Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 57

Filters come in a variety of types including low pass, band pass, high pass, and band stop. Multiport filters form diplexers or multiplexers, which are used to separate or combine signals of different frequency from a common port to a port associated with the different frequencies of interest. Diplexers are sometimes called duplexers, but duplexing is a function of the operation of a communication system. That is, a system that can transmit and receive at the same time is said to operating in a duplex mode. A diplexer is used to support the duplex operation by keeping the transmit signal from saturating the receiver.

The structure and variety of filters are almost endless, but they all share these common attributes: low loss in the pass band, low reflection in the pass band, high reflection, and high loss in the stop band. In nearly every case, the goal of the design is to minimize unwanted loss, and this quality of a filter is often referred to as the Q of the filter. In microwave cases, filters are designed to operate into a matched impedance, so there is always loss associated with power from the source being absorbed by the load. The Q of a filter in operation is fixed by the loading of the ports and can never be infinite. The quality of a filter is usually defined by its unloaded Q, which accounts for the (desired) power loss from the source to the load.

For many filters, the desired qualities are a trade‐off between creating a maximally flat passband and creating a maximally sharp cutoff. Thus, the measurement of the transmission response of the filter is critical in evaluating the quality of a filter design. For most filters used in communications, the transmission responses is desired to be equally flat (rather than maximally flat) across the passband, resulting in filters that have Chevyshev‐type response (equal ripple) in the passband (Zverev 1967). The desire for sharp cutoffs has led to many filters employing an elliptic response, which provides for finite zeros in the transmission response. Stopband performance of high‐performance filters can also require careful consideration in measuring, with some requirements going beyond 130 dB of isolation over selected regions of the stop band. These extreme isolation requirements put tremendous burdens on the design of the filter, as well as the design and use of the measurement systems.

In modern communications systems using complex modulation, the phase response of the filters is also critical, and a significant design parameter is controlling the phase of the filter to follow a linear response, with a key measurement parameter being deviation from linear phase. Closely aligned to that is maintaining a constant group delay through the passband. Equalization techniques are utilized that can remove higher‐order phase responses, such that another measure of filter phase response is deviation from parabolic phase, where the phase is fitted to a second‐order response, and the deviation of the phase from this second‐order response is the measurement criteria. Some filters are used as part of a feed‐forward or matched system network where their phase response as well as absolute phase and delay must be carefully controlled.

The reflection response of filters is also a key measurement parameter. To the first order, any signal that is reflected is not transmitted so that high reflections lead to high transmission loss. However, the loss due to reflection for most well‐matched filters is much less than the dissipation loss. Still, low reflections at the test ports are required to avoid excess transmission ripple from concatenated components, and even moderate reflections from filters in a high‐power transmission path can cause damage to the preceding power amplifier. Thus, very low return loss is often a critical parameter of filters and also a difficult parameter to measure well. This becomes especially true in the case of diplex and multiplex filters, where the loading of any port affects the return loss of the common port.

For high‐power applications, the filter itself can become a source of IM distortion, and the attribute passive inter‐modulation (PIM) has become common in the measurement of these high‐power filters. Poor mechanical contacts between components in a filter, poor plating on a filter, or the use of magnetic materials in the plating or construction of the filter can lead to hysteresis effects that cause IMD to be created in an otherwise passive structure. The level of IMD typically found in these filters is less than −155 dBc, but this can be a difficult spec to meet without careful design and assembly.

Most of these high‐performance communication filters are designed using coupled‐resonator designs (Cameron et al. 2007; Hunter 2001). 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).