What are the working principles of AM radio?
1. The technical specifications of the AM radio are in the range of: 525 ~ 1605kHz, and the intermediate frequency is: 465kHz.
Sensitivity: ≤ 1.5 MV / s / N = 261B
Selectivity: ≥ 20 dB, + 9 kHz.
Static current: no signal < 20mA.
Loudspeaker: φ 57mm 892.
Output power: ≥ 180mW 10% distortion.
Power supply: 3 V / 2 / 5.
2. How AM radios work
A typical block diagram of the working principle of a superheterodyne AM radio.The circuit principle is composed of AC frequency circuit (including local oscillator circuit and mixing circuit), if amplifier circuit liquid detector, ACC circuit preamplifier circuit and power amplifier circuit. Antenna receives amplitude modulation high frequency signal, the amplitude of amplitude modulation signal changes according to the change of audio signal, the track of amplitude change is the waveform of audio signal, also known as envelope. The radio converts the high frequency amplitude modulation signal received by the antenna into a fixed medium color signal.After detection, the signal is preamplified and power amplified. Then drive the speaker to sound.
The input loop is also called the tuning loop or the selection loop. The function of the input loop is to receive radio waves from the air and select the desired radio signal from all these signals. Input loop requirements: high efficiency, high selectivity, covered by segments,the cover coefficient is suitable and the voltage transmission coefficient is even in the band overcurrent range. This is also the transistor radio used more one of the input single resonant circuit input,
Intertemporal, people come to use into the shift antenna bureau variable capacitor C forward antenna.
The iron body core of the coil of the oxygen body core is added to the K machine, which makes it become a Gaoton electric signal by measuring the electric waves of the sensor line of the oxygen body core. When the amplitude-modulated signal induces the B and C human training circuits, the necessary electrical signals are selected to enter the base of the VT (9018H) pole tube.
2) Medium sequence amplifier circuit
If amplifier circuit is the main part of the whole machine to ensure the selectivity and pass frequency band. It is a key component to the radio.
3) Geophone and ACC circuit
The function of the geophone in the AM radio is to "detect" the modulation of the audio modulation signal on the high frequency carrier, also known as the amplitude demodulator. There are many kinds of geophone and classification method, but in modern AM radio, semiconductor diode large signal detector is used without exception. the PN junction of the VTT / 9018H) transistor is used as the detector.
The ACC automatic gain control circuit is an automatic control method that makes the gain of the receiving circuit adjust automatically with the signal strength, that is, the audio signal is controlled in real time, so as to ensure the same of the far and near stations.
4) low amplifier with low current, so it can only use high resistance
Amplitude modulation radio in the detector output audio signal is generally only a few groups of low discharge circuit.
5) Power amplifier circuit
The low-frequency signal has reached a voltage of 1 to several volts after the preamplifier, but its load capacity is still very poor, it can not directly promote the loudspeaker sound, but also need to carry out power amplification. Power amplifier circuit not only needs to output a large voltage, but also needs to output a large current. The push-pull power amplifier circuit is composed of 9013H) and VTN (9013H). The impedance matching performance of this circuit is good, the requirement for this parameter of push-pull tube is also low, and it works at a lower level.
Large power can be output at voltage.VD, VD2N4148) consists of 1.3V + 0.1V steady voltage, fixed frequency conversion circuit, middle amplifier circuit and second amplifier circuit with low base voltage, stable working current at all levels to maintain sensitivity.
This article is from Allicdata Electronics Limited.