US Patent No. 4,875,019 October 17, 1989
A preamplifier system adapted for a Loran receiver includes a buffer amplifier receiving the signal from the antenna at its input and a tuned circuit connected between the input of the buffer amplifier and ground. The tuned circuit includes a parallel combination of a resistance, capacitance and inductance with values selected such that a resonant frequency is provided substantially at or near the carrier frequency of the Loran signal to pass it while higher and lower frequencies, constituting extraneous noise signals, are substantially attenuated. In particular, low frequency, e.g. 60 Hz signals and DC from sources such as precipitation static, are shunted to ground through the parallel inductor in the tuned circuit so that these low frequency components do not affect the performance of the receiver. A secondary filter section is connected to the output of the buffer amplifier and has components that may be selected in value such that the overall transfer function of the preamplifier, including the tuned input circuit, has a desired bandpass characteristic. Thermal noise contributed by the parallel resistor in the tuned circuit may be reduced by feeding back the output of the buffer amplifier through the resistor to effectively reduce its noise contribution while leaving the effective input resistance the same. Notch filters may be incorporated with the input tuned circuit to reject strong extraneous signals within the passband.