Adjustable LNB working power supply meets DiSEqC requirements
Abstract: This article briefly introduces the DiSEqC standard and gives a DiSEqC compatible power supply circuit designed using the MAX1771. The application circuit can provide the required 22kHz pulse position modulation (PPM) signal and 13V or 17V output selection. The circuit also uses a comparator to detect the signal sent by the satellite antenna.
The circuit in Figure 1 provides a digitally switchable 13V or 17V power supply for a low-noise unit (LNB), which is typically used as an antenna feed in satellite receivers. Changes in operating power supply "tell" whether the remote LNB circuit should set the antenna to be clockwise polarized or counterclockwise, thereby reducing the interface and cable connection to the antenna.
Figure 1. This circuit is designed for a low-noise unit in a satellite receiver. This DiSEqC-compatible power supply implements data communication by switching the supply voltage between 13V and 17V.
The circuit shown in the figure also supports a communication bus that is being formed and considered more comprehensively, called the DiSEqC standard (for digital satellite equipment control). The open DiSEqC standard was developed by the European Wireless Communications Satellite Organization and promises to develop into a world standard for communication between satellite receivers and satellite peripheral equipment. More detailed information and circuits can be obtained through the DiSEqC website.
DiSEqC provides a 22kHz pulse position modulation signal with an amplitude of about 0.6V, superimposed on the LNB DC power supply. The encoding method allows remote electronic devices to perform more complex functions-such as changing the down-conversion frequency or mechanically adjusting the antenna direction. IC1 is a PFM-type boost controller, which controls the boost conversion from 5V to 13V or 17V by controlling an external FET. The digital input controls the position of the analog switch to set the voltage. This position determines the amount of feedback to IC1, which affects the voltage output. In this way, the logic input is low corresponding to 13V, and the logic is high corresponding to 17V output. IC2 is a single switch in a tiny SOT23-5 package, which is very suitable for this simple switching task.
The components on the right side of the schematic provide compatibility with the DiSEqC standard. The comparator in IC3 constitutes a receiver to detect the data sent from the slave LNB device (DiSEqC standard allows bidirectional data flow). This output can be connected to the IRQ or port pin of a microcontroller (not shown) to complete the decoding.
The DiSEqC transmitter consists of a transistor Q1 and an LED (D1). D1 is not only used to send instructions, but also used as a constant voltage source to force a constant current corresponding to about 40mA to pass through Q1. During the 22kHz burst signal encoding from the microcontroller, the low-power part turns off the LED by absorbing its driving current, and also forcibly turns off Q1. The 40mA switching current flows through R5, producing a 600mV voltage output swing that meets the specifications Breadth.
C4, L2 and R5 form a resonant circuit, and its impedance at 22kHz is 15Î© according to the specification. The DC resistance of the inductor must be below 0.5Î© to provide a maximum load current of 0.5A. The circuit can also work at 12V with higher efficiency. When working at 12V, please check the MAX1771 data sheet to select the appropriate L1 and R1.
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