Directional Coupler Topology

Introduction

Directional couplers are used for monitoring signals.  They can be installed within a trunk line and allow accurate sampling of the RF signal while minimally impacting performance of the system.  They are available in various configurations including single directional, bidirectional and dual directional.

Schematic
Functional Diagram
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: Input Power Shutdown Protection

A coupler may be used to monitor an input signal.  Consider an active device that required the input power not to exceed +10dBm, otherwise damage may occur.  A coupler may be placed at the input, directly before the device to be protected. 

The coupled signal may them be fed into an RMS detector, which converts the coupled power into a voltage that is directly proportional to the input power level. 

An op-amp comparator circuit is used to drive a TTL signal when the input power is too high.  This TTL signal may then be used to shut off the device, thereby protecting it from input power over load.

Bidirectional

Often times it is advantageous to monitor power in the forward and reverse directionals, simultaneously.  For this, a bidirectional coupler may be used. 

The bidirectional coupler is constructed similarly to the single directional coupler with the termination omitted.  This allows a signal travelling in the reverse direction to be coupled to the reverse port.  It is important to know that the forward and reverse ports are electrically and physically connected to each other.  Thus, there is no isolation offered between the forward and reverse ports.  

This is normally not a problem if the VSWR at either the forward or reverse port is acceptable.  However, if the VSWR is poor, it may cause inaccurate measurements to appear at the opposite port due to reflections.

That being said, an advantage of the bidirectional coupler is that is allows simultaneous measuring of forward and reverse signals within the same physical space as the single directional version.  Hence, a single part can satisfy two needs.

Schematic
Functional Diagram
Application: High Power External Load

The user may “bring their own load” to connect to the reverse port, effectively converting the coupler to a single directional coupler with higher reverse power handling capability.  However, care must be taken to ensure the load has excellent VSWR performance, as it will impact directivity of the coupler.

Dual Directional

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.

Schematic
Application: Amplifier Output Power Monitoring with Detector and Shutoff

The dual directional coupler may be used to monitor and control the output power level of an amplifier.

In this circuit, the dual directional coupler is placed immediately after the power amplifier.  Both the forward and reverse power is monitored simultaneously.

The forward monitor may be used in a negative feedback loop to regulate the amplifier’s output power.  In this example:

  • The RMS detector converts the coupled power to a proportional voltage.
  • An op-amp scaling circuit converts the voltage slope to drive the voltage-variable attenuator (VVA).
  • The VVA increases attenuation proportional to the voltage applied, which reduces the amplified signal.

The reverse monitor may be used to protect your expensive amplifier from an open/short or bad load VSWR condition at the output.

  • If a reflected signal of high power is coupled to the reverse port, it creates a proportional voltage at the RMS detector out.
  • This voltage is fed into an op-amp comparator, which outputs a TTL signal when the input voltage is above the threshold.
  • The TTL signal controls the power supply voltage to the power amplifier, shutting it off. 
  • The power supply shutdown could be implemented using a relay or MOSFET. 
Comparison of Bidirectional versus Dual Directional Couplers

A common application of a dual directional coupler is to monitor 

Subject

Bidirectional

Dual Directional

Coupling Flatness

Con

  • The coupler will generally perform better in one direction over the other.  
  • Because the coupling line is shared, we are forced to “split the difference” between forward and reverse performance.

Pro

  • Independent coupling lines may be optimized separately, lending to better overall performance in both directions.

Directivity Performance

Con

  • Generally not as good as dual or single directional due to lack of isolation between forward and reverse ports.

Pro

  • Forward and reverse channels are independent, hence high directivity can be achieved.

Main Line Insertion Loss

Pro

  • Less than equivalent dual directional model because of shorter path length.

Con

  • Longer path length adds loss.

Measurement Accuracy

Con

  • Heavily dependent on well matched parts at monitoring ports.
  • To minimize this issue, additional components (such as attenuators) may be needed at coupled ports.

Pro

  • Inaccuracies are contained at their respective ports.  e.g., a mismatch at the reverse port will not affect measurements at the forward port.

Size and Weight

Pro

  • Smaller compared to dual.  Same size as single coupler.

Con

  • About twice the size as a single coupler.

Cost

Pro

  • Smaller parts and less of them, reducing cost.

Con

  • Additional components and higher performance cost more.