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Application Notes: Directional Couplers

Werbel Microwave creates, maintains, designs and develops our own line of directional couplers using a variety of technologies to help you achieve uour goals. These include stripline, coaxial and ferrite couplers.

Applications of directional couplers are many.

Test and Measurement

Unequal Signal Splitting

A common requirement in DAS (distributed antenna systems) is unequal splitting.  This is commonly achieved with directional couplers of values between 5 and 15 dB.  For example, an antenna further away from the repeater would require a stronger incoming signal to overcome cable losses.  

Precision Attenuation

Traditionally attenuators are used in series with sensitive monitoring equipment such as power sensors, the purpose of which is to limit the incoming power to  protect the instrument.  This gets the job done, but adds at least two junctions in series with the signal path, which may introduce reflections, thus reducing accuracy.  Worse yet, if a short develops, it could permanently damage the sensor.

A better approach is to use a directional coupler as an attenuator.  In the example of a 20-dB coupler, power is replicated at the coupled port 1/100th of the signal strength.  The coupler may be terminated as normal at the far end, suppressing reflections.  Because the conductors are physically separate from each other, the measurement probe is not within the direct path of a potentially damaging high power signal.

Feedback and Gain Control Loops

Directional Couplers are commonly used in amplification systems to monitor and control output power.  A dual directional coupler is very useful for this purpose.  Both coupled ports feed into detectors.  Gain control is accomplished through voltage conversion, often by way of operational amplifiers or a micro controller.

 

The forward port would be used to monitor output power of the amplifier, while the reverse port would be used to monitor in the case of an accidental open circuit or disconnection of the output.  Sustained reflected power would cause damage to the amplifier. 

 

Some amplifier systems easily cost $10k and above!  By monitoring reverse power, a voltage detector can send a signal to immediately shut off the amplifier until the problem is cleared.

Signal Monitoring, Tap Points

 

Sometimes it is useful to have the ability to introduce test points in an RF network for periodic measurements.  However breaking into a system can be costly both in terms of maintenance cost and equipment downtime. 

 

Consider the cost of pulling a piece of equipment out of service to take measurements.  Technician hourly rate and loss of use of the equipment can add significant costs.

A directional coupler of high value, such as 30 dB may be installed as a permanent tap point with the coupled port terminated.  Because 30 dB correlates to only 1/1000 or 0.1% coupling ratio, it is practically invisible to the network it is connected to.  This would add minimal, if any loss to the system and allow spontaneous breaking into the network without any interruption of service.

In short, a less-than $200 part, if properly designed in, can save thousands of dollars of technician hourly costs and loss of revenue due to equipment downtime.

The information contained in this article is provided for informational purposes only and shall not constitute advice of any kind.

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