AWR eBooks

RF Electronics Chapter 7: RF Filters Page 215 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 7.9. Passband Frequency Response of the filters of Figures 7.2, 7.4, 7.6 and 7.8. The selection of the correct filter type is critical to ensure that the filter used satisfies both the transient response requirements and the passband insertion loss and stopband attenuation requirements for the filter. In digital communication applications, the Bit Error Rate (BER) after demodulating is the critical parameter. To reject signals in an adjacent channel intuition suggests the use of a Chebyshev filter. However, computer simulations carried out by the author [4] for a 3 rd Generation Mobile Radio system, have shown that when there is a large unwanted signal in an adjacent channel, the lowest demodulated BER is obtained with a Bessel type filter response for the Channel filters. Reflectionless Filters The filters shown above have a low return loss in the pass band, but attenuate the signal by reflecting the signal and having a high return loss in the stop band. The hybrids discussed in chapters 3 and 4 all require a resistive termination at all ports, to provide the required performance. In some applications, such as RF amplifiers, as discussed in chapters 8 and 9, the high return loss can lead to instability of amplifiers. It is possible to design filters to have a low return loss, so that the filter absorbs rather than reflects the signals to be filtered out. These filters are called reflectionless filters. Mini-Ccircuits [5] now make a range of reflectionless low pass and bandpass filters. Having the IF port of a mixer terminated in a reflectionless low pass filter, results in a much cleaner output spectrum as described in [6]. The diplexer shown in figure 2.9 and 2.13 is in fact a reflectionless filter for the combined port (port 1), having a very low reflection coefficient at all frequencies. When used as a high pass filter, the below cut-off frequency signals appear at port 2. When used as a low pass filter, the above cut-off frequency signals appear at port 3. Port 1 will thus have a low reflection coefficient, but ports 2 and 3 do not. The frequency dependent signal steering principle of the diplexer can be used to design a filter to have low reflections at both input and output terminals. Since the design principles are different from the reflectionless filter of [5-8] and figure 7.14, these filters are called "low reflection filters" here. A low reflection low pass filter must thus include two high pass filters, one to absorb all the unwanted signals at the input and another to absorb them at the output. Similarly, RF Electronics: Design and Simulation 215 www.cadence.com/go/awr

• Cover