LED Indicator: two status LED (Green/Blue 2 in 1 LED) Antenna: 2 dual band dipole antennas (2.4G/5G Hz) Audio DAC: 106dB SNR Accessories: 5V/1A Adaptor.
The circuit consists of an inductive coil, L anda capacitor, C.The capacitor stores energy in the form of an electrostaticfield and which produces a potential ( static voltage)across its plates, while the inductive coil stores itsenergy in the form of an electromagnetic field. Thecapacitor is charged up to the DC supply voltage, V byputting the switch in position A.When the capacitor is fully charged the switch changes toposition B.The charged capacitor is now connected in parallel acrossthe inductive coil so the capacitor begins to dischargeitself through the coil. The voltage across C startsfalling as the current through the coil begins to rise. Thisrising current sets up an electromagnetic field around thecoil which resists this flow of current. When thecapacitor, C iscompletely discharged the energy that was originally storedin the capacitor, C asan electrostatic field is now stored in the inductive coil, L asan electromagnetic field around the coils windings. As there is now no external voltage in the circuit tomaintain the current within the coil, it starts to fall asthe electromagnetic field begins to collapse. A back emf isinduced in the coil ( e= -Ldi/dt) keeping the current flowing in theoriginal direction.
This current now charges up thecapacitor, C withthe opposite polarity to its original charge. C continuesto charge up until the current reduces to zero and theelectromagnetic field of the coil has collapsed completely.The energy originally introduced into the circuit throughthe switch, has been returned to the capacitor which againhas an electrostatic voltage potential across it, althoughit is now of the opposite polarity.
The capacitor now startsto discharge again back through the coil and the wholeprocess is repeated. The polarity of the voltage changes asthe energy is passed back and forth between the capacitorand inductor producing an AC type sinusoidal voltage andcurrent waveform. This then forms the basis of an LCoscillators tank circuit and theoretically this cycling backand forth will continue indefinitely. However, every timeenergy is transferred from C to L orfrom L to C lossesoccur which decay the oscillations. This oscillatory action of passing energy back and forthbetween the capacitor, C tothe inductor, L wouldcontinue indefinitely if it was not for energy losses withinthe circuit. Electrical energy is lost in the DC or realresistance of the inductors coil, in the dielectric of thecapacitor, and in radiation from the circuit so theoscillation steadily decreases until they die awaycompletely and the process stops. Then in a practical LC circuitthe amplitude of the oscillatory voltage decreases at eachhalf cycle of oscillation and will eventually die away tozero.
The oscillations are then said to be 'damped' with theamount of damping being determined by the quality orQ-factor of the circuit.Damped Oscillations. The frequency of the oscillatory voltage depends upon thevalue of the inductance and capacitance in the LC tankcircuit. We now know that for resonance to occur inthe tank circuit, there must be a frequency point were thevalue of X C,the capacitive reactance is the same as the value of X L,the inductive reactance ( X L = X C)and which will therefore cancel out each other out leavingonly the DC resistance in the circuit to oppose the flow ofcurrent. If we now place the curve for inductive reactanceon top of the curve for capacitive reactance so that bothcurves are on the same axes, the point of intersection willgive us the resonance frequency point, ( ƒr or ωr )as shown below.Resonance Frequency.
To keep the oscillations going in an LC tankcircuit, we have to replace all the energy lost in eachoscillation and also maintain the amplitude of theseoscillations at a constant level. The amount of energyreplaced must therefore be equal to the energy lost duringeach cycle. If the energy replaced is too large theamplitude would increase until clipping of the supply railsoccurs. Alternatively, if the amount of energy replaced istoo small the amplitude would eventually decrease to zeroover time and the oscillations would stop.
To produce a constant oscillation, the level of the energyfed back to the LC networkmust be accurately controlled. Then there must be some formof automatic amplitude or gaincontrol when the amplitude tries to vary from areference voltage either up or down. To maintain a stableoscillation the overall gain of the circuit must be equal toone or unity. Any less and the oscillations will not startor die away to zero, anymore the oscillations will occur but the amplitudewill become clipped by the supply rails causing distortion.Consider the circuit below.Basic Transistor LC Oscillator Circuit. A is used as the LC oscillators amplifierwith the tuned LC tankcircuit acts as the collector load. Another coil L2 isconnected between the base and the emitter of the transistorwhose electromagnetic field is 'mutually' coupled with thatof coil L.Mutual inductance exists between the two circuits.
Thechanging current flowing in one coil circuit induces, byelectromagnetic induction, a potential voltage in the other(transformer effect) so as the oscillations occur in thetuned circuit, electromagnetic energy is transferred fromcoil L tocoil L2 anda voltage of the same frequency as that in the tuned circuitis applied between the base and emitter of the transistor.In this way the necessary automatic feedback voltage isapplied to the amplifying transistor. The amount of feedback can be increased or decreased byaltering the coupling between the two coils Land L2.When the circuit is oscillating its impedance is resistiveand the collector and base voltages are 180 o outof phase. In order to maintain oscillations (calledfrequency stability) the voltage applied to the tunedcircuit must be 'in-phase' with the oscillations occurringin the tuned circuit. Therefore, we must introduce anadditional 180 o phaseshift into the feedback path between the collector and thebase. This is achieved by winding the coil of L2 inthe correct direction relative to coil L givingus the correct amplitude and phase relationships for the Oscillators circuitor by connecting a phase shift network between the outputand input of the amplifier. The LC Oscillator is therefore a'Sinusoidal Oscillator' or a 'Harmonic Oscillator' as it ismore commonly called.
LC oscillators can generatehigh frequency sine waves for use in radio frequency (RF)type applications with the transistor amplifier being of aBipolar Transistor or FET. Harmonic Oscillators come in manydifferent forms because there are many different ways toconstruct an LC filter network and amplifier with the mostcommon being the Hartley LC Oscillator, ColpittsLC Oscillator, Armstrong Oscillator and ClappOscillator to name a few.Example No1.