Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 102

Directional couplers are more often used in higher‐microwave frequency ranges because of the difficulty of maintaining good bridge performance at high frequencies. Directional coupler design is a broad topic, and much literature has been devoted to structures that can be used as couplers. However, for use in VNAs, there are some particular characteristics that are critical. In general, commercial directional‐couplers are designed to maintain a flat coupling factor over their bandwidth, and the bandwidth is limited by this coupling factor. Couplers used for VNA reflectometers require wide bandwidths, so rather than a flat response, they are often designed with an equal‐ripple or Chebyshev response. Ripple in the loss or coupling factor is not much concern in a modern VNA, where calibration techniques can remove almost any frequency response error. Isolation is an important criteria in VNA couplers. One attribute about directional‐couplers that distinguish them from bridges is that they are ideally lossless devices such that all the power applied is either coupled (to the coupled port or the internal load) or transmitted through the coupler. The relationship between insertion loss and coupling factor is

(2.8)

Directional couplers typically come in one of three forms: waveguide couplers, microstrip couplers, and stripline couplers.

Waveguide couplers are most common at mm‐wave frequencies but have the inherent limitation of narrowband operation due to the narrowband nature of waveguides. The structure of waveguide couplers is a 4‐port device with the main arm connected in such a way as to have irises (or holes) to a second waveguide. The second waveguide can have two ports or one port internally terminated. The nature of the coupler is symmetrical. In theory either port can be the coupled port; in practice a load is often embedded in the coupled arm. Because of the fundamental function of a waveguide coupler, the forward coupled wave comes out of the waveguide port nearest the test port. This often causes confusion in the symbols used.

A microstrip or stripline coupler uses a different electric‐magnetic (EM) configuration to perform coupling, and the coupled arm of these couplers is the one farthest from the test port. Microstrip couplers often suffer from the fact that there is some dispersion in microstrip lines, and since the even‐ and odd‐mode waves in the coupled lines experience different effective dielectric constants, they will have different velocities of propagation. This makes it more difficult to create microstrip couplers with good isolation. For this reason, many VNA couplers are in the form of stripline (or slabline, which is similar to stripline but with a rectangular center conductor thickness), suspended in air. These couplers are designed to have very stable coupling and isolation factors. For a VNA, it is not so important what the exact directivity is, as long as it is completely stable. Figure 2.18 shows an example of a directional‐coupler used in VNAs. The test port connector is one attribute that differentiates this from a commercially available directional‐coupler that might be used as component in a different system. This connector is designed to be firmly mounted to the VNA front panel and withstand numerous connections and reconnections. This coupler has an integrated load and so exposes only three ports.

Figure 2.18 A directional coupler used in VNAs.