AM suppression with low AM-PM conversion with the aid of a variable-gain amplifier

This paper proposes the use of a variable-gain amplifier instead of a hard limiter for amplitude modulation (AM) suppression with low AM-PM (phase modulation) conversion. A hard limiter shows phase shift variations through input-amplitude dependent changes in output waveform, combined with bandwidth limitations, It is shown that these can be kept small only for limiter bandwidths much larger than the input frequency. A linear amplifier with variable gain used for AM suppression does not suffer from this problem. A CMOS variable-gain amplifier with gain-insensitive phase shift has been designed for this purpose. The benefits and limitations of the technique are explored with reference to an experimental 2.5 mu m BICMOS chip for a television IF demodulator. Experimental and simulation results indicate that the AM-PM conversion can be kept below 0.5 degrees at 40 MHz over an input amplitude range of 20 dB, where typical hard limiters show 3-5 degrees. This is achieved with an amplifier bandwidth of 80 MHz, while a hard limiter would need a bandwidth of more than 600 MHz to obtain similar results.