(1) Multisim-based radio transmission system: oscillator design

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1. This series is divided into five articles , including (1) oscillator design, (2) amplitude modulator design, (3) high frequency power amplifier design, (4) low frequency power amplifier design and (5) buffering The design of the device and the software used are Multisim14 .
2. The next series is a superheterodyne receiving system based on Multisim, so stay tuned.
3. Free to share the original files of the Multisim circuit design of the entire radio transmission 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 radio transmission system uses free space as the transmission channel, and loads the signals to be transmitted into high-frequency oscillations and transforms them into electromagnetic waves and sends them to the remote receiving point.
The overall framework of the radio transmission system is shown in the figure.
In order to improve the frequency stability, an improved capacitor three-terminal oscillator- Schiller oscillator is used , and a buffer is added behind it to weaken the influence of the latter stage on the main oscillator. The amplitude modulator is the core of the transmitter, and an analog multiplier is used to modulate the carrier signal and the input signal. Although AM modulation has low power utilization and poor anti-interference ability, its receiving equipment is simple and is still widely used in radio transmission systems. The high-frequency power amplifier amplifies the power of the modulated signal for signal transmission.

Oscillator design

Schiller circuit

The main oscillator is the core component of the AM transmitter, which is mainly used to generate a high-frequency sine wave signal with stable frequency, large amplitude and small waveform distortion as the carrier signal . Because the experiment has higher requirements for the stability of the oscillation frequency, the Schiller oscillator is used . The schematic diagram of the Schiller circuit is shown in the figure.

The Schiller circuit is based on the Krapper circuit with a capacitor C 4 C_4 in parallel across the inductor L $C_4$. The circuit condition is still $C_3<<$$C_1$，$C_4<<$$C_1$，$C_3$With $C_4$Same order of magnitude. The total capacitance of the loop is: in the

parallel resonant loop, the formula for the resonant frequency can be calculated:

from the above analysis, it can be seen that the Schiller oscillator has the characteristic of weak coupling between the transistor and the loop , and the frequency stability is high . The output oscillation voltage amplitude remains basically stable, can work in a wide frequency band, and is suitable for generating carrier waves.

Multisim circuit and analysis

The carrier can be expressed as $S(t)=Acos(w_c)$, Where $A$Is the carrier amplitude, $w_c$Is the carrier angular frequency, the initial phase of the carrier is $0$.

As shown in the figure above, set the carrier frequency to $664.7kHz$。
1. $V1$It is the power supply voltage, which provides a higher voltage at the collector. In order to generate oscillation, a switch should be connected to the connection. The switch is disconnected before oscillation, and the switch is closed during oscillation.
2. $R_2$With $R_3$Is a bias resistor, provides a bias voltage for the base, $R_4$Has the effect of negative feedback. Fine-tune $R_2$With $R_3$, Change the quality factor of the circuit $Q$, Make the triode work in the enlarged state.
3. Add a high-frequency choke to the collector to prevent the carrier from affecting the collector current.
4. The circuit parameter calculation process is as follows.
First set $C_2=1nF$、 $C_3=33nF$, Because it is known that $C_4<<$$C_2$, First set $C_4=47pF$，$C_5$It is a variable capacitor, easy to adjust.
From the following formula, we get: $L_2=0.737mH$， $C_5=25.192pF$。

Simulation results

During simulation, adjust the variable capacitor and variable resistance appropriately according to the number displayed by the frequency counter until the expected frequency is reached. The waveform diagram and simulation results at this time are shown in the figure. After starting the vibration, the frequency gradually stabilizes. When the waveform is relatively stable, the frequency stability is calculated as:

Design of Amplitude Modulator

Please see the next article (2) Multisim-based radio transmission system: the design of amplitude modulator.

Design of high frequency power amplifier

Please see the next article (3) Multisim-based radio transmission system: design of high-frequency power amplifier.

Design of low frequency power amplifier

Please see the next article (4) Multisim-based radio transmission system: design of low-frequency power amplifier.

Buffer design

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