Briefly introduce the working principle and characteristics of the oscillation circuit

To put it simply, an oscillating circuit is one that can generate an oscillating current whose magnitude and direction change with the cycle. The circuit that generates this oscillating current is called an oscillating circuit. The LC loop is the simplest oscillating circuit. Oscillating current cannot be generated by rotating a coil in a magnetic field. It is a high-frequency alternating current that can only be generated in an oscillating circuit. So what is the specific working principle of an oscillating circuit? In the following article, the editor will give you a detailed introduction, hoping to help you learn!

The physical model of the oscillation circuit meets the following three conditions:

1. The inductor coil L concentrates the inductance of the entire circuit, and the capacitor C concentrates the capacitance of the entire circuit. There is no latent capacitance.

2. The resistance of the entire circuit is R = 0 (including coils and wires). From an energy perspective, no other form of energy is converted into internal energy, that is, the heat loss is zero.

3. The LC oscillation circuit does not radiate electromagnetic waves to the outside world when electromagnetic oscillation occurs. It is a closed circuit in the strict sense. Only the mutual conversion between the magnetic field energy of the coil and the electric field energy of the capacitor occurs inside the LC circuit. Even the changing electric field generated in the capacitor and the changing magnetic field generated in the coil do not excite the corresponding magnetic field and electric field according to Maxwell's electromagnetic field theory, and radiate electromagnetic waves to the surrounding space.

The general oscillation circuit consists of four parts: the amplifier circuit, the positive feedback network, the frequency selection network and the amplitude stabilization circuit. The amplifier circuit is essential to meet the amplitude balance condition, because during the oscillation process, there will inevitably be energy loss, resulting in oscillation attenuation. Through the amplifier circuit, the power supply can be controlled to continuously provide energy to the oscillation system to maintain constant amplitude oscillation, so the amplifier circuit is essentially a transducer, which plays the role of supplementing energy loss.

The positive feedback network is essential to meet the phase balance condition. It returns part or all of the output power of the amplifier circuit to the input end to complete the self-excitation task. In essence, it plays an energy control role. The function of the frequency selection network is to make the feedback signal passing through the positive feedback network, only the selected signal can make the circuit meet the self-excitation oscillation condition. For signals of other frequencies, since they cannot meet the self-excitation oscillation condition, they are suppressed. Its purpose is to make the circuit generate a single-frequency sine wave signal.

If the frequency selection network is composed of R and C components, it is called an RC sine wave oscillation circuit; if it is composed of L and C components, it is called an LC sine wave oscillation circuit; if it is composed of a quartz crystal, it is called a quartz crystal oscillation circuit. The function of the amplitude stabilization circuit is to stabilize the amplitude of the oscillation signal. It can use a thermistor or other limiting circuit, or it can use the nonlinearity of the amplifier circuit's own components. In order to better obtain a stable equal-amplitude oscillation, a negative feedback network is sometimes required.

When analyzing the working principle of an oscillation circuit, first check whether the circuit has an amplifier circuit, a feedback network, a frequency selection network, and an amplitude stabilization link, then check whether the static operating point of the amplifier circuit can ensure the normal operation of the amplifier circuit, and then analyze whether the circuit meets the self-excited oscillation conditions, that is, the phase balance condition and the amplitude balance condition.

The oscillation conditions of an oscillation circuit include two parts: equilibrium conditions and starting conditions.

The balance condition of an oscillation circuit is the condition for the oscillation circuit to maintain equal amplitude oscillation. The balance condition of an oscillation circuit includes two parts: amplitude balance condition and phase balance condition. The reason why an oscillation circuit can have an output signal without an external input AC signal is that it uses its own positive feedback signal as the input signal.

Therefore, in order for the oscillation circuit to maintain equal amplitude oscillation, the amplitude and phase of its feedback signal Vf must be the same as its net input signal Vid. The amplitude balance condition of the oscillation circuit is AF = 1; the phase balance condition of the oscillation circuit is cpA + (pf = + 2n, 7r (n = 0, 1, 2, 3--). In the formula, cpA represents the phase shift of the basic amplifier circuit, and cpA represents the phase shift of the positive feedback network. For an oscillation circuit, the amplitude balance condition and phase balance condition of the oscillation circuit must be met at the same time so that the oscillation circuit can maintain equal amplitude oscillation.

When the oscillation circuit starts to work, current disturbances are generated in the circuit at the moment the power is turned on. These current disturbances may be current mutations caused by the moment the power is turned on, or they may be noise signals inside the transistor or circuit. This current disturbance contains weak sinusoidal signals of multiple frequencies, which are the initial input signals of the oscillation circuit.

When the oscillation circuit starts working, if AF>1 is satisfied, the sine wave signal with the same frequency as the frequency selection network can be amplified and fed back to the input of the amplifier circuit through the amplification and frequency selection of the oscillation circuit, while the signals of other frequencies are suppressed by the frequency selection network. In this way, the oscillation circuit can establish oscillation from small to large after the power is turned on, until the oscillation amplitude is determined when AF =1. Therefore, AF>1 is called the starting condition of the oscillation circuit.

By utilizing the nonlinearity of the transistor or adopting negative feedback in the circuit, the oscillation circuit can be transitioned from AF>1 to AF=1, thereby achieving the purpose of stabilizing the amplitude.

If the maintenance condition and the starting condition of the oscillation circuit are combined, written as AF≥1, this is the amplitude balance condition of the oscillation circuit. In other words, to ensure that the oscillation circuit can generate and maintain equal amplitude oscillation, it must meet the starting condition while meeting the maintenance condition. In summary, the oscillation condition of the oscillation circuit is AF≥1:(;PA +(pf=t:2n-rr(n =0,l,2,3--)o

When designing an oscillation circuit, the following characteristics must also be taken into consideration.

1 Output level stability

Output level stability with respect to time, temperature, and supply voltage.

2 Oscillation waveform distortion

This is the distortion rate of the sine wave output. If it is a pure sine wave, the distortion rate is zero.

In high-frequency oscillation circuits, in addition to the above characteristics, the frequency variable range and oscillation frequency range must also be considered during design.

3Frequency stability

The quality of the oscillation circuit is determined by the frequency stability, which is an important characteristic of the oscillator. The frequency fluctuation can be expressed by the following numerical values.

Frequency: Change over time

After the power is turned on, the frequency fluctuates over time. The fluctuation is greatest during warm-up.

Frequency temperature coefficient

The frequency change relative to temperature change is expressed in ppm/°C.

Frequency: Power supply voltage variation

The frequency change when the power supply voltage changes, expressed as %/V.

We know that the oscillation circuit is composed of four parts, namely the amplifier circuit, positive feedback network, frequency selection network and amplitude stabilization circuit. When we analyze the working principle of the oscillation circuit, we should first check whether each link of the circuit is perfect, and then check whether the static operating point of the amplifier circuit is correct, whether the amplifier circuit can work normally, and then analyze whether the circuit meets the self-excited oscillation conditions. Only after all aspects are checked and confirmed, it will be easier to understand when we analyze the working principle of the oscillation circuit.