Figure 2.17 RF performance of a directional bridge.
Bridges of this type have been used successfully up to 27 GHz.
2.2.4.2 Directional Couplers
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.
2.2.4.3 1+Gamma
Another proposed reflectometer structure is a 1+gamma structure, whose name comes from the block‐diagram architecture, shown in Figure 2.19. As the name implies, the signal at the b1 receiver is a combination of the incident (a1) and reflected (gamma) signal.
Figure 2.19 Block diagram of a 1+gamma reflectometer.
In this configuration, the signal in the test or b1 receiver never goes to zero; rather, it is minimum with a short, maximum with an open, and nominal 1 when there is a load attached. Also, the signal variation between an open and short is about 14 dB less than that for a bridge or directional‐coupler. Put another way, the reflection gain of the 1+gamma bridge is lower than for a directional‐coupler or bridge. Consider the Smith chart in Figure 2.20; an open, short, and load (all non‐ideal with fringing capacitance and series inductance) are shown for each on a 1+gamma reflectometer.
Figure 2.20 Smith chart showing reflections of a 1+gamma bridge with an open, short, and load.
The value of attenuation in the reference channel is adjusted to set the value of the open circuit reflection to 1. For a directional‐coupler, the load gives a zero reflection (ideally), and the short gives a −1 reflection. For the 1+gamma bridge, the open is also 1, but the short is +0.6, and the load is +0.75; thus, the difference between the open and the short moves from 2 to only 0.4. These reflections are mapped to the full Smith chart through the error correction math, in such a way that the values from the reflections, and any instability, are multiplied by 5. Also, since the load condition has a large signal in the b1 receiver, any instability in that signal is apparent as a directivity error, which is also multiplied by 5. In theory, if directivity is defined as the average of the open/short response relative to the load response, then the directivity of a 1+gamma reflectometer is about 0 dB (remember that directivity for a coupler or bridge is always positive, often 20 dB or more).
Theoretically, any directivity error can be corrected for by a calibration, but in practice, certain unstable errors can cause uncorrectable errors when the directivity is poor. Thus, 1+gamma structures have largely disappeared from use. Also, this same multiplying effect causes any slight drift in the test port cable to cause a considerable change in the measured reflection coefficient, after calibration.
2.2.5 VNA Receivers
The final RF components in a VNA block diagram are the test and reference receivers. Dynamic range is a key specification of a VNA and is sometimes referred to as the difference between the maximum signal level that the receiver can accept while still operating and the noise floor of the receiver. In most cases, the maximum damage level is significantly above the maximum operating level of the receiver, which is usually limited by the input compression level of the receiver. The maximum operating level is set by the structure of the components, but for most modern VNAs, it is around the −5 dBm at the receiver mixer input, or about +10 dBm at the test port, after considering the coupling factor of the test port coupler. The noise floor of the receiver is set primarily by the type of mixing down‐converter used, of which the two principal types are sampling down‐converters (or samplers) and mixers.
2.2.5.1 Samplers
The