(4) Multisim-based superheterodyne receiving system: design of detector

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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

Please see the previous article (3) Multisim-based superheterodyne receiving system: design of intermediate frequency amplifier.

Detector design

Diode large signal envelope detection

Fundamental

This system adopts diode large signal envelope detection , which consists of input circuit and diode $V_D$Japanese $RC$The low-pass filter consists of three parts.
It is suitable for demodulating large signals with larger carrier components, using the unidirectional conductivity characteristics of the diode and the charge and discharge process of the detection load to achieve detection.

Charge and discharge process

The input signal is an amplitude modulated wave, when the carrier is positive for half a cycle, the diode is forward conducting, and the input high-frequency voltage passes through the diode to the capacitor $C$Charging, the charging time constant is $\tau =RC$. Because $\tau =RC$Smaller , charging quickly , the voltage on the capacitor builds up quickly, and the output voltage increases quickly.
When the input signal reaches the peak value, it begins to drop. When the voltage across the diode is equal, the diode $V_D$Cut off, capacitance $C$Pass the charge stored during the on-time through $R$Discharge. Due to the discharge constant $\tau =RC$Larger , slower discharge .

Inert distortion and negative peak cutting distortion

$R$、$C$The value is too large, so that $R$、$C$When the discharge speed corresponding to the discharge time constant of is less than the AM envelope drop speed , the output waveform will not change with the input signal envelope, resulting in distortion .
In order to avoid inert distortion , each parameter should be satisfied.

Due to the different AC and DC load resistance of the detection circuit, negative peak cutting distortion may occur . In order to avoid negative peak cutting distortion, each parameter should meet

Multisim circuit and analysis

From the analysis of inert distortion and negative peak cutting distortion, we take $R_1=3k\Omega$，$C_2=8nF$。

Simulation results

Buffer design

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