1. What is the difference between a 3-port and a 4-port coupler?

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."

2. What advantage does a 3-port coupler have over a 4-port?

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.

3. 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?

Yes, but with care to provide good impedance match at all 4 ports of such a "bi-directionalcoupler." 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 monitoringby leaking forward and reflected signals into one another's paths. An alternative approach whichovercomes this limitation is to use two 3-port couplers back-to-back; this combination is calleda "dual directional coupler."

4. Is a coupled port an input or an output?

It can be used as either. The coupling factor determines the attenuation between main-line and coupled line signals in both cases.

5. How is directivity measured when the 4th port is internally terminated?

Measure the loss from the main-line input to the coupled port with the main-line output terminated. Then reverse the main-line connections. The difference in dB readings is the directivity.

6. How good does the directivity of a coupler have to be, as a reflectometer, to measure the return loss of an unknown impedance?

A mathematical determination requires knowledge of phase relationships in a given case, but as a guide, less than 1dB error will result when the directivity is 20dB greater than the unknown's return loss.

7. How is power rating determined for pulsed-RF operation?

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.

8. What performance degradation occurs if a 50-ohm directional coupler is used in a 75ohm system?

The most pronounced effect is a reduction in directivity because the internal 50-ohm termination would give a VSWR of 1.5:1. The directivity would be limited to the corresponding return loss, 14 dB.

9. What effect does DC current through the main-line have upon performance?

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.

10. Does the input power rating apply equally to the main-line and coupled port?

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.

11. If the external load on the output port is mismatched, where does the reflected power go?

Most goes back to the input port. A portion of it, corresponding to the coupling factor, is dissipated in the internal termination.

12. How do similar coupler designs in different case styles or with different connectors compare in performance?

Performance is similar, except at high frequencies, where the rule generally is "the smaller,the better."

13. How important is grounding of pins?

All ground pins should be connected to ground with short path length to obtain full directivity and VSWR performance.

14. Mention few characteristics of Directional Coupler.

Key characteristics of a directional coupler include,
•    flat coupling over a broad bandwidth
•    low main-line loss, as low as 0.1 dB
•    directivity as high as 55 dB and
•    a wide range of coupling values, from 6 dB to 30 dB.

15.Mention few applications of DIRECTIONAL COUPLER.

Directional couplers are general purpose tools used in RF and microwave signal routing for isolating, separating or combining signals. They find use in a variety of measurement applications:

•    Power monitoring
•    Source leveling
•    Isolation of signal sources
•    Swept transmission and reflection measurements

16. Mention the key specifications of a DIRECTIONAL COUPLER.

The key specifications for a directional coupler depend on its application. Each of them should be carefully evaluated to ensure that the coupler meets its intended use.
•    Directivity
•    SWR
•    Coupling factor
•    Transmission loss
•    Input power

17. Define DIRECTIVITY.

The difference in dB of the power output at a coupled port, when power is transmitted in the desired direction, to the power output at the same coupled port when the same amount of power is transmitted in the opposite direction. In the case of a bi-directional coupler, an alternative definition is the difference in dB of the power output of the two coupled ports, when power is transmitted in a constant direction on the main-line. Reflectionless terminations are assumed to be connected to all ports.

18. Define VSWR.

The standing-wave ratio at any port of a coupler is specified for the case of reflectionless  terminations at all other ports. As with any device, VSWR is a measure of the quality of  match relative to a given characteristic impedance.

19. Define COUPLING COEFFICIENT.

The ratio in dB of the incident power fed into the main port to the coupled port power when all  ports are terminated by reflection less terminations.

20. Define COUPLING TOLERANCE.

The coupling tolerance is the allowable unit-to-unit variation in nominal coupling.

21. Define COUPLING FLATNESS.

The coupling flatness is the maximum peak-to-peak variation in coupling coefficient that may be expected over a specified frequency range.

22. Define COUPLING LOSS.

coupling Loss is the reduction in power available to the main-line output due solely to power transferred to the coupled line. It is equal to the theoretical minimum main-line insertion loss.

23. Define MAIN-LINE LOSS.

The main loss line is the change in load power, due to the insertion of the coupler in a  transmission system, with reflectionless terminations connected to the ports of the coupler.  The main-line loss includes the effect of power transferred to the coupled line.

24. Define AVERAGE POWER.

The average power is the CW average power handling capability with one-way transmission  through the main line of the coupler under matched load conditions.

25. Define BI-DIRECTIONAL COUPLER.

It is another name for a 4-port coupler; that is, a single coupler having no  internal termination. It is intended to allow forward and reflected signals to be  sampled simultaneously.

26. Define DUAL DIRECTIONAL COUPLER.

The dual directional coupler is the combination of two 3-port couplers having their main lines cascaded, and their internally terminated ports facing each other at the interface between the couplers. This provides bi-directional coupler action, but with independent use of the coupled ports: A mismatched load applied to either of them will not affect the other.