Very narrow-band tunable filters and amplifiers using a small signal varactor up-converter

This paper presents a method for realizing tunable microwave components with bandwidths that can be less than 0.5% by the use of a varactor up-converter (UC). In the past the UC has been used to up-convert an input signal at frequency f_S to a signal at frequency f_S+f_P, where the varactor is “pumped” at frequency f_P. In this paper we will use the property of the UC that when it is terminated at frequency f_S in an impedance Z(ω_S) where ω_S=2πf_S, this impedance is “imaged” at the output frequency f_S+f_P. For example: if a bandpass filter with a passband from 97.5 to 102.5 MHz is terminated in a negative resistance and connected to the input terminals of a UC pumped at a frequency of 900 MHz, then the negative resistance will be ”imaged” at the output of the UC over the frequency band from 997.5 to 1002.5 MHz. Thus the 5% bandwidth of the input circuit results in a 0.5% bandwidth at the output of the UC. If the pump frequency is tuned to 870 MHz, then the negative resistance will be “imaged“ over the passband from 967.5 to 972.5 MHz. Thus by varying the pump frequency the passband can be tuned while preserving the bandwidth. Equations describing this “imaging” are derived in this paper for the small signal UC.