AWR eBooks

RF Electronics Chapter 7: RF Filters Page 241 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. For the cavity: d = 0.66S Eqn. 7.15 B = S Eqn. 7.16 H = 1.6S Eqn. 7.17 f S Q 2363 0 Eqn. 7.18 For the highest Q value, the wire diameter of the helix is selected to be half the pitch of the helix. The gap between the wires is thus the same as the wire diameter. For the helix: f S N 6 . 40 Eqn. 7.19 f 5 10 6 . 6 Eqn. 7.20 f S Z 2070 0 Eqn. 7.21 0 0 1 1 4 Q Q Z R d b Eqn. 7.22 The doubly loaded input Q d , is defined as ½ Q 1 for the input tapping point calculation and ½Q n for the output tapping point calculation, and represents the total load seen by the resonator if there is a very small insertion loss as is normally the case. The electrical length from the bottom of the helix to the point where the input or output connects to the helix is given by: 2 0 2 ) ( Z R R Sin tap b Eqn. 7.23 The gap between the shield and the top of the helix is given by: 91 . 1 071 . 0 d h K Eqn. 7.24 When the bandwidth increases, the gap between the top of the shield and the top of the coil increases. For wideband filters, no shield is required, thus reducing the cost of construction. In that instance, it is also possible to vary the coupling by varying the spacing between the resonators. Example 7.2: 100 MHz, 1 MHz Bandwidth Filter Design a helical filter to have a 1 MHz bandwidth at 100 MHz. A diecast box of 112 mm width and 50 mm depth and 90 mm height is available. This can be split into two cavities 56 x 50 x 90 mm. The average side S is 53 mm. The H/S ratio is 90/53 = 1.7, which is close enough to the h = 1.6S value from equation 7.17. The K and Q values required can be obtained from table 7.1. Since for two resonators and Butterworth filters the K and Q values are independent of the insertion loss, equations 7.4 and 7.5 can be used to calculate the required K and Q values. RF Electronics: Design and Simulation 241 www.cadence.com/go/awr

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