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Directional coupler concepts and indicators
Mar 21, 2018

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Couplers are often used to sample a specified flow of microwave signals. When there is no internal load, the directional coupler is often a four-port network.

A directional coupler is a low-loss device that accepts one input signal and outputs two signals that theoretically have the following characteristics

a) The output amplitudes are not equal.

The output of the main line is a large signal, which can basically be considered as a through-pass, and the output of the coupled line is a smaller signal.

b) The theoretical loss on the main line determines the signal level of the coupled line, ie, the degree of coupling.

c) High isolation of main lines and coupled lines.

Directly speaking, the role of the coupler is to divide the signal unevenly into 2 points (called the trunk end and the coupling end, and others are called the pass-through end and the coupling end).

Main indicators: Coupling degree, power loss, isolation, directionality, input/output VSWR, power tolerance, frequency range, and in-band flatness. The following table shows some typical specifications of the broadband cavity coupler (reference):

Coupling: The direct difference between the power of the signal output from the coupling port and the power of the input signal through the coupler. (usually theoretical values such as: 6dB, 10dB, 30dB, etc.)

Coupling degree calculation method: If the input signal A is 30dBm and the coupling output signal C is 24dBm, then the coupling degree = C-A = 30-24 = 6dB, so this coupler is a 6dB coupler. The actual coupling may fluctuate between 5.5 and 6.5.

Power loss: divided into coupling loss and insertion loss.

Coupling loss: The ideal coupler input signal is A, and the coupling part to B, then the output port C must be reduced.

Calculation method: The “dBm” power of all ports is converted into “milliwatts” as a unit. For example, the power at the A input is originally 30dBm, the converted “milliwatts” is 1000mW, and the output at the coupling end is 25.5dBm (first Assuming that a 6dB coupler is used and the actual coupling of the 6dB coupler is 6.5dB), converting 25.5dBm to milliwatts is: 316.23mW. Assuming that this coupler has no other losses, the remaining power should be 1000-316.23 = 683.77 milliwatts, all output from the output. Converting 683.77 milliwatts into "dBm" = 28.349, the coupler's coupling loss = power at the input (dBm) - power at the output (dBm) = 30dBm - 28.349dBm = 1.651dB, this value means that the coupler has no extra Coupling loss in the case of loss (device loss).

Insertion Loss: Refers to the value obtained by subtracting the coupling loss from the value of the signal power that passes through the coupler to the output.

Calculation method: Take 6dB coupler as an example. In the actual test, assume that the input A is: 30dBm, the coupling degree is measured: 6.5dB, the ideal value of the output is 28.349dBm, and the signal of the actual output is assumed to be 27.849dBm, then the plug Loss = theoretical output power - measured output power = 28.349-27.849 = 0.5 dB;

Isolation: refers to the isolation between the output port and the coupling port; generally this indicator is only used to measure the microstrip coupler, such as 5-10dB is 18~23dB, 15dB is 20~25dB, and 20dB (including the above) is: 25~30dB; the isolation of the cavity coupler is very good so there is no requirement for this index.

Calculation method: When the input termination is matched to the load, the signal is input from the output, and the reduced amount of the measured coupling is the isolation.

Directionality: refers to the value obtained by subtracting the coupling degree from the value of the isolation between the output port and the coupled port. Since the directionality of the microstrip gradually decreases as the degree of coupling increases, there is basically no direction beyond the final 30 dB. Sexuality, so the microstrip coupler does not have this index requirement, the directionality of the cavity coupler is generally: 1700~2200MHz: 17~19dB, 824~960MHz: 18~22dB.

Calculation Method: Directionality = Isolation - Coupling

For example, 6dB isolation is 38dB, and the coupling degree is 6.5dB, then directionality=isolation-coupling=38-6.5=31.5dB.

VSWR: Refers to the matching of input/output ports. The requirements for each port are generally: 1.2~1.4.

Power tolerance: refers to the maximum operating power tolerance that can be passed on this coupler for a long period of time (without damage). The general microstrip coupler is: 30~70W average power, and the cavity is: 100~200W average power. The power marked on the coupler also refers to the maximum input power of the input port, and the output port and the coupling port cannot be input with the marked maximum power.

Frequency range: Generally, the nominal value is 800~2200MHz. In fact, the required frequency band is: 824-960MHz plus 1710~2200MHz. The middle frequency band is not available. Some power dividers also exist in the 800-2000MHz and 800-2500MHz frequency bands

In-band flatness: refers to the difference between the maximum and minimum values of the degree of coupling over the entire available frequency band.


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