VSAT frequency source using direct digital synthesizer

DETAILED DESCRIPTION The objective of this invention is to simplify the overall operation and design of the carrier frequency generation components of the VSAT using a direct digital synthesizer (DDS) as a key element.
This invention is a part of a new type of Ku band VSAT terminal and system which uses minimum-shift-key (MSK) modulation for the remote-to-hub ("inroute") instead of the traditional phase-shift-key (PSK) modulation.
MSK is a "constant envelope", "continuous phase" modulation.
The frequency generation device of this invention reduces the two synthesizers of the prior art to a single synthesizer located in the IDU.
In this invention, the indoor-located synthesizer generates a reference signal which is conveyed to the outdoor unit.
In the outdoor equipment, this signal frequency is converted indirectly ("multiplied") by a fixed number (e.
g.
128) using a phase-locked-loop to produce a signal of the desired frequency.
The DDS of this invention operates such that the IDU generated signal has sufficient resolution (e.
g.
, 78 Hz) and sufficiently low phase noise (e.
g.
, 0.
039.
degree.
rms) in order to make this design practical.
The DDS also provides a scaled modulation which moves the carrier up and down in frequency at a controlled rate using fine steps of less than 1 Hz.
The DDS creates a high resolution, low frequency with spurious outputs not exceeding -70 dBc.
The further objects of this invention include the ability to achieve fine resolution using only a single synthesizer, to achieve an output having the necessary spurious levels for satellite communication, and to use a relatively simple cable for transmission between the IDU and ODU.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.