(2) Multisim-based radio transmission system
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1. This series is divided into five chapters , 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 − 1605 k H z 535−1605kHz\,535−1605kHz
2. IF signal frequency 465 k H z 465kHz\,465kHz
3. Modulation signal frequency 500 Hz − 10 k Hz 500Hz-10kHz\,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 a 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
Please see the previous article (1) Multisim-based radio transmission system: oscillator design.
Design of Amplitude Modulator
AM modulation
In amplitude modulation, the information that needs to be transmitted is used to control the amplitude of the high-frequency carrier oscillation voltage .
Suppose the carrier signal voltage is:
uc (t) = U cmcoswct u_c(t)=U_{cm}cosw_ct\,uc(t)=Ucmcoswct, The
mathematical expression of ordinary AM wave is:
u AM = U cm (1 + macos Ω t) coswct u_{AM}=U_{cm}(1+m_acosΩt)cosw_ct\,uAM=Ucm(1+macosΩt)coswct，
其中ma = ka U Ω / U cm m_a = k_aU_Ω / U_ {cm} \,ma=kaUOh/ Ucm, It can be seen from the waveform of the AM wave that ma = (U max − U min) / (2 U cm) m_a=(U_{max}-U_{min})/(2U_{cm})\,ma=( Umax−Um i n) / ( 2 Ucm), When
undistorted modulation, ma ≤ 1 m_a≤1\,ma≤1, If ma ＞ 1 m_a＞1\,ma＞1, The envelope shape of the modulated wave is different from that of the modulated signal, and over-amplification occurs.
Differential pair circuit
The circuit shown in the picture is a differential pair circuit. The DC potential difference between terminal 4 and terminal 1 is not zero. At this time, it is an ordinary amplitude modulation wave.
When U cm ＞ ＞ (2 k T) / q U_{cm}＞＞(2kT)/q\,Ucm＞＞(2kT)/qWhen is large signal input, the output current is:
Multisim circuit and analysis
To complete AM modulation, the input loop directly couples the carrier signal and the modulation signal and directly adds them to the nonlinear device , uses a multiplier to generate a new frequency, and finally uses a band-pass filter to take out the frequency components of the AM wave to suppress unwanted frequencies ingredient.
The amplitude modulation circuit is shown in the figure, the input carrier frequency is set to fc = 664.7 k H z f_c=664.7kHz\,fc=6 6 4 . 7 k H z, The modulation signal frequency is set to f 0 = 1.65 k H z f_0=1.65kHz\,f0=1.65kHz. This system uses MC1596 analog multiplier . Since there is no MC1596 in Multisim , we can encapsulate it according to the internal circuit structure of MC1596 .
Experimental results and analysis
The simulation results are shown in the figure, and the modulation amplitude is calculated:
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.