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Click here to see another parabolic equalizer example (6-18 GHz)
New for April 2022. Here we will describe a gain equalizer that can operate from 2-18 GHz. Such a device could be handy in flattening out the gain curve of a 2-18 GHz TWT, among other applications. The gain "cost" of this component is less than 1.5 dB at the band edges (this is the ideal case... your real results will not be as good).
Why aren't there any parablic equalizers out there as COTS parts? Maybe their use is so limited no one thinks it is worth it to develop a product line. If you serch EverythngRF, all you will find are linear linear equalizers. We are on our own here!
Below is a schematic we developed using infinite monkey technique. It has two identical open stubs that are quarter-wave at center frequency, separated by a quarterwave line. The stubs are connected to the main line using two resistors of equal value. The resistor value is what sets the depth of the parabola. By the way, is the transmission coefficient really an actual parabola versus frequency? Probably not, but using that name gives a good sense of what it does.
Below is a plot of the equalizer with five different resistor values that we found worked out to provide approximately 1dB steps in the parabola depth.
188 ohms: 1dB
128 ohms: 2 dB
98 ohms: 3 dB
80 ohms: 4 dB
67 ohms: 5 dB
57 ohms: 6 dB
Below is teh predicted performance with these values, with the markets on the 188 ohm S21 curve. Note that through all of this range, return loss of better than 10dB is provided.
If you wanted to go crazy you could probably create a two or three-bit digital parabolic equalizer MMIC where the resistors are switched using FETs. While you are designing that, please do a linear, negative-slope equalizer MMIC as well. If you ever use this design, be sure to give us credit for it. You're welcome!
Next, we will show an example of how this equalizer can fis the gain of a component. Stay tuned!