VHF & UHF PASSIVE COMPONENTS FOR RF POWER AMPLIFIERS

Passive components like two-way/four-way power dividers and combiners, dual quadrature couplers, directional couplers, low-pass and high-pass filters play a critical role in realising a high power amplifier. The technology of high power passive components changes from lumped element circuits at HF and VHF frequencies to stripline and microstrip technology above 500 MHz. The performance of the power dividers and combiners governs the structure, gain, and efficiency of the amplifier.
The important characteristics of the power dividers and combiners are frequency coverage, power handling, amplitude imbalance, phase imbalance, isolation between ports, insertion loss, input and output voltage standing-wave ratio (VSWR). In phase and quadrature phase power dividers and combiners are widely used in high power RF amplifiers. Directional Couplers are essential for the purpose of power monitoring and ensuring protection of the amplifier under high load VSWR conditions. High power low-pass filters are used for suppression of the unwanted harmonics generated by power amplifiers.
DRDO has developed various types of high power passive components for power amplifier applications in the frequency range of 20-1000 MHz. The technology has been transferred to both public sector and private sector agencies to establish the indigenous production base.
Following are some of the high power passive components designed and developed by DRDO.


Dual Directional Couplers


100-500 MHz dual direction coupler

Dual directional coupler is iDEAL for sampling and monitoring both forward and reflected RF powers at the same time. Couplerís coupling response plays a critical role in ensuring the gain flatness of the amplifier.
DRDO has developed dual directional couplers to enable the levelling of the amplifier output to a preset value across the 20-100 MHz and 100-500 MHz band and handles 2000 W and 1200 W CW power respectively. The couplers are essential for power monitoring and power control of RF amplifiers.

Four-way In Phase Power Combiners/Dividers


Four-way in phase power combiners/dividers have been designed for combining four equi-phase signals of 50 W, 150 W and 500 W each having frequency range of 500-1000 MHz, 100-500 MHz and 20-100 MHz, respectively.
The combiners/dividers are widely used in power amplifiers for power com-bining purposes where the envisaged power output is higher than 200 W, 500 W or 2000 W.
The subassembly com-prises transmission line transformers, microstrip circuit and lumped element inductors, chip capacitors and chip resistors.

Two-way In Phase Power Combiner/Divider

Two-way in phase power combiner/divider has been designed for combining two equi-phase signals of 300 W each anywhere in 100-500 MHz. It is being widely used in power amplifiers for power combining purposes where the envisaged power output is higher than 500 W.

Quadrature Coupler


 

The quadrature coupler divides an input signal into two equal amplitude isolated outputs having a quadrature phase relationship in 100-500 MHz band. Conversely, the unit may be used to combine two quadrature phased, equal amplitude signals into a single output.
The coupler is iDEAL for driving two power amplifier modules of 100 W each and then combining their outputs and protects the input driving module under load mismatch conditions.

Dual Quadrature Coupler


 

Dual quadrature coupler divides an input signal into four equal amplitude signals with signals from alternate ports having 90O phase difference. It comprises lumped L and C network for realising 90O phase shift. Signals from adjacent ports will be in phase.
The coupler offers higher isolation among ports than in phase power combiner/divider. Used in power combining of identical power modules.

Hybrid Coupler


 

Hybrid coupler divides input signal into two equal amplitude isolated outputs having a quadrature phase relationship in 500-1000 MHz band. Conversely, the unit may be used to combine two quadrature phased, equal amplitude signals into a single output. The coupler has been designed using microstrip technology, and is iDEAL for driving two power amplifier modules of 100 W each and then combining their outputs. It protects the input driving modules under load mismatch conditions.

Low-Pass Filter


 

The VHF filters are designed for suppression of harmonics present at the output of a high power class AB amplifier. These are made up of lumped inductors (L) and special type capacitors which comprises brass discs separated by Teflon dielectric.