modulated by passing it through a nonlinear device with the modulating signal, then sidebands are produced. To retrieve the modulation signal again the signal is 'demodulated'.

But your application of 'dismodulation' seems to imply that the TX wave has had its amplitude varied by some low frequency in the region 0.6 to 6Hz, and you want to get rid of that low frequency component. So why not call the procedure 'amplitude clamping' ?

As I cannot find a definition of 'parasitic modulation' anywhere I assume you mean amplitude modulation.

[mikebg]

DISMODULATION

Posted by Pebe
"Separate Oscillator. Can anyone point me to a circuit that uses a driven TX output stage - rather than a self-oscillating one?"
Pebe, below is the worst working TX circuit with driven output stage used by a great company. Output stage operates as class C amplifier with current limit by equivalent emitter resistance Re. It pumps tank circuit with current pulses, which are independent from energy absorbtion in ground. Pumping TX tank with calibrated current pulses causes parasitic modulation of TX field when sensig head moves. Spectrum of modulating signal are frequencies from 0,1 to 6Hz. The same spectrum occupates modulating signal of the target, so you need to remove the parasitic modulation if you want to find deeper targets.
Pebe, I use one term expressing the removal of parasitic modulation. If you do not like "dismodulation", then offer some other.
Now you know a simple method to suppress parasitic modulation at mode Hi Power, when Q11 is on. Put a Shottky diode and adjust tank to overflow to supply rails when sensing head is over salty water.
However in mode Low Power, when Q11 is off, we can not use the supply rails as amplitude limiter because the amplitude should be lower than rails voltage. But you know a complicated metod for removal of parasitic modulation - the PID controller.
The (R)EMI group has a detailed description of the PID controller by Tietze and Shenk as a PDF file 996KB, but it is in German. If anyone wants this information in German, let me know.

[pebe]

You insist on continually referring to 'dismodulation'. Not only is it not a technical term - it is not even in the English dictionary! So you are the only one who knows what you mean. Why do you use this nebulous waffle?

If you want to communicate, please use standard electronic terms, otherwise I cannot take you seriously.

[mikebg]

It would forever be correcting for an insignificant variation in amplitude.

Pebe, Eureka!
You already rediscovered the "dismodulation".
In modern metal detectors there are two or more demodulators. Often demodulators are called "detectors" because their job is to detect changes of the received signal. Whether a signal is modulated or not, decide demodulators. The signal from deep target appears as very low modulation index because it is against a background of two strong signals AIR and GND. Synchronous demodulators in metal detector are adjusted so as to eliminate signal GND, which generally appears modulated. AIR signal differs in phase from the GND signal, therefore demodulators are sensitive to its modulation. The AIR signal should be "dismodulated" so that we can detect deeper targets, because its modulating spectrum coincidents with modulating spectrum of target signal.
Theory of PID control is fundamental and used everywhere in electronics: AGC, AFC, frequency correction of operational amplifiers.
In the aforementioned book by Tietze and Schenk, is described the tuning of PID controller to achieve minimal settling time.

[pebe]

"A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems. A PID controller calculates an "error" value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process control inputs. The PID parameters used in the calculation must be tuned according to the nature of the system." - Wikipedia
This is not the best definition, but works. An excellent explanation for beginners was made by Tietze and Schenk:
"Semiconductor Circuit Technology" Tietze and Schenk, Springer Verlag Publishers.
The TX circuit diagram of Garrett in posting #9 contains P-I controller. The proportional part is an amplifier with U1B. The integral part is a capacitor C2 connected as feedback of amplifier. It makes gain extremely high at zero frequency. This reduces error at setpoint to minimum.

The control system described in Wikipedia relates to servo-operated systems commonly found in digital proportional radio control systems for models and robots. It ‘looks ahead’ and computes present and targeted servo position and speed (taking into account servo overshoot) and comes up with the most suitable correcting signal at any one time.

Whether the term PID should be applied to #9 circuit is debatable. While initially correcting for an error it behaves as a simple AGC system, as used in radio receivers for over 70 years. Once in lock, the full 1 million gain of the op-amp comes into action. I think the addition of that feature is ‘gilding the lily’ and I cannot see how the circuit benefits from such a high gain. It would forever be correcting for an insignificant variation in amplitude.

[mikebg]

"A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems. A PID controller calculates an "error" value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process control inputs. The PID parameters used in the calculation must be tuned according to the nature of the system." - Wikipedia
This is not the best definition, but works. An excellent explanation for beginners was made by Tietze and Schenk:
"Semiconductor Circuit Technology" Tietze and Schenk, Springer Verlag Publishers.
The TX circuit diagram of Garrett in posting #9 contains P-I controller. The proportional part is an amplifier with U1B. The integral part is a capacitor C2 connected as feedback of amplifier. It makes gain extremely high at zero frequency. This reduces error at setpoint to minimum.

[pebe]

Mikebg,

Pebe, before modification and after modification, the transistor operates as current pump. This is not oscillator circuit, but an amplifier class C. It transforms the input voltage (square wave) in current pulses with duty cycle 50%. The current of pulse is determined by expression Ic=(Vz-0.6)/R2. The LC tank circuit transforms current pulses in oscillation.

Energy return in original circuit is possible via collector junction of transistor and ZD, however this requires voltage excess 1.2V. The SD reduces this excess to 0.4V only.

I suggested you put component values on your first circuit. Instead, you ignored my suggestion and came up with a second circuit. So I assumed you comments referred to that and I replied accordingly. But now it seems you are commenting on the first one. And now your last post shows yet another, different, circuit!

Why don’t you number your circuits and refer to their numbers when making comments? That way, we would both know we are talking about the same thing.

What you say for oscillator, refers to TX circuit diagram of Fisher 440 in posting # 4. The task of "feedback" in this circuit is to dismodulate the amplitude of oscillation. For this purpose a transistor is connected as diode and operates as amplitude detector. It generates control signal for amplitude dismodulation. The dismodulator is P (proportional) controller. Garrett uses P-I controller, but for best results it should be P-I-D controller.

You are talking about ‘dismodulation’ again. As you obviously mean ‘varying’ when you talk about ‘modulation’, do you mean non-varying or ‘amplitude level stabilisation’ when you talk of dismodulation.

As a newcomer to metal detection, I am not familiar with P-I and P-I-D controllers. Could you briefly explain what they are?

[mikebg]

To pebe

Pebe, before modification and after modification, the transistor operates as current pump. This is not oscillator circuit, but an amplifier class C. It transforms the input voltage (square wave) in current pulses with duty cycle 50%. The current of pulse is determined by expression Ic=(Vz-0.6)/R2. The LC tank circuit transforms current pulses in oscillation.
Energy return in original circuit is possible via collector junction of transistor and ZD, however this requires voltage excess 1.2V. The SD reduces this excess to 0.4V only.
What you say for oscillator, refers to TX circuit diagram of Fisher 440 in posting # 4. The task of "feedback" in this circuit is to dismodulate the amplitude of oscillation. For this purpose a transistor is connected as diode and operates as amplitude detector. It generates control signal for amplitude dismodulation. The dismodulator is P (proportional) controller. Garrett uses P-I controller, but for best results it should be P-I-D controller.

[WM6]

WM6, look into your English dictionary to see that "modulate" means change. If we use the term "dumping" of TX tank circuit, then the real process is CHANGE OF DUMPING due motion of TX coil relative to a halfspace core (ground) and change electromagnetic properties of the core (because the soils are different). A thorough visual explanation will be made with pieces that we have now begun to plot.

Probably you use general dictionary. Here we use AM or Amplitude Modulation as electronic term. So you have to use dictionary of electronic term not general dictionary. On web you can find free online electronic dictionaries. Attenuation of signal is not the same as modulation, although in both cases the changes are going on.