Originally posted by mikebg
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Revision of TX used in Classic family
This posting is a continuation of my posting # 597.
Objective of the revision is to analyze, experiment and redesign TX circuit.
A TX for metal detectors must meet the following requirements:
1. Work equally well with different by diameter TX coils.
2. Work with enough high power (Ampere-turns) at given supply voltage.
3. Work with maximal efficiency at limited mass of wire used in the TX coil.
Maximal power and efficiency can be achieved: (a) by reducing the resistance of the TX coil (for example coil r = 1,5
Ohm as shown below), (b) by using large tank capacitance formed with MKP type capacitors (for example tank C = 1uF
as shown below) and (c) by quick operating automatic control maintaining stabile amplitude of oscillation near to
saturation of pumping transistor (Q1 below, but thre is no control :-(.
4. Do not get amplitude and angle modulation when operator changes distance of the TX loop to the ground or when
the loop passes over ground with different properties.
The modulation of TX by the ground is very harmful for operation of metal detectors because the AIR signal appears
as an additional GND signal which differs in phase from the true GND signal, but is parasite modulated in the same way.
The problem is even more complicated because the true GND signal is generated by a modulated by ground
transmitter.
What block diagram should have TX for metal detectors?
The most suitable block diagram of TX which can suppress amplitude and angle modulation was shown in the forum.
In theory, the resistance of environment is the true load for TX end stage. In practice, the most part of tank energy dissipates in resistance of TX coil, which forms a LC "tank circuit" or "tuned circuit". In German is used term "Schwingkreis", in Russian - "колебательный контур".
To suppress angle modulation, the tank circuit is not part of regenerative oscillating loop.
To suppress amplitude modulation, a P-I-D controller uses signal from an ampllitude detector connected to TX tank
circuit. The end stage of TX pumps the LC tank circuit with current pulses having less than 25% duty cycle. The quick operating P-I-D controller increases width of pumping pulses when the search coil decreases height (or passes over more absorbing ground).
HOW TX OF CLASSIC FAMILY WORKS?
The answer is: very bad.
Transistor Q1 operates as an amplifier class C. Instead P-I-D (proportional-integral-derivative) controller is used proportional controller (amplifier) with Q2.
Transistor Q2 also should operate as an amplitude detector, but it can not make this because the circuit is incompetent designed. It should maintain stabile amplitude of oscillation as a self-biasing oscillator. The controller can not operate because transistor Q1 pumps the LC tank circuit with current pulses having 50%
duty cycle as shown below. This makes maximal possible power and the controller can not increase it more.
Incompetent designed is not only the block diagram (P-controller instead P-I-D controller) .Bad designed are the amplitude detector, which can not detect amplitude (see the voltmeter below) and not suitable starting operating points
(showed below with blue numbers). In addition to this:
(a) resistor R7 decreases TX power and efficiency,
(b) resistor R10 eliminates operation of amplitude detector, eliminates self-biasing action, decreases amplification of the P-controller, and
(c) a large capacitance C21 charges and discharges slowly.
At the end of this list of drawbacks, the emitter junction of Q2 (used as amplitude detector) has properties as a Zener diode.
However there is no breakdown of emitter junction in this design because the amplitude of oscillation is too small - almost 4.3V at
possible 8V. However that means low utilization of supply voltage which is 4.3 : 8 = 0.5375 or only 54%.
The SPICE can not simulate the Zener action of emitter junction because SPICE model of transistor Q2 contains no information for emitter breakdown voltage. I measured about 11V Zener voltage for most widely used transistors. If the amplitude detector where designed correctly, the average voltage showed by voltmeter below should be negative. The positive voltage shows that amplitude detector can not operate and TX circuit can not suppress the amplitude modulation when TX coil moves.
Please simulate the circuit with different SPICE programs. My program CircuitMaker shows that the current in tank circuit has
amplitude 174 mA. That means RMS value 123 mA and the TX power is too small - almost 23 mW.
CONCLUSION
Below is shown an incompetent designed circuit diagram of TX, which is not suitable for sensitive metal detectors.
What follows?
In the next my post will be described how a competent design can improve operation of this circuit. We will obtain suppression of AM and triple more power.
This posting is a continuation of my posting # 597.
Objective of the revision is to analyze, experiment and redesign TX circuit.
A TX for metal detectors must meet the following requirements:
1. Work equally well with different by diameter TX coils.
2. Work with enough high power (Ampere-turns) at given supply voltage.
3. Work with maximal efficiency at limited mass of wire used in the TX coil.
Maximal power and efficiency can be achieved: (a) by reducing the resistance of the TX coil (for example coil r = 1,5
Ohm as shown below), (b) by using large tank capacitance formed with MKP type capacitors (for example tank C = 1uF
as shown below) and (c) by quick operating automatic control maintaining stabile amplitude of oscillation near to
saturation of pumping transistor (Q1 below, but thre is no control :-(.
4. Do not get amplitude and angle modulation when operator changes distance of the TX loop to the ground or when
the loop passes over ground with different properties.
The modulation of TX by the ground is very harmful for operation of metal detectors because the AIR signal appears
as an additional GND signal which differs in phase from the true GND signal, but is parasite modulated in the same way.
The problem is even more complicated because the true GND signal is generated by a modulated by ground
transmitter.
What block diagram should have TX for metal detectors?
The most suitable block diagram of TX which can suppress amplitude and angle modulation was shown in the forum.
In theory, the resistance of environment is the true load for TX end stage. In practice, the most part of tank energy dissipates in resistance of TX coil, which forms a LC "tank circuit" or "tuned circuit". In German is used term "Schwingkreis", in Russian - "колебательный контур".
To suppress angle modulation, the tank circuit is not part of regenerative oscillating loop.
To suppress amplitude modulation, a P-I-D controller uses signal from an ampllitude detector connected to TX tank
circuit. The end stage of TX pumps the LC tank circuit with current pulses having less than 25% duty cycle. The quick operating P-I-D controller increases width of pumping pulses when the search coil decreases height (or passes over more absorbing ground).
HOW TX OF CLASSIC FAMILY WORKS?
The answer is: very bad.
Transistor Q1 operates as an amplifier class C. Instead P-I-D (proportional-integral-derivative) controller is used proportional controller (amplifier) with Q2.
Transistor Q2 also should operate as an amplitude detector, but it can not make this because the circuit is incompetent designed. It should maintain stabile amplitude of oscillation as a self-biasing oscillator. The controller can not operate because transistor Q1 pumps the LC tank circuit with current pulses having 50%
duty cycle as shown below. This makes maximal possible power and the controller can not increase it more.
Incompetent designed is not only the block diagram (P-controller instead P-I-D controller) .Bad designed are the amplitude detector, which can not detect amplitude (see the voltmeter below) and not suitable starting operating points
(showed below with blue numbers). In addition to this:
(a) resistor R7 decreases TX power and efficiency,
(b) resistor R10 eliminates operation of amplitude detector, eliminates self-biasing action, decreases amplification of the P-controller, and
(c) a large capacitance C21 charges and discharges slowly.
At the end of this list of drawbacks, the emitter junction of Q2 (used as amplitude detector) has properties as a Zener diode.
However there is no breakdown of emitter junction in this design because the amplitude of oscillation is too small - almost 4.3V at
possible 8V. However that means low utilization of supply voltage which is 4.3 : 8 = 0.5375 or only 54%.
The SPICE can not simulate the Zener action of emitter junction because SPICE model of transistor Q2 contains no information for emitter breakdown voltage. I measured about 11V Zener voltage for most widely used transistors. If the amplitude detector where designed correctly, the average voltage showed by voltmeter below should be negative. The positive voltage shows that amplitude detector can not operate and TX circuit can not suppress the amplitude modulation when TX coil moves.
Please simulate the circuit with different SPICE programs. My program CircuitMaker shows that the current in tank circuit has
amplitude 174 mA. That means RMS value 123 mA and the TX power is too small - almost 23 mW.
CONCLUSION
Below is shown an incompetent designed circuit diagram of TX, which is not suitable for sensitive metal detectors.
What follows?
In the next my post will be described how a competent design can improve operation of this circuit. We will obtain suppression of AM and triple more power.
Best regards!
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