Q. What is the difference between a 3-port and a 4-port coupler?
A. A directional coupler is basically a 4-port network. The main-line and auxiliary line each have 2 ports: a 3-port coupler has one end of this auxiliary line, the “isolated port,” internally terminated. When all 4 ports are made available to the user, the device is called a “bi-directional coupler.”
Q. What advantage does a 3-port coupler have over a 4-port?
A. Directivity of a coupler is strongly affected by the impedance match provided by the termination at the isolated port. Furnishing that termination internally ensures high performance.
Q. Can a 4-port coupler be used to sample forward and reflected power simultaneously, by placing measuring instruments at both ports of the coupled line?
A. Yes, but with care to provide good impedance match at all 4 ports of such a “bi-directional coupler.” A coupler’s directivity can be no better than the return loss of the terminations at the far-end main-line and coupled line ports; poor directivity causes inaccurate power monitoring by leaking forward and reflected signals into one another’s paths. An alternative approach which overcomes this limitation is to use two 3-port couplers back-to-back; this combination is called a “dual directional coupler.”
Q. Is a coupled port an input or an output?
A. It can be used as either. The coupling factor determines the attenuation between main-line and coupled line signals in both cases.
Q. How is directivity measured when the 4th port is internally terminated?
A. A mathematical determination requires knowledge of phase relationships in a given case, but as a guide, less than 1 dB error will result when the directivity is 20 dB greater than the unknown’s return loss.
Q. How is power rating determined for pulsed-RF operation?
A. The first limitation to peak pulse power which a coupler can handle is determined by dividing the average power rating by the duty factor of the pulse train. This applies when the duration of each pulse is much shorter than the thermal time constant of the internal terminating resistor. If the pulse duration is greater than about a millisecond, the peak-power should be reduced.
A further limitation may be necessary to avoid saturation of the transformer core used in the coupler to avoid compression and distortion of the signal. For specific applications, consult the factory.
Q. What performance degradation occurs if a 50Ω directional coupler is used in a 75Ω system?
A. The most pronounced effect is a reduction in directivity because the internal 50Ω termination would give a VSWR of 1.5:1. The directivity would be limited to the corresponding return loss, 14 dB.
Q. What effect does DC current through the main-line have upon performance?
A. Most Mini-Circuits’ couplers have their ports at DC ground. Therefore DC current would not pass through from in to out. However, for those models designed to pass DC current, when the current is large relative to RF current, signal compression and distortion due to core saturation may occur.
Q. Does the input power rating apply equally to the main-line and coupled port?
A. Not for a 3-port coupler. The coupled port rating is less, because power applied to the coupled port is almost entirely dissipated in the internal termination.
Q. If the external load on the output port is mismatched, where does the reflected power go?
A. Most goes back to the input port. A portion of it, corresponding to the coupling factor, is dissipated in the internal termination.
Q. How do similar coupler designs in different case styles or with different connectors compare in performance?
A. Performance is similar, except at high frequencies, where the rule generally is “the smaller, the better.”
Q. How important is grounding of pins?
A. All ground pins should be connected to ground with short path length to obtain full directivity and VSWR performance.
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