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The phase-locked loop (PLL) has been widely used in traditional tunable RF synthesis design to maintain a “fixed” phase relationship between a generated (output) frequency and that of a reference signal. Although a properly designed PLL is an effective method for maintaining a fixed frequency, it does not allow for fully monotonic phasing in applications where phase coherence is critical.

The typical operation of analog VGA’s is to use dc voltage as the “variable” to control the amplification and hence overall signal level. This type of VGA typically requires a driver circuit to control the voltage swing but benefits in the ability to achieve fi ner degrees of attenuation and typically a much wider dynamic range.