Constant amplitude voltage controlled variable bandwidth crystal filters

A transistorized crystal I.F. stage featuring variable bandwidth with constant midband amplitude has been developed. No mechanical tracking devices or mechanical bandwidth controls are employed: the constant amplitude feature is electronic, while the bandwidth is proportional to a D.C. control voltage. At narrow bandwidth the response curve is Gaussian with relatively steep skirts; as the bandwidth nears its maximum value the top of the curve first flattens, then exhibits a double peak at either side of midband frequency. The bandwidth of the 100KC stage actually constructed varied from 8 to 1400 cycles at the 3 DB points as the control voltage varied from 0 to 3 volts. The circuit exhibited a midband amplitude variation of less than 3% over the entire bandwidth range. Circuit operation has been analyzed both qualitatively and theoretically, and empirical results are given.