Best circuit for search oscillator (quiz)

[mikebg]

Best circuit for search oscillator (quiz)

Here are different circuits of LC oscillators, where L components are used as sensor for metal: Colpitts, Hartley, Meissner, Armstrong etc. When L is close to metal target, the frequency of oscillation is shifted. This principle of operation is used in "Frequency shift metal detectors". The rest of the metal detector circuit provides to operator information about the direction and magnitude of the resulting frquency shift. Since in most cases the rest of circuit contains reference oscillator or BFO (Bear Frequency Oscillator), the metal detector is called BFO MD.

The quiz is: WHAT OSCILLATOR CIRCUIT PROVIDES MOST FREQUENCY SHIFT AND WHY?

HINT: For metal detecting are used two types inductive sensors: monocoil and two or more coils (separate RX & TX coil) positioned in induction balance. To answer to quiz, you should know which type sensor is the best for metal detecting and why.

[WM6]

........... or BFO (Bear Frequency Oscillator), the metal detector is called BFO MD.

You mean Beat?

[mikebg]

You mean Beat?

Who is mr Beat? Do you mean that somebody beats the reference oscillator?

Despite my keyboard is QUERTZ (for Germany) instead QUERTY, the keys "R" and "T" are neighbors in it.
However, the error cause is that from July Morning since, no one is checking my drafts and my postings before I send them, even me.

An additional hint to the quiz:
SEARCH OSCILLATOR SHOULD BE MORE SENSITIVE TO NONFERROUS TARGETS.
Here is the Phase Characteristics of such target (phase difference of electromotive voltage induced in the RX coil to the current flowing in the TX coil.

[WM6]

Who is mr Beat?

Do you mean that somebody beats the reference oscillator?

Probably lost gemini brother of Mr. Bear.

Not to beat oscillator, but frequency by other frequency.

As I understand you are speaking about Figure 8 scheme?

[mikebg]

As I understand you are speaking about Figure 8 scheme?

Oscillator circuit of Fig. 7C has the same properties as the circuit of Fig. 8. The difference is only in used amplifier. The amplifier in Fig. 7C has infinite input and output impedances, but the amplifier in Fig. 8 has zero input and output impedances.
At what frequency will oscillate such an oscillator if we set each LC tank on the same resonance frequency?
What will happen if both tanks differ in resonance frequency?
HINT: To answer you should know the phase characteristic of a LC tank.

[WM6]

The amplifier in Fig. 7C has infinite input and output impedances, but the amplifier in Fig. 8 has zero input and output impedances.

Or vice versa?

At what frequency will oscillate such an oscillator if we set each LC tank on the same resonance frequency?

At half as much as LC tank resonate.

What will happen if both tanks differ in resonance frequency?

Probably drift on paralel LC tank?

[mikebg]

1. Or vice versa?



2. At half as much as LC tank resonate.



3. Probably drift on paralel LC tank?

Not correct answers.
1. Input impedance at inverting input of an opamp is in theory zero. At noninverting input is infinity.
2 and 3. HINT for correct answer: at oscillation frequency the phase lag in open loop is 360 deg.

[WM6]

1. Input impedance at inverting input of an opamp is in theory zero. At noninverting input is infinity.

This (zero and infinity) suit to which tipe of oscillator - to which of oscillator figure?

[simonbaker]

Not correct answers.
1. Input impedance at inverting input of an opamp is in theory zero..

Only true if negative feedback loop employed, I think?

SB

[mikebg]

Only true if negative feedback loop employed, I think?

SB

Simon, the negative feedback is shown in Fig. 8 as resistor R1! The sign "gain infinity" means that this is an opamp, however because of feedback, voltage amplification for inverting input is minus R1/r. Since there is also positive feedback in noninverting input, the circuit oscillates.
The circuit diagram in Fig. 7C is different, because it contains an amplifier shown with its voltage gain K. It can not operate with tank circuits if input and output impedances of amplifier are zero. Both capacitances become useless when their tank circuits are shorted by zero input and output impedances of such amplifier.
I will post the last hint for answer but it requires drawing. The question is:
AT WHAT FREQUENCY WILL OSCILLATE A SYSTEM HAVING VERY STEEP PHASE CHARACTERISTIC OF OPEN LOOP? (the drawing will be in next my posting)

[mikebg]

Steep or Slant?

Attached is phase characteristic of an open loop system consisting of an amplifier, RX coil connected to its input and TX coil connected to its output. Let the circuit diagram is as in Fig. 7C or in Fig. 8. The system is open because there is balance between TX and RX coil and there is no target. The phase characteristic has very steep region because both tank circuits are tuned to the equal resonance frequency, for example in the diagram below is shown 1kHz.
What happens if there is target with phase characteristic shown in posting #3?
We should sum both characteristics to obtain phase characteristic of the new system which now is closed by target and can oscillate. Despite target has slant (not so steep) character in this region, the common characteristic remains steep. Target adds a phase lag 45 deg in the steep region as shown in posting #3, however the condition for oscillation is satisfyed again for about 1kHz.
CONCLUSION: To increase frequency shift of an oscillator caused by non ferrous target, we need slant phase characteristic of open loop system.
How to make phase characteristic slant
One method is to tune RX and TX tanks for different resonance frequencies.
That means we converted a narrow band amplifier to operate as wide band amplifier. However to increase bandwidth of such system we can use dumping resistors in tuned circuits.

[mikebg]

Ferrite target

Remains to see what happens when the target is dry (nonconductive) mineralized soil, lossless "hot rock" or ferrite core. It has absolutely flat phase characteristic with phase led allways 90 deg independent on frequency as shown in the attached figure.
We can tune feedback system to operate with degeneration (negative feedback) if soil or target have ferromagnetic properties. However this will reduce sensitivity to nonferrous targets.

[WM6]

AT WHAT FREQUENCY WILL OSCILLATE A SYSTEM HAVING VERY STEEP PHASE CHARACTERISTIC OF OPEN LOOP? (the drawing will be in next my posting)

Hi mikebg, i think you are only one that can explain this. So do not hesitate to tell us all secrets (graphically if possible please!)

[simonbaker]

Simon, the negative feedback is shown in Fig. 8 as resistor R1! The sign "gain infinity" means that this is an opamp, however because of feedback, voltage amplification for inverting input is minus R1/r. Since there is also positive feedback in noninverting input, the circuit oscillates.
The circuit diagram in Fig. 7C is different, because it contains an amplifier shown with its voltage gain K. It can not operate with tank circuits if input and output impedances of amplifier are zero. Both capacitances become useless when their tank circuits are shorted by zero input and output impedances of such amplifier.
I will post the last hint for answer but it requires drawing. The question is:
AT WHAT FREQUENCY WILL OSCILLATE A SYSTEM HAVING VERY STEEP PHASE CHARACTERISTIC OF OPEN LOOP? (the drawing will be in next my posting)

I see -- amps in 7C not ideal op amps!

Good probs to think about.

-SB

[mikebg]

Graphical analysis - page 1

Hi mikebg, i think you are only one that can explain this. So do not hesitate to tell us all secrets (graphically if possible please!)

Graphical analysis of search oscillator for BFO metal detector with computer aided testing.
What was invented when was born carbon microphone.
page 1
1. What is an electronic oscillator?
A not popular answer is: “The oscillator is system consisting of an amplifier and feedback. It oscillates when is satisfyed Barkhausen criteria".
May be the most popular saying of radio amateurs:
“AN AMPLIFIER CAN OSCILLATE, BUT AN OSCILLATOR CAN AMPLIFY”
arises when was born regenerative radio invented by Edwin Armstrong (1890-1954).
However the first oscillation in electronic device is acustical and arises in 19th century at born of telephone. Then carbon microphone is invented by Thomas Edison.
The circuit of first sound oscillator is shown below. The carbon microphone operates as transistor (variable resistor) connected by load to battery. This kind microphone is active component, that means it operates as amplifier transforming energy from battery in current signal to load. The generated current drives headphone with noise. The headphone operates as transmitter which excites the microphone with sound waves. Oscillation frequency depends on distance between microphone and headphone, travelled by sound waves in feedback path. The phase lag of feedback signal depends also on velocity of sound wave. We can change frequency if swap leads of microphone or telephone.
Headphone operates as transmitter, emitting sound waves. However in 19th sentuty this telephone part is called "receiver" because receives current from the correspondent and transforms it in sound.
The microphone in oscillating circuit receives sound energy, but in 19th century it is called "transmitter" because transmitts sound signal by wires to the recipient. Since we will design search oscillator for BFO metal detector, it will be interesting to test how environment changes frequency of oscillation. The oscillating system can operate as sound radar for detecting obstacles. You have all parts for this experiment in your computer.