Equalizers are used in many coaxial cable transmission systems (e.g. CATV applications). Their main application is to adjust for an (overall) flat amplitude response.

This is necessary, because coaxial cables have a higher loss at higher frequencies. To compensate for that, lower frequencies must be damped.

Also when using rf amplifiers, their frequency response very often has a higher gain at lower frequencies. In order to "equalize" that, the lower frequency range has to be attenuated.

In both cases, an equalizer does the job. And yes, it must be carefully chosen / designed to achieve a flat amplitude response of the overall system.

The Design starts with a 1-pole highpass. This is in fact just a capacitor. As this will reflect the lower frequencies, a lowpass with a 50 Ω termination has been added to absorb that reflected energy and present a broadband matching to the outer world. In order to make this circuit symmetrical, the lowpass is split in two portions. (One at each port). We used RFSim99 to simulate the circuit. When the slope deems satisfactory, the ideal inductors are replaced with real world components. For this, we used the physical model and used the data given in the datasheet to optimise the frequency response.

The measurements below have been made with a hp8753.



S21 from 10 MHz to 1 GHz.




S11 from 10 MHz to 1 GHz. S22 is similiar, as the circuit is symmetrical.

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