What are Absorptive Filters?

Normal passive electromagnetic filters are “reflective”. That means the out-of-band electromagnetic energy is reflected towards its source. Equipment such as transmitters, mixers, amplifiers or high precision oscillators can be sensitive to such returning energy, leading to a degraded signal-to-noise ratio and dynamic range. Non-reflective “Absorptive filters” guarantee that no energy is reflected towards its source.

Wainwright Instruments offers absorptive filters that consist of a band pass filter and a band reject filter, with their respective passband and stopband set to the same frequencies. To function as an absorptive filter, a power absorbing “Dummy Load” must be added. Alternatively, instead of a dummy load, one may also use measuring equipment with well-defined input impedance (Power meter, SPA or VNA) to monitor harmonics and non-harmonic spurious emissions.

Absorptive filters are also called reflectionless filters.