(3) Multisim-based superheterodyne receiving system: design of intermediate frequency amplifier

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1. This series is a superheterodyne receiving system based on Multisim. It is divided into five chapters , including (1) the design of local oscillator, (2) the design of mixer, (3) the design of intermediate frequency amplifier, and (4) detection The software used for the design of the device and (5) the design of the buffer is Multisim14 .
2. The previous series is based on Multisim radio transmission system, divided into five parts , including (1) oscillator design, (2) amplitude modulator design, (3) high frequency power amplifier design, (4) low frequency The software used for the design of the power amplifier and the design of the buffer (5) is Multisim14 , please refer to my previous article for details!
3. Free to share the original files of the Multisim circuit design of the entire radio transmission system and the original files of the Multisim circuit design of the superheterodyne receiving system, just leave a comment.
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System Requirements

1. Carrier signal frequency $535-1605kHz$
2. IF signal frequency $465kHz$
3. Modulation signal frequency $500Hz-10kHz$

Fundamental

The main task of the superheterodyne receiving system is to demodulate the original useful signal from the modulated AM wave . The principle block diagram is shown in the figure. The specific operation process of the system is as follows.
1. The input circuit selects one of the signals sent by many radio broadcasting stations in the air and sends it to the mixing circuit. The mixer changes the frequency of the input signal to an intermediate frequency, but its amplitude change law does not change. Regardless of the frequency of the input high-frequency signal, the frequency after mixing is fixed, and China stipulates it as 465kHZ .
2. The intermediate frequency amplifier amplifies the intermediate frequency amplitude modulation signal to the size required by the detector.
3. The audio signal carried by the intermediate frequency amplitude modulation signal is taken down by the detector and sent to the low frequency amplifier.
4. The low-frequency amplifier will amplify the voltage of the detected audio signal, and then the power amplifier will amplify the audio signal to the level that its power can drive the speaker or earphone .
5. Finally, the audio electrical signal is converted into sound by the speaker or earphone.

Design of local oscillator

Please see the previous article (1) Multisim-based superheterodyne receiving system: design of local oscillator.

Mixer design

Please see the previous article (2) Multisim-based superheterodyne receiving system: mixer design.

Design of IF amplifier

Fundamental

The intermediate frequency amplifier circuit is a well-tuned circuit with filtering properties, so the receiver has a high selectivity index.
In the superheterodyne receiving system, the radio signal frequency becomes an intermediate frequency signal at any value , and then enters the intermediate frequency amplification stage, so the radio stations of different frequencies can be evenly amplified. The unwanted adjacent radio signals and the difference frequency between some interference signals and the local oscillator signal is not the predetermined intermediate frequency, so they are "turned out."

Multisim circuit and analysis

The design index of the intermediate frequency amplifier is: center frequency $f=465kHZ$, Bandwidth $2△f_{0.707}=8kHZ$。

Simulation results

Detector design

Please see the next article (4) Multisim-based superheterodyne receiving system: the design of the detector.

Buffer design

Please see the next article (5) Multisim-based superheterodyne receiving system: buffer design.