This article describes the design and implementation of L-band frequency synthesizer based Local Oscillator to down-convert the uplink signal in a Telecommand Satellite Digital Receiver. An Integer-N PLL scheme was implemented to achieve a compact and low power module. The Local Oscillator when tested with the Receiver successfully achieved all performance parameters for satisfactory operation of the total system.

There has been an ever-increasing demand for low cost, low power, miniaturized frequency sources that can be conveniently configured to generate any predefined frequency without major changes in the circuit and without elaborate testing. A frequency synthesizer is a system that can generate several distinct frequency signals from a single reference source or time base.

In a satellite system, a synthesizer finds applications as a transmitter carrier; as a local oscillator for up/down conversion; or as a frequency multiplier. Important features of a frequency synthesizer are frequency stability (ppm/°C), phase noise (dBc/Hz), frequency resolution (Hz), signal power (dBm), switching time (sec), spectral purity (dBc), power consumption (W), size and cost. In a satellite system size, weight and power play a dominant role in the choice of any particular architecture.

Techniques used for frequency generation include conventional methods using frequency multipliers, Direct Digital Synthesizers DDS, Integer-N PLL, Fractional-N PLL, Sample Phase Lock Detector SPLD. The choice of a technique for any given application is governed by the factors given above.

For example if sub-hertz resolution is desired then DDS based architecture is the ideal choice. Our requirement was to generate two 7dBm signal sources at 1888.8 MHz and 188.88 MHz with ± 2 ppm frequency stability over a temperature of -10°C to +50°C. The Local Oscillator was also subjected to vibration and thermovac tests before qualifying it for flight. We required a phase noise of –95 dBc/Hz at 1 kHz offset and a spectral purity of better than –40dBc. Integer-N PLL architecture was chosen for deriving these signals in a compact and low power fashion.


Integer-N PLL Local Oscillator for Satellite Telecommand Receiver
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