Electronic Design

Adjustable LNB power supply is DiSEqC-compatible

The circuit shown provides a digitally switchable 13 V or 17 V for the low-noise block (LNB) typically found in satellite receivers at the antenna feedhorn. This variation of supply voltage “tells” the remotely located LNB electronics whether it should set the antenna polarization clockwise or counterclockwise, thereby eliminating the need for an interface and cable connection to the antenna.

The circuit also supports an emerging and more sophisticated communications bus called the DiSEqC standard (for Digital Satellite Equipment Control). Though developed by the European Telecommunications Satellite organization (EUTELSAT), the open DiSEqC standard promises to become the de facto world standard for communications between satellite receivers and satellite peripheral equipment. More details and circuits are available at the DiSEqC web site: http://www.eutelsat.org.

DiSEqC provides a 22-kHz pulse-position-modulated signal of about 0.6-V amplitude, superimposed on the LNB’s dc power rail. Its coding scheme allows the remote electronics to perform more complex functions, such as varying the downconversion frequency or physically rotating the antenna assembly.

IC1 is a PFM boost-converter controller that controls an external FET to provide the step-up conversion from 5 V to either 13 V or 17 V. The digital input Voltage Control sets the position of an analog switch that determines the amount of feedback to IC1 and hence the output voltage level. Thus, an input logic low selects 13 V and a logic high selects 17 V. IC2, a single switch that comes in a tiny SOT23-5 package, is ideal for this simple switching task.

Components on the right side of the schematic provide compatibility with the DiSEqC standard. The comparator in IC3 forms a receiver that detects data transmitted from a slave LNB assembly (the DiSEqC standard specifies bidirectional data flow). This output connects to the IRQ or port pin of a microcontroller (not shown) for decoding.

The DiSEqC transmitter consists of transistor Q1 and an LED (D1) that acts as a transmit indicator and as a constantvoltage source that forces a relatively constant current of about 40 mA through Q1. During encoded 22-kHz bursts from the microcontroller, the low portions turn off the LED by sinking its drive current, which forces Q1 off as well. The 40-mA switched current flows through R5, providing 600-mV output swings as required by the specification.

C4, L2, and R5 form a resonant circuit whose impedance at 22 kHz is 15 Ω as required by the specification. The inductor’s dc resistance must be 0.50 or lower to accommodate the 0.5-A maximum load currents. The circuit also operates on 12 V, and does so with greater efficiency. When operating at 12 V, consult the MAX1771 data sheet for suitable values of L1 and R1.

See associated figure

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