The principle of the radio analysis

Last Update Time: 2019-01-05 11:13:09

1. The block diagram of the superheterodyne radio is relative to the direct amplification of the radio.

 The superheterodyne radio first converts the high frequency signal to an intermediate frequency signal through the frequency conversion, and the frequency of this signal is higher than that of the audio frequency signal, and the frequency is fixed at 465 kHz. Because 465 kHz is taken from the difference between the frequency of the local oscillation signal and the external high-frequency signal, it is called superheterodyne. That is, f ( intermediate frequency ) f0 ( local oscillator ) fs ( high frequency amplitude modulation signal )

465 kHz

1000 ~ 2070 kHz

535 ~ 1605 kHz


( 1 ) Series resonance is generated when the natural oscillation frequency of the receiving circuit ( CA, T1 ) LC series resonance circuit is equal to an external electromagnetic wave frequency, thus, the amplitude modulation emission signal of a certain station is received, and is coupled to the next stage circuit through the coil.


( 2 ) Frequency conversion circuit ( V1, CB, T2, T3 ) function : the high-frequency amplitude modulation signal of the antenna loop becomes frequency-fixed IF amplitude modulation signal. Principle : using the nonlinear characteristics of the transistor ( V1 ), the frequency of the input signal is synthesized, and the output signals with different frequencies are obtained, and select the desired signal through the frequency selection circuit. In a superheterodyne radio, a transistor is used to simultaneously generate the local oscillator signal and complete the mixing, which is called a frequency conversion circuit.


( 3 ) Effect of IF amplifier circuit ( V2, T4, V3, T5 ) : amplify IF signal.

Requirements :

1)A sufficient medium amplifier gain ( 60 dB ) is often used in two stages;

2)There is a suitable passband ( 10 kHz ). The frequency band is too narrow, and the amplification gain of each frequency component in the audio signal is different, and the distortion will be generated; the frequency band is too wide, the anti-jamming performance will be weakened, and the selectivity will be reduced. In order to realize the amplitude-frequency characteristic of the mid-level amplifier, the intermediate stage is composed of a frequency selective amplifier with a LC parallel resonant circuit as the load, and the stage adopts the transformer coupling mode.

Note : the intermediate frequency transformer used in this comprehensive experiment is not interchangeable and the manufacturer has adjusted well, do not adjust again.


( 4 ) Detection circuit ( V4, C8, C9, R9, W )

The use of V4 in the circuit is equivalent to a diode. Principle : when V4 loses a positive half cycle peak, V4 is turned on, C6 is charged, and the output voltage of V4 is less than that on C6, v4 cutoff, C6 discharge, the discharge time constant is much larger than the charging time constant, so that the voltage on C6 has little change on discharge. At the next peak, V4 is turned on, C6 continues to charge... so that the envelope of the IF signal containing the audio information can be detected.


( 5 ) Low amplifier and power amplifier ( V5, V6, V7, T6 ) : amplify the amplitude and power of the audio signal, driving the loudspeaker. Low put : V5. Power Amplifier : complementary symmetrical power amplifier mainly composed of V6 and V7.


This article is from Allicdata Electronics.