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RF Electronics Chapter 7: RF Filters Page 246 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Similar to the Helical filter, the larger the filter cavity, the lower the insertion loss of the filter and the bigger the power handling capability of the filter. The Round Rod Interdigital filters are reasonably large and as a result have a low insertion loss and a high power handling capability, permitting them to be used at the output of transmitters. These filters normally have bandwidths less than 10%. The filters become very large for frequencies below 300 MHz. The coupling into the filter can be achieved by coupling loops as shown in figure 7.58, or by direct tapping to the resonator rod. In most instances the resonator rods length can be adjusted. This provides a coarse tuning adjustment. In the figure 7.58, the Allen key locking screws for the resonator rods can be seen at the base of each of the resonators allow the course adjustment to be performed. A small variable capacitor, in the form of an insulated screw that slides inside the hollow resonator rod, as shown in figure 7.58, result in filters that can easily be fine-tuned to the correct frequency. The filter in figure 7.58 was designed using equations 25 to 28. Applying the course- tuning and fine-tuning and adjusting the coupling loops enabled a filter to be obtained, which satisfied the design specifications exactly. Transmitter Amplifier Receiver Amplifier Pass Transmitter Frequency Stop Receiver Frequency Pass Receiver Frequency, Stop Transmitter Frequency Diplexer Transceiver Figure 7.59. The use of a diplexer to provide full duplex operation. Round rod interdigital filters are often used as a diplexer in mobile radio transceivers, as shown in figure 7.59. The filter connected to the receiver has a very high, typically greater than 60 dB, attenuation at the transmitter frequency. This ensures that the transmitter output, which is typically 25 W, does not damage the receiver input, which is typically designed to accurately demodulate signals of 1 nW. The use of the diplexer will thus permit full duplex operation, simultaneous transmission and reception, of the transceiver. Mobile phones have similar diplexers. Some older phones use diplexers made using a ceramic material with a typical dielectric constant of 30, allowing a 30 reduction in each dimension of the filter as shown on the left of figure 10.2. The resulting filter will thus have 0.6% of the volume of the air-filled filter and have a fraction of the weight. More recent phones, use surface acoustic wave (SAW) or bulk acoustic wave (BAW) filters to obtain an even smaller size and weight, whilst keeping the same functionality shown in figure 7.59. PCB Interdigital Filters With circuit miniaturisation, and improved Microstrip simulation tools, interdigital filters are often designed using Stripline or Microstrip circuits for the resonating elements. The power handling capacity of these filters is small, so that they can only be used as filters in receivers or low power parts of communication equipment. The design procedure is illustrated with the following example. RF Electronics: Design and Simulation 246 www.cadence.com/go/awr

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