Superheterodyne radio: refers to the process of generating a fixed intermediate frequency signal from an input signal and a local oscillator signal. If all high-frequency signals received by the radio are converted into fixed intermediate frequency carrier frequency (only the carrier frequency changes, but the signal envelope is still the same as the original high-frequency signal envelope), and then the fixed intermediate frequency is amplified, detected and low-amplified, it becomes a superheterodyne radio. In this receiver, it is necessary to add a first-stage converter, usually called frequency converter, between the high-frequency amplifier and the intermediate-frequency amplifier, whose basic task is to convert the high-frequency signal into a fixed intermediate frequency. Because the intermediate frequency (465 kHz in China) is lower than the high frequency signal before conversion (radio frequency) and the frequency is fixed, the signal of any radio station can be amplified equally. In addition, the amplification of intermediate frequency is easy to be higher than self-excitation, so a superheterodyne radio with high sensitivity can be made. Because foreign radio stations have to be converted into intermediate frequency through "frequency conversion" to pass through the intermediate frequency amplification loop, the selectivity of radio stations can be improved. Main structure 1. Frequency conversion stage The frequency conversion stage of superheterodyne radio station includes mixer and local oscillator. The high-frequency amplitude modulation signal received by the receiving antenna is sent to the mixer of the frequency conversion stage through the selection of the tuning input loop. The high-frequency constant-amplitude oscillation current generated by the local oscillator (the constant-amplitude high-frequency signal generated by the frequency conversion stage itself) is also sent to the mixer. Usually, the frequency of the local oscillator is higher than the frequency of the external signal, and the higher value should be kept at a certain value, that is, the intermediate frequency. The result of mixing the two signals in the mixer produces a new frequency signal, that is, the basic function of the mixer is to beat the carrier frequency of the input signal with the carrier frequency of the local oscillator and get a "difference frequency" signal at its output, that is, an "intermediate frequency" signal. This is the "heterodyne effect". China's radio IF is set at 465 kHz. The difference frequency signal of 465 kHz still belongs to the high frequency range, but it is called "intermediate frequency" signal because it is lower than the carrier frequency of foreign signals. The external high-frequency amplitude modulation signal only changes the carrier frequency after frequency conversion, which requires that the modulation law of the original signal cannot be changed and it is still modulated in the new intermediate frequency signal, so the intermediate frequency signal output by the frequency conversion stage is still an amplitude modulation signal. The working process of this circuit is described as follows: Lab is a coil wound on a magnetic bar, Lab, Ca and Cat form a high-frequency tuning loop, and Lb, Cb, Cbt and C3 form a local oscillation loop. The high frequency amplitude modulation signal received by the magnetic antenna is added between the base and emitter of the frequency converter by the coupling coil Lcd through the selection of the high frequency tuning loop; The high-frequency constant-amplitude signal generated by the local oscillator (fixed intermediate frequency higher than the external signal frequency) is also applied between the base and emitter of the frequency converter through C2, C 1 and R2. We know that the emitter junction (P-N junction between emitter and base) of a semiconductor triode is a nonlinear element, so when external signals and local oscillation signals are added to the emitter-base loop, they are mixed, resulting in the required difference frequency (465 kHz). We select the amplified IF signal through the IF resonant circuit (commonly known as the middle ring) composed of L3 connected in the collector circuit, and send it to the IF amplifier through the secondary output of L3. In order to make the difference between the local oscillation frequency and the high-frequency resonance frequency of the tuning loop always be a fixed intermediate frequency (465 kHz), when changing the resonance frequency of the tuning loop (when selecting a radio station to listen), the oscillation frequency of the oscillation loop must be adjusted at the same time, which is called "unified tuning". In order to simplify the tuning procedure, in the radio, the above two circuits are regulated by a coaxial double variable capacitor (Ca, Cb). The commonly used double variable capacitors are of equal capacitance type. For example, there are 270PF×2, 365PF×2 and other specifications. When the variable capacitor with equal capacitance and double connection is used, a small capacitor Cbt must be connected in parallel with the variable capacitor CB in the local oscillator circuit, and the CbT should be properly selected so that the two circuits can be better tuned. C3 is a buffer capacitor, which is used to compensate the tuning deviation of the high-end and low-end of the frequency band. Resistors R 1 and R2 form a bias circuit. L2 is a medium wave oscillating coil. L3 is "mid-week". IF amplifier and IF amplifier are extremely important components of superheterodyne radio station, and the quality of IF amplifier has a decisive influence on the sensitivity, selectivity and fidelity of radio station. The working frequency of the IF amplifier in the radio is 465 kHz. Using the resonant circuit as the load can greatly improve the sensitivity and selectivity of the radio. The radio IF amplifier circuit of the experimental suite is shown in Figure 3. The intermediate frequency signal converted to 465 kHz by the frequency conversion stage is coupled to the base of Q2 through the intermediate frequency transformer L3, amplified by Q2, and then coupled to Q3 by the second intermediate frequency transformer L4 for second intermediate frequency amplification. Q3 is not only the amplifier tube of the second intermediate amplifier, but also the detection stage. The amplified IF signal of Q3 is detected by the unilateral conductivity of the base PN junction of Q3. R3 is the bias circuit of the first amplifier Q2, and one of the tasks of C4 is to bypass the IF signal. R4, R3 and +0 are bias circuits of the second intermediate discharge tube Q3. C5 and C6 are bypass capacitors, and the audio signal is coupled to the low amplifier stage through C7. The IF transformer is used to couple the IF amplifier of each pole. Because the output impedance of the triode is low, considering the impedance matching, the power supply is connected from the primary center head of the intermediate frequency transformer. At the same time, the secondary is mostly non-adjustable, and the number of turns is small, so as to adapt to the small input impedance of the triode connected to the next stage. Detection and automatic gain control In superheterodyne radio, diode detectors are usually used. In fig. 3, the bipolar unilateral conductivity of Q3 is used as a detection diode, C5 and C6 are intermediate frequency filter capacitors, and W 1 is a detector load and also used as a volume control potentiometer. The detected audio signal is sent to the low-frequency amplifier by the sliding arm of the potentiometer through the DC blocking capacitor C7. When the radio receives different radio signals, the volume often changes greatly. The radio signal is too strong, even causing distortion. These phenomena can be avoided by installing automatic gain control. The automatic gain control circuit consists of R3 and C4. After detection, a part of the audio signal is sent back to the base of the first amplifier Q2 through R3. Due to C4 filtering effect, the AC component in the audio signal is filtered out, while the DC component is retained. What is actually sent back to Q2 base is the DC component in the audio signal. When the detected audio signal increases, IC3 of Q3 increases and the collector potential of Q3 decreases. By reducing the base potentials of R3 and Q2, the collector current of Q2 is reduced, and the amplification factor of Q2 is reduced, thus keeping the detected audio signal basically unchanged, thus achieving the purpose of automatic gain control. The power amplifier circuit Q4 is the driving stage, and its collector current is large, so it can output certain audio power to drive the final power amplifier. Input transformer L5 plays the role of impedance matching and phase inversion. It outputs signals with equal magnitude and opposite phases, and pushes transistors Q5 and Q6 to perform analog pull-in power amplification. Q5 and Q6 are connected in series to form an OTL push-pull power amplifier circuit. R7, R8, R9 and R 10 are bias resistors, so that Q5 and Q6 have a certain collector current when there is no signal input to eliminate cross distortion. The inverted signal provided by L5 secondary turns on Q5 and Q6 alternately, and the amplified complete signal is output at the collector of Q6, which is coupled to the speaker through DC blocking capacitor C9. Analysis of the overall circuit of the superheterodyne six-tube radio: the signals induced by the magnetic antenna are sent to the resonant circuits Lab and Ca, which are tuned to the received signal frequency, and other interference signals are suppressed accordingly. Then the high frequency signal is sent to the base of frequency conversion stage Q 1 through the coupling of Lcd. The oscillating voltage of the frequency conversion stage is injected into the emitter of Q 1 through C2. Lb and Cb form an oscillation loop, and the feedback is realized by Lc. So this is a frequency conversion stage in which the oscillating voltage is injected from the emitter and the signal is injected from the base. R 1 and R2 are bias elements, and C 1 is used for high frequency bypass. After frequency conversion, the signal is converted into an intermediate frequency signal of 465 kHz, which is taken out by the intermediate frequency transformer L3 resonating at 465 kHz and sent to the first intermediate frequency amplifier stage composed of Q2. The first intermediate amplifier stage adds automatic gain control and consists of R3 and C4. C4 is a large-capacity electrolytic capacitor, and its main function is to filter out the detected audio current. The IF signal amplified by Q2 is taken out by L4 and sent to the second IF amplifier stage. R4, R3 and W 1 are the bias resistors of the second intermediate amplifier stage, and C5 and C6 are bypass capacitors. The signal after two-stage intermediate amplification is detected by the bipolar unidirectional conduction of Q3. The audio signal on potentiometer W 1 is coupled to the preamplifier stage consisting of Q4 to C7. The detected DC component is added to the base of IF amplifier Q2 through R3 for automatic gain control. The audio signal amplified by Q4 is sent to the push-pull power amplifier stage composed of Q5 and Q6 through L5, and finally more audio power is output to push the speaker to sound. R5 is the bias resistance of Q4; R7, R8, R9 and R6+00 are the bias resistors of the push-pull amplifier stages of Q5 and Q6. C 10, R6 and C 1 1 form a power supply decoupling circuit; Capacitor C8 is used to improve sound quality; Cat and Cbt are fine-tuning capacitors at the top of double variable capacitors; The resonant capacitors of the local IF transformers L3 and L4 are made together with the IF transformers, so the position of the resonant circuit capacitors is no longer designed in the printed circuit board. L5 is the input transformer and JK is the external headphone jack.