Digital satellite finder

By N.S.Sivaramakrishnan

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This article describes a satellite finder that we can build ourselves to assist with aligning the satellite TV dish without having to take our receiver outside. We are proposing a signal strength measurement meter which will greatly help in optimizing the dish antenna alignment precisely, once after the satellite is located.

Satellite finder implementation
Figure 1: Satellite finder implementation

The utilization of satellite finder is shown in figure 1. The low noise block (LNB) down-converter used for satellite reception (digital or analogue) does not just receive a single channel; it receives the complete range from all transponders that are operating on a particular satellite. Together with the high gain a modern LNB feeds a lot of RF energy to the receiver when the dish is correctly aligned. Our digital satellite finder measures the amount of RF energy over a wide frequency range by summing the power from all transponders and producing Received Signal Strength Indicator (RSSI) output, by means of proportionate DC voltage for display.

Since most of the existing satellite finders are using analog scale only, and some of the satellite finders are partially digital (digital handling with analog display) and rarely available digital satellite finders are with high cost, Our innovative design explores latest technology for RF power measurement (RSSI) with digital read out, economically, and provides fruitful results to the dish TV installers.

RF Power Detector

We are using the IC LTC 5508 from Linear Technology, for RF power measurement. The LTC5508 operates with input power levels from –32dBm to 12dBm.This IC is an RF power detector for RF applications operating in the 300MHz to 7GHz range. A temperature compensated Schottky diode peak detector and buffer amplifier are combined in a small SC70 package, to detect the RF power by means of dc output voltage. The detected voltage is buffered and supplied to the VOUT pin. Maximum power of RF signal produces peak output voltage as shown in the figure 3.

A logic low on the SHDN (pin1) disables the circuit and Logic high enables the circuit. This is done by a SPST switch (SW1) and resistor R2. The SHDN pin has an internal 150k pull down resistor to ensure that the part is in shutdown when no input is applied. This IC can be operated from the Supply Voltage, 2.7V to 6V. VCC is applied to the pin 4 and the capacitor C3 is used as bypass capacitor. A coupling capacitor C2 is used to connect to the RF signal source to the IC. The frequency range is 300MHz to 7GHz. This pin has an internal Schottky diode detector and a peak detector capacitor. The application circuit of LTC5508 is shown in figure 2.

LTC 5508 application

Figure 2. Application Circuit of LTC 5508

The LTC5508 RF detector integrates several functions to provide RF power detection over frequencies ranging from 300MHz to 7GHz. These functions include an internally compensated buffer amplifier, an RF Schottky diode peak detector and level shift amplifier to convert the RF input signal to DC, a delay circuit to avoid voltage transients at VOUT when coming out of shutdown and a gain compression circuit to extend the detector dynamic range. The buffer amplifier has a gain of two and is capable of driving a 2mA load. The buffer amplifier typically has an output voltage range of 0.25V to 1.75V.

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