Digital premodulation filter

DETAILED DESCRIPTION It is an object of the present invention to provide a digital premodulation filter circuit for generating various forms of continuous phase modulation (CPM).
Examples of such CPM are minimum shift keying (MSK), Gaussian minimum shift keying (GMSK), tamed frequency modulation (TFM) and four-level FM (4-FM) and additional modulation methods which a user may define.
The premodulation filter of the present invention is electronically configurable by a user to perform a desired modulation type, thus providing a more versatile instrument than previous methods which are typically dedicated to a single modulation type.
According to the present invention, the premodulation filter utilizes a finite impulse response filter (FIR) technique to digitally filter an input serial bit data stream.
Each modulation format, such as TFM, has a unique impulse response function g(t) which is the response to a single input data bit represented by a single impulse function.
The premodulation filter preforms a linear combination of g(t) responses resulting from the input data stream represented by a series of bimodal impulse functions.
The output of FIR digital filter in the form of parallel digital words is coupled to a digital to analog converter (DAC).
The output of the DAC is then passed through an analog filter to provide a smooth analog voltage input to a modulator to vary, for example, the frequency or phase of an oscillator, such as a voltage controlled oscillator (VCO) to provide a digitally modulated carrier output.
The prior art premodulation filters described hereinabove are typically dedicated to a single digital modulation format and are not easily user configurable.
In the premodulation filter of the present invention a number of frequency or phase pulse-shape function g(t) arrays are stored in ROM (or RAM) allowing the user to easily choose any desired modulation format.
A user may adjust operational parameters to optimize further performance or to design a custom modulation format for use with a users particular digital communication system