Not All Gain Equalizers are Created Equal

Feb. 5, 2019

With RF, microwave, and millimeter-wave systems, the component and system losses, along with frequency dependent gain and losses of devices and components, lead to uneven frequency versus gain slopes in many systems. This is often undesirable, especially in high precision sensing and test and measurement applications. For example, with wideband radar technologies, a precisely controlled and flat gain across multiple frequency octaves is needed to reduce error. Gain equalizers are active or passive components that are designed to correct the uneven gain curves of a device or system, and enhance their gain flatness.

Gain equalizers are relied upon in electronic warfare (EW), signal intelligence (SIGINT), test and measurement applications. Applications that cascade several gain stages may also require gain equalizers to flatten the compounded negative gain slope that many RF amplifiers exhibit.

There are several key parameters for Gain Equalizers, including their impedance, VSWR, insertion loss, linearity, power handling, return loss, bandwidth, control capability, and packaging. For modern radar, test and measurement, and communication systems that rely on complex modulation schemes with high modulation orders, phase noise and phase balance may be additional considerations. The main specifications that define a gain equalizer is its insertion loss versus frequency. Power handling is a limiting performance parameter of many gain equalizers, as the signal energy absorbed by the gain equalizer is converter to heat, and the packaging and interconnect method must be able to handle sustained temperatures without derating.

Gain equalizer circuits can be as simple as a simple passive attenuator and inductors/capacitors with a negative sloping loss curve that compliments the negative sloping gain curve of an amplifier. More complex gain equalizers are active circuits that demonstrate a voltage variable insertion loss. Often these voltage variable gain equalizers have a negative loss slope which increases or decreases as higher DC bias/control voltages are applied. Gain equalizers are available as surface mount technology (SMT) packages, similar to chip attenuators, as inline coaxial assemblies, and as monolithic microwave integrated circuit (MMIC) devices.

Given that gain equalizers are often implemented as attenuators with a gain slope designed to complement the gain slope of a device or system, minimum insertion loss may be overlooked as a consideration. However, many systems can only afford to sacrifice enough gain in order to achieve a desired gain flatness, and any additional loss may reduce overall system performance. Hence, the minimum insertion loss can be seen as an additional loss beyond the loss necessary to equalize the gain. With radar as an example, reduced transmitter output power or receiver sensitivity from a gain equalizer with excessive insertion loss would reduce the range of the radar.

Parasitic capacitances and inductances in an adjacent circuit can also influence the loss curve of a gain equalizer, and degrade performance. This is less likely with coaxial packaged gain equalizers, as the interconnect method, such as wire bonding or die attach, for SMT and MMIC chip attenuators intrinsically add parasitic capacitance, inductance, and resistance.

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