Wideband Coupler

A wideband coupler typically has an operating bandwidth that covers one or more octaves. These type of couplers are very useful for many applications. Some of the applications include:

• Instrumentation – Most RF test instruments and equipment are designed to be broadband and they usually the use of a broadband coupler for to sample signals for processing.
• Wideband receivers – These receivers operate over a wide frequency range and they require the use of a coupler that has a wide enough bandwidth to use together with it.
• Wideband radar – Wideband radar systems can operate from 2-18 GHz. In such cases, a 2-18GHz wideband coupler need to be used.

Wideband Coupler: Three Parameters That Needs Checking

Each and every CATV or MATV distribution system has splitters, directional couplers and other passive RF components. These components may lead to the development of excessive signal constriction and losses, or even poor insulation between inputs and outputs. These components can be measured with the use of a Frequency Selective Voltmeter (FSV). Today, we’ll be tackling about couplers and how to test them.

Couplers

Couplers, it can be directional hybrid or a wideband coupler, are utilized to extract a little amount of signal distribution from the distribution cable to feed the subscriber taps. It continues to function as such while it maintains the right characteristic impedance of the distribution cables.

In addition, couplers are passive devices used to test high frequency signals. It takes a single signal as the input and provides two more for outputs – one would be the coupled output. Couplers have a lot of applications and are mostly utilized for testing signals, signal injection, signal generation, and are utilized to measure incident/reflected power to identify VSWR and have many more other applications.

Each coupler has three parameters that are needed to be inspected: the insertion loss, isolation and tap loss.

Tap Loss: The Most Important among the Three

The most crucial parameter in each and every coupler is the tap loss. Tap loss measures how low signal levels are at the tap output, compared to the signal level of the input. The typical tap loss values range from 3 dB to 28 db. Couplers are then positioned at different locations throughout a distribution system based on the tap loss and signal level required.

For example, if a signal level on a line is 28dBmV, a directional coupler having a tap loss amounting to 28 dB would be utilized to give the signal 0dBmV required for a subscriber tap.

Measuring these Parameters

Tap loss is measured using an FVS. Just apply a known signal level to the input of the directional coupler, cutting off the output terminal of the coupler with a 75 ohm terminator. Afterwards, measure the signal level at the tap output with the FVS. The result (the difference of input signal and tap signal output) is the coupler’s tap loss.

For insertion loss, apply a known signal level to the coupler’s input port, while tap output is being cut off in 75 ohms. Then measure the signal at the coupler’s output using the FSV. The difference between the input and output would result to the coupler’s insertion loss. A coupler usually has a small insertion loss, sometimes around 1.5dB.