AWR eBooks

RF Electronics Chapter 7: RF Filters Page 219 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Cascading these filters preserves the low return loss and increases the stopband attenuation. However, including a conventional high pass filter as part of such a cascaded filter, results in a passband return loss similar to that of the conventional high pass filter. Low reflection and reflectionless lowpass, bandpass and bandstop filters can be made by using the relevant low-pass to highpass, low-pass to band-pass, low-pass to band-stop transformations described later in this chapter. These filters require close to twice the number of reactive components, compared to conventional filters. Reflectionless filters are also very useful for ensuring stability of amplifiers described in chapter 8, by reducing the gain to signals outside the required bandwidth, whilst maintaining near ideal termination for the amplifier. Higher order low reflection and reflectionless filters can be designed using the principles outlined in [7] for reflectionless filters and using the optimisation outlined in figures 7.10 to 7.14 for low reflection filters. In practice, both designs techniques will result in reflections less than -20 dB. RF Lowpass Filter design Lowpass filters at RF and microwave frequencies, can be designed and constructed using conventional LC filter design principles. However, care should be taken to ensure that: 1 The impedance of the leads is kept as small as possible. In particular, the impedance of the leads on capacitors connecting the capacitors to ground, or the length of any PCB layout track connecting the components to the ground-plane must be kept as small as possible, since its inductance will affect the frequency response of the filter. 2 Components must be used that have self-resonant frequencies well above the cut off frequency of the filter if a lowpass filter is used and well above any useable frequency if a high pass filter is used. Most commonly used capacitors have self-resonant frequencies well below 1 GHz. It is feasible to design conventional LC lowpass filters with cut off frequencies in excess of 1 GHz. Surface-mount components generally have a higher cut-off frequency than leaded components. When the frequency becomes too high for either of them to be used, inductors can be replaced with thin transmission lines of the length required to give the desired inductance and capacitors can be replaced by wide transmission lines. As an example, consider the design of a 7 th order Butterworth lowpass filter with a cut off frequency of 2 GHz. From the equations 7.1 and 7.2 for Butterworth filters or from tables, by de-normalising and using a cut off of 2 GHz and input impedance of 50 , the component values shown in table 7.2 are obtained for the filter. Table 7.2. LC filter values and corresponding Microstrip line size. L C Value Required Line Length Line Width L1 1.771 nH 3.54 mm 0.5 mm C2 1.985 pF 4.2 mm 10 mm L3 7.170 nH 13.5 mm 0.5 mm C4 3.183 pF 6.6 mm 10 mm L5 7.170 nH 13.5 mm 0.5 mm C6 1.985 pF 4.2 mm 10 mm L7 1.771 nH 3.54 mm 0.5 mm RF Electronics: Design and Simulation 219 www.cadence.com/go/awr

• Cover