A circuit that can automatically convert DC power into an AC signal with a certain amplitude and frequency without an external signal is called an oscillating circuit or an oscillator. This phenomenon is also called self-excited oscillation. In other words, a circuit that can generate an AC signal is called an oscillator circuit.
An oscillator must include three parts: amplifier, positive feedback circuit and frequency selection network. The amplifier can amplify the input signal applied to the input terminal of the oscillator so that the output signal maintains a constant value. The positive feedback circuit ensures that the feedback signals provided to the oscillator input are in the same phase, only in this way can the oscillation be maintained. The frequency selection network only allows a specific frequency f0 to pass through, so that the oscillator produces a single frequency output.
Whether the oscillator can oscillate and maintain a stable output is determined by the following two conditions; one is that the feedback voltage uf and the input voltage Ui must be equal, which is the amplitude balance condition. The second is that uf and ui must have the same phase, which is a phase balance condition, that is to say, positive feedback must be guaranteed. In general, the amplitude balance condition is often easy to achieve, so when judging whether an oscillating circuit can oscillate, it mainly depends on whether its phase balance condition is established.
Oscillators can be divided into ultra-low frequency (below 20 Hz), low frequency (20 Hz-200 kHz), high-frequency (200 kHz-30 MHz) and ultra-high frequency (10 MHz-350 MHz) according to the oscillation frequency. ) and so on. According to the oscillation waveform, it can be divided into two types: sine wave oscillation and non-sinusoidal wave oscillation.
Sine wave oscillators can be divided into LC oscillators, RC oscillators and quartz crystal oscillators according to the components used in the frequency selection network. Quartz crystal oscillators have high frequency stability and are only used in demanding occasions. In general home appliances, various LC oscillators and RC oscillators are widely used.
The frequency selection network of the LC oscillator is the LC resonant circuit. Their oscillation frequencies are relatively high, and there are three common circuits.
(1) Transformer feedback LC oscillation circuit
Figure 1 (a) is the transformer feedback LC oscillation circuit. Transistor VT is a common emitter amplifier. The primary of the transformer T is an LC resonant circuit for frequency selection, and the secondary of the transformer T provides a positive feedback signal to the input of the amplifier. When the power is turned on, a weak transient current appears in the LC loop, but only the current with the same frequency as the loop resonant frequency f0 can generate a higher voltage at both ends of the loop, and this voltage is sent to back to the base of transistor V. It can be seen from Figure 1(b) that as long as there is no error in the connection, the feedback signal voltage is in the same phase as the input signal voltage, that is to say, it is positive feedback. Therefore, the oscillation of the circuit strengthens rapidly and finally stabilizes.
The characteristics of the transformer feedback LC oscillation circuit are: wide frequency range, easy to start oscillation, but the frequency stability is not high. Its oscillation frequency is: f0=1/2πLC. It is often used to generate tens of kilohertz to Tens of megahertz sine wave signal.
(2) Inductive three-point oscillator circuit
Figure 2 (a) is another commonly used inductive three-point oscillator circuit. In the figure, inductance L1, L2 and capacitance C form a resonant circuit that plays a role in frequency selection. Feedback voltage is taken out from L2 and added to the base of transistor VT. It can be seen from Figure 2(b) that the input voltage and feedback voltage of the transistor are in phase and meet the condition of phase balance, so the circuit can start to oscillate. Since the three poles of the transistor are respectively connected to the three points of the inductor, it is called an inductive three-point oscillator circuit.
The characteristics of the inductive three-point oscillation circuit are: wide frequency range, easy to start oscillation, but the output contains more high-order modulation, and the waveform is poor. Its oscillation frequency is: f0=1/2πLC, where L=L1+L2+2M. It is often used to generate sine wave signals below tens of megahertz.
(3) Capacitive three-point oscillator circuit
There is also a commonly used oscillator circuit is a capacitor three-point oscillator circuit, see Figure 3 (a). In the figure, the inductance L and the capacitors C1 and C2 form a resonant circuit for frequency selection, and the feedback voltage is taken out from the capacitor C2 and added to the base of the transistor VT. It can be seen from Figure 3(b) that the input voltage of the transistor and the feedback voltage are in phase, satisfying the condition of phase balance, so the circuit can start to oscillate. Since the three poles of the transistor in the circuit are respectively connected to the three points of the capacitors C1 and C2, it is called a capacitor three-point oscillator circuit.
The characteristics of the capacitor three-point oscillation circuit are: high frequency stability, good output waveform, The frequency can be as high as 100 MHz or more, but the frequency adjustment range is small, so it is suitable for a fixed frequency oscillator. Its oscillation frequency is: f0=1/2πLC, where C=C1C2C1+C2.
The amplifiers in the above three oscillation circuits are all common emitter circuits. The oscillator with common emitter connection has higher gain and is easy to start oscillation. It is also possible to connect the amplifier in the oscillating circuit into a common base circuit. The oscillator with common base connection has a relatively high oscillation frequency and good frequency stability.
The frequency selection network of the RC oscillator is an RC circuit, and their oscillation frequency is relatively low. There are two commonly used circuits.
(1) RC phase shift oscillator circuit
Figure 4(a) is RC phase shift oscillator circuit. The 3-section RC network in the circuit plays the role of frequency selection and positive feedback at the same time. It can be seen from the AC equivalent circuit in Figure 4(b): Because it is a single-stage common-emitter amplifier circuit, the output voltage Uo and the output voltage Ui of the transistor VT have a phase difference of 180°. When the output voltage passes through the RC network, it becomes the feedback voltage Uf and is sent to the input terminal. Since the RC network only produces a 180° phase shift for the voltage of a certain frequency f0, only the signal voltage of the frequency f0 is positive feedback. and cause the circuit to oscillate. It can be seen that the RC network is not only a frequency selection network, but also a part of the positive feedback circuit.
The characteristics of the RC phase-shift oscillation circuit are: the circuit is simple and economical, but not stable High and difficult to adjust. It is generally used as a fixed frequency oscillator and occasions where the requirements are not too high. Its oscillation frequency is: when 3 sections RC.
When the network parameters are the same: f0=12π6RC. The frequency is generally tens of kilohertz.
(2) RC bridge oscillator circuit
Figure 5 (a) is a common RC bridge oscillator circuit. The series-parallel circuit of R1C1 and R2C2 on the left side of the figure is its frequency selection network. This frequency-selective network is in turn part of the positive feedback circuit. This frequency selection network has no phase shift for a signal voltage with a specific frequency f0 (the phase shift is 0°), and the voltages of other frequencies have phase shifts of different sizes. Since the amplifier has 2 stages, the feedback voltage Uf taken from the output terminal of V2 is in phase with the input voltage of the amplifier (2-stage phase shift 360°=0°). Therefore, when the feedback voltage is sent back to the input terminal of VT1 through the frequency selection network, only a voltage with a specific frequency of f0 can meet the phase balance condition and start to oscillate. It can be seen that the RC series-parallel circuit plays the role of frequency selection and positive feedback at the same time.
In fact, in order to improve the working quality of the oscillator, a series voltage negative feedback circuit composed of Rt and RE1 is also added to the circuit. Among them, Rt is a thermistor with a negative temperature coefficient, which can stabilize the oscillation amplitude and reduce nonlinear distortion to the circuit. diangon.com From the equivalent circuit in Figure 5(b), we can see that this oscillating circuit is a bridge circuit. R1C1, R2C2, Rt and RE1 are the four arms of the bridge respectively, and the input and output of the amplifier are respectively connected to the two diagonal lines of the bridge, so it is called an RC bridge oscillator circuit.
The performance of the RC bridge oscillator circuit is better than that of the RC phase shift oscillator circuit. It has high stability, small nonlinear distortion and convenient frequency adjustment. Its oscillation frequency is: when R1=R2=R、 f0=12πRC when C1=C2=C. Its frequency ranges from 1 Hz to 1 MHz.
Amplitude modulation and detection circuit
Broadcasting and radio communication use modulation technology to add low-frequency sound signals to high-frequency signals for transmission. The process of restoring in the receiver is called demodulation. Among them, the low-frequency signal is called the modulation signal, and the high-frequency signal is called the carrier. Common continuous wave modulation methods include amplitude modulation and frequency modulation, and the corresponding demodulation methods are called wave detection and frequency discrimination.
Let’s introduce the amplitude modulation and detection circuit first.
(1) Amplitude modulation circuit
Amplitude modulation is to make the amplitude of the carrier signal change with the amplitude of the modulating signal, and the frequency of the carrier remains unchanged. A circuit that can perform the function of amplitude modulation is called an amplitude modulation circuit or an amplitude modulator.
Amplitude modulation is a nonlinear frequency conversion process, so its key is to use nonlinear devices such as diodes and triodes. According to the loop in which the modulation process is performed, the triode amplitude modulation circuit can be divided into three types: collector amplitude modulation, base amplitude modulation and emitter amplitude modulation. Let’s take the collector amplitude modulation circuit as an example.
Figure 6 is the collector amplitude modulation circuit, the equal amplitude generated by the high frequency carrier oscillator The carrier is added to the base of the transistor through T1. The low-frequency modulation signal is coupled to the collector through T3. C1, C2, C3 are high-frequency bypass capacitors, R1, R2 are bias resistors. The collector’s LC parallel circuit resonates at the carrier frequency. If the static operating point of the triode is selected in the curved part of the characteristic curve, the triode is a nonlinear device. Because the collector current of the transistor varies with the modulation voltage, the two signals in the collector realize amplitude modulation due to nonlinear effects. Since the LC resonant circuit is tuned on the fundamental frequency of the carrier, the amplitude modulated wave output can be obtained in the secondary of T2.
(2) Detection circuit
The function of the detection circuit or detector is to extract the low-frequency signal from the AM wave. Its working process is just the opposite of AM. The detection process is also a frequency conversion process, and nonlinear components are also used. Commonly used are diodes and triodes. In addition, in order to extract low-frequency useful signals, filters must be used to filter out high-frequency components, so the detection circuit usually includes two parts: nonlinear components and filters. Let’s take a diode detector as an example to illustrate its work.
Figure 7 is a diode detection circuit. VD is a detection element, and C and R are low-pass filters. When the input modulated wave signal is large, the diode VD works intermittently. During the positive half cycle, the diode conducts and charges C;When the half cycle and the input voltage are small, the diode is cut off, and C discharges R. The voltage obtained at both ends of R contains a lot of frequency components. The high-frequency part is filtered by the capacitor C, and then the DC-blocking effect of the DC-blocking capacitor C0 is used to obtain the restored low-frequency signal at the output terminal.
Frequency modulation and frequency discrimination circuit
Frequency modulation is to make the carrier frequency change with the amplitude of the modulation signal, while the amplitude remains unchanged. Frequency discrimination is to demodulate the original low-frequency signal from the frequency modulation wave, and its process is just the opposite of frequency modulation.
(1) Frequency modulation circuit
The circuit that can complete the frequency modulation function is called a frequency modulator or frequency modulation circuit. The commonly used frequency modulation method is the direct frequency modulation method, that is, the method of directly changing the frequency of the carrier oscillator with a modulation signal. Figure 8 shows its general idea. In the figure, a variable reactance element is connected in parallel with the resonant circuit. The change of the parameters of the variable reactance element is controlled by the low-frequency modulation signal, so that the frequency of the carrier oscillator changes.
(2) Frequency discrimination circuit
The circuit that can complete the frequency discrimination function is called discrimination Frequency detector or frequency discrimination circuit, sometimes also called frequency detector. The method of frequency discrimination is usually divided into two steps. The first step is to convert the equal-amplitude FM wave into an FM-AM wave whose amplitude varies with frequency. The second step is to use a general detector to detect the amplitude change and restore it to a low-frequency signal. Commonly used discriminators include phase discriminators, ratio discriminators, etc.