Radio and television transmitter is a comprehensive electronic system, it not only includes the wireless transmission of video and audio channels, but also includes the channel detection and automatic control circuit, so in the design, it must not only ensure that the technical index of the wireless channel is in the normal range, but also must design advanced sampling detection and protection alarm circuits, so as to ensure the normal operation of the transmitter. Thus the on-line automatic monitoring and control of transmitter can be realized. In recent years, with the development of multi-channel power synthesis technology of high power all-solid-state TV transmitter, more and more manufacturers adopt modular structure design, so a single power amplifier module is the basic measuring unit of the whole transmitter. This paper focuses on the discussion of the detection and control circuit of a single module, so as to realize the automatic monitoring of the online state of the transmitter.
First, the working principle
rf hybrid combiner in the power amplifier module, the main detection and control parameters are power supply voltage, the working current of each amplifier tube, output power, reflected power, over temperature and over excitation protection, etc. Figure 1 is the block diagram for realizing the above detection and control functions. It consists of sampling amplifier circuit, V/F transform, isolation circuit, F/V transform, A/D conversion, AT89C51, display circuit and output protection circuit, etc.
1. Isolation circuit
In the power amplifier module, due to the application of high-power devices, often the output power of a single module is relatively large, so there is a large high-frequency interference to small signals, such as poor processing, will affect the work of the post-stage ADC circuit, resulting in inaccurate detection data, display data jumping phenomenon, and even misaction. The photoelectric coupler is used here for isolation. Because the photoelectric coupler has the characteristics of small volume, long service life, wide operating temperature range, strong anti-interference performance, no contact, and the input and output are electrically completely isolated, the analog circuit and the digital circuit are completely isolated, so as to ensure the system works safely and reliably in the environment of high voltage and high power radiation.
2, LM331 frequency voltage converter
V/F conversion and F/V conversion adopt integrated block LM331, LM331 is a high performance and price integrated chip produced by NS company, which can be used as a precision frequency voltage converter. The LM331 uses a new temperature-compensated band-gap reference circuit for extreme accuracy over the entire operating temperature range and at supply voltages as low as 4.0V. At the same time, its dynamic range is wide, up to 100dB; Good linearity, the maximum nonlinear distortion is less than 0.01%, working frequency is low to 0.1Hz fashion has good linearity; High conversion precision, digital resolution up to 12 bits; The external circuit is simple, only a few external components can be easily constituted V/F or F/V conversion circuit, and easy to ensure the conversion accuracy.
When the input Vi+ input a positive voltage, the input comparator outputs a high level, so that the R-S trigger is set, the output high level, the output drive tube is on, the output f0 is a logical low level, and the power supply Vcc also charges the capacitor C2 through the resistor R2.
When the charging voltage at both ends of capacitor C2 is greater than 2/3 of Vcc, the timing comparator outputs a high level, resets the R-S trigger, outputs a low level, and the output driver tube is cut off. The output f0 is a logical high level. At the same time, the complex zero transistor is switched on, and capacitor C2 quickly discharges through the complex zero transistor. The electronic switch causes capacitor C3 to discharge against resistor R3.
When the discharge voltage of capacitor C3 is equal to the input voltage Vi, the input comparator outputs a high level again, so that the R-S flip-flop is set, and the cycle is repeated to form a self-excited oscillation. The output pulse frequency f0 is proportional to the input voltage Vi, thus realizing the voltage-frequency transformation. The relation between input voltage and output frequency is as follows: fo=(Vin%26; # 215; R4)/(26 2.09%; # 215; R3%26; # 215; R2%26; # 215; C2) from the formula known resistance R2, R3, R4, and C2 directly affect the conversion result f0, so the precision of the components to have certain requirements, can be selected according to the conversion accuracy. Resistor R1 and capacitor C1 constitute a low-pass filter, which can reduce the interference pulse in the input voltage and improve the conversion accuracy.
Similarly, a frequency-voltage conversion circuit can also be formed by M331.
rf hybrid combiner https://www.cenrf.com/Hybrid-Coupler
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