Understanding 90-degree Hybrids

Introduction

90-degree Hybrids, also known as Quadrature Hybrids, are used in splitting and combining signals.  They are often used in power amplifiers to allow two transistors to operate in parallel for combined power or redundancy (more on that later).

The term “hybrid” derives from its handling of the input signal.  It is constructed like a directional coupler while behaving like a power divider/combiner.  An additional characteristic unique to the device is its 90-degree phase shift between ouptuts.

Single Directional or Unidirectional

The most common type of coupler.  When a signal generator is applied at the input, the coupler is said to be operating in the forward direction.  The signal is replicated at a much lower power level at the coupled port.  The bulk of the applied signal continues to the output port, to the rest of the network.

The structure is optimized to sample signal in the forward direction.  Technically speaking, the device does work in reverse, however the reverse power is absorbed into the termination.  As long as the termination is very well-matched, a practically imperceptible signal will appear at the coupled port.

Application: Power Amplifier

Two 

Application: Redundancy

In the dual directional coupler, the forward and reverse coupled outputs are independently isolated.  Effectively, there are two single couplers connected back-to-back, in an single enclosure.  The key difference between the dual directional and the bidirectional couplers is that in the dual directional, each of the coupling lines is independently connected to its own termination.  Thich means that a mismatch on one coupled port will not affect the other. This is a useful characteristic in amplifier power monitoring applications where a good VSWR cannot always be guaranteed at the detector input.

Comparison of 90-degree Hybrids versus Couplers and Dividers

It has been my experience that the differences between the two types are not always well understood.  After all, from a functional perspective, they appear interchangeable.  The difference lies in the structure and application.  I will do my best to explain with a comparison of both types, with their advantages and disadvantages. 

Subject

90-degree Hybrid

Couplers and Dividers

Operating Bandwidth

Depends on Application

  • The hybrid will perform well over an optimized band, typically an octave.
  • The hybrid may be broadband with compromises to performance and cost.

Pro

  • Couplers and Dividers may be very wide band.

Power Handling

Pro

  • External termination can handle high power without limiting frequency of operation.

Con

  • Wilkinson splitters often have small resistors that cannot dissipate high power compared to an external load.
  • Larger internal resistors may be used but this often limits upper frequency of operation.

Insertion Loss

Pro

  • Typically, lower than an equivalent Wilkinson splitter because there are no resistors to introduce parasitic reactance.

Con

  • Longer path length adds loss.
  • Resistors introduce reactive elements which can add loss.

Size and Weight

Pro

  • Typically smaller than a splitter/combiner of similar frequency and bandwidth.

Con

  • Wilkinson splitter/combiners tend to be larger for similar parameters.

Ease of Installation

Pro

  • The ports are conveniently aligned with the nature of the structure and typical application.

Con

  • The added length of a Wilkinson splitter/combiner may be difficult to implement.

Cost

Con

  • Hybrids require more precision fabrication and tuning compared to splitters, increasing cost.

Pro

  • Splitters are easier for high volume production, reducing cost.