Directional filter for superheterodyne applications

A downconverter was required from 8.5 GHz and 9.0 GHz to 30 MHz. The receiver consisted of a double superheterodyne system that initially converted the input signal to 2 GHz and this was followed by a second mixer to 30 MHz. Isolators were used on the signal and LO ports of the mixer to ensure its correct match to 50 ohms. The minimum isolation between the LO and IF ports on the first mixer was only 10 dB. This precluded the use of a low pass filter to select the IF since the LO power would be reflected back into the mixer and cause unwanted modulation effects. Active isolators based on FET technology were considered but required biasing and produced unacceptably high noise figures. The design of diplexers using low pass, high pass and band pass filters in MIC technology were too complex. The travelling wave directional filter offers an attractive solution to this problem and is the subject of this article