The Trilithic VF-4 Tunable Filter provides the preselection needed for the accurate measurement of composite triple beat, cross modulation and other spurious signals in a working CATV system. Measurements made without the use of a filter are prone to error because spectrum analyzers and signal level meters overload in the presence of many carriers. Input overload causes instruments to generate spurious signals which interfere with measurements and which may be misinterpreted as problems in the CATV system. A VF-4 filter placed between the test instrument and the system test point blocks all unnecessary signals, passing only the carrier to be analyzed to the instrument's input. By eliminating input overload, a VF-4 can improve an instrument's reliable measurement range by as much as 30 dB.