Because of manufacturing tolerances, these filters cannot be manufactured to specification from the start; they require tuning of the resonators as well as the inter‐resonator couplings. Techniques to optimize the response of these filters are highly sought and a key aspect of the filter measurement task, requiring fast precise response of the transmission and reflection response in real time.
Another type of filter commonly found in the intermediate frequency (IF) paths of receivers is a surface acoustic wave (SAW) filter. The frequency of these SAW filters has been steadily increasing, and they are sometimes found in the front end of a receiver. SAW filters can be made to high orders and can have large delays (in the order of microseconds). Because of these long delays, special measurement techniques are required when attempting high‐speed measurements. Another type of acoustic wave filters are the film bulk acoustic resonator (FBAR) filters, which are small in size and have been used as RF/TX duplexers in handset cell phones.
Ceramic coupled resonator filters are also used extensively in cell phone and radio applications. Because of manufacturing tolerances, the filters are often required to be tuned as part of the manufacturing process, and tuning consists of grinding or laser‐cutting electrodes until the proper filter shape is obtained. This presents some difficulty in coupled resonator filters as the tuning is often “one way,” and once the resonator frequency has been increased, it cannot be reduced again. This has led to the need for high‐speed measurements to ensure that the latency between measurement and tuning is as small as possible.
Some examples of filters are shown in Figure 1.32.
Figure 1.32 Examples of microwave filters: cellular phone handset filter (upper left), thin film filter (upper right), and cellular phone base station filters (bottom).
1.10 Directional Couplers
Directional couplers separate the forward and reverse waves in a transmission system (see Section 1.3). A directional‐coupler is classically defined as a 4‐port device, often with a good load on the fourth port, as shown in Figure 1.33; but in practice a load element is almost always permanently attached. The directional‐coupler has four key characteristics: insertion loss, coupling factor, isolation, and directivity. In fact, directivity is related to the other three factors in a specific way.
(1.90)
Figure 1.33 Directional couplers.
Most couplers have a nearly lossless structure so that the directivity is nearly equal to the isolation/coupling, but for lossy structures, such as directional bridges, the earlier definition provides the proper description. In fact, consider the case of a directional‐coupler with 20 dB of coupling, 50 dB of isolation, and 0.05 dB of insertion loss, setting the directivity at nearly 30 dB. If a 10 dB pad is added to the input, as shown in Figure 1.34, the isolation is increased by 10 dB, the loss is increased by 10 dB, and the coupling stays the same. Thus, the simple but incorrect definition of directivity as isolation/coupling would yield an increase of 10 dB.
Figure 1.34 The effect of attenuation at the input of a coupler.
In fact, a better way of looking at directivity is the ability of the power at the coupled port to represent a change in reflection at the test port. Again considering Figure 1.34, if a signal of 0 dBm is injected into the input port and a full reflection (an open or short) is applied to the test port, the coupled port will show a power of about −30 dB (10 dB loss, plus a full reflection, plus 20 dB coupling; here the isolation term is ignored for the moment). If a load is applied to the test port, the signal at the input sees a 10 dB loss and 50 dB isolation for a value at the coupled port of −60 dBm. The difference between the open and the load is 30 dB; hence, the directivity is 30 dB, and adding the pad at the input has no effect.
In practice, the output match of a directional‐coupler is critical, and the test port mismatch can dominate the directivity. This signal flow is demonstrated in Figure 1.35. Mismatch at the output of the directional‐coupler affects directivity on a one‐for‐one basis. This mismatch is combined with the coupler input mismatch to create the overall source‐match. The source‐match affects the power measured at the coupled port when measuring large reflections at the test port. This “source mismatch” causes some reflected signal from the test port to re‐reflect from the input port, reflect a second time off the test port termination, and add or subtract to the main reflection, causing error in the coupled port power.
Figure 1.35 Coupler with mismatch after the test port flow graph.
However, the output mismatch is a direct error and causes reflection back into the coupler, thereby adding directly to the coupler directivity.
1.11 Circulators and Isolators
While most passive components are linear and bilateral (that is, the forward loss equals the reverse loss), a particular class of devices based on the ferromagnetic effect doesn't follow this rule. These devices comprise circulators and isolators. A circulator is a 3‐port device, with low loss in one direction between ports, say from port 1 to 2, port 2 to 3, and port 3 to 1. But it has high loss (called the isolation) in the reverse direction, from port 2 to 1, 3 to 2, or 3 to 1. An isolator is a special case of a circulator with a good load applied to port 3, such that it becomes a 2‐port device. Circulators pose a particular measurement difficulty as the isolation between ports depends upon the match applied to a third port. Thus, for good measurement quality, the isolation measurement requires a good effective‐match on the ports.
Further, circulators are often tuned by magnetizing a permanent magnet attached to the circulator, and it's desired that the measurement system can determine the isolation of all three ports in a single connection step, and with good speed. Thus, multiport (more than two ports) systems were developed to simplify the connections, and multiport calibration techniques were developed to satisfy the need for high‐quality correction.
Even though they are passive devices, circulators and isolators are sometimes tested for their high‐power response, such as compression and IMD. The ferromagnetic effect has hysteresis properties that can produce IMD and compression when driven with sufficiently high powers.
