1. Microwave Filters Filters allow some frequencies to go through while block the remaining In receivers, the system filters the incoming signal right after reception (to avoid interference and nonlinear operation of LNA) In transmitter, filters suppress much of the transmitted generated harmonics, wide band noise, IMD products, and out-of-band conversion frequencies In detector, mixer and multiplier applications, the filters are used to block unwanted high-frequency products Filters are used to separate or combine radio frequencies They are used to select or confine the RF/microwave signals within assigned spectral limits and so improve the use of electromagnetic spectrum

2. Filter Characteristics Important parameters: ■ SWR or S11 ■ Insertion Loss (S21 or S12) Passive Reciprocal ■ Attenuation in the stopband (S21) ■ Group delay ■ Power Handling Capability ■ Size & Weight ■ Tunability

3. Filter Classification Classification may be according to one of the following:  Frequency selection (LP, HP, BP, or BS)  Response (Chebysheve, Maximally flat,…..etc)  Technology (Lumped, Waveguide, SAW, …etc)  Frequency band (Narrow band or Broadband)  Reflection type or absorbing type

4. Filter Types: Attenuation (dB) Frequency 60 40 20 0 LP HP BP BS fcfc fcfc f1f1 f2f2

5. Response: P LR Maximally flat Equal ripple  /  c P LR is the power loss ratio Reflection type or absorbing type:  The majority of filters achieve frequency selection by reflection  A small class of filters achieve attenuation by absorption

6. Filters technology include:  Lumped elements  SAW  Planar & Uniplanar (MIC)  Coaxial type  Dielectric Resonators  Metallic resonators or Waveguide  MMICs Classification by Band of Operation:  Narrow band or Broad band  IF filters, L-band, C-band, …

7. In receiver: Filters reject signals outside the operating band and so protect the receivers from any out of band signals, attenuating undesired mixer products and setting the IF bandwidth of the receiver In transmitters: Filters control the spurious response of the mixers, select the desired side bands, and confine the radiation from high power transmitters within assigned spectral limits

8. Filter Design (2) Filter order n (according to the required frequency response For design purpose, insertion loss method is generally preferred for the flexibility and accuracy. (1) Select response (Chebyshev or Maximally flat)

9. (3) LP prototype: g1g1 g3g3 g5g5 g7g7 g2g2 g4g4 g6g6 g 0 = 1 g n + 1 = g 8 = 1 g1g1 g2g2 g3g3 g4g4 g5g5 g6g6 g7g7 g 0 = 1 g n + 1

12. Impedance Scaling: L’ = R o L C’ = C/R o R’s = R o R’ L = R o R L Frequency Scaling of LP prototype: Replacing  by  /  c L L/  c C C/  c

13. LP to HP LP to BP LP to BS Transformations:

14. High Frequency Limitations of lumped Elements Filters Lumped elements Filters:  Work well at low frequencies < 1GHz  Available only for a limited range of values  Difficult to implement at high frequencies  Parasitic effects of lumped element components have a significant impacts on elements performances  Can be fabricated with limited values using MMICs technology to be used at frequencies below 20 GHz but in this case the elements Q-factor is reduced and large loss is expected especially for narrow band applications

15. Microwave Filters Richard’s Transformation Lumped elements Transmission line stubs S.C /8 at  c L=Zo L O.C C=1/ Zo C Lumped elements filter can be implemented as sections of transmission lines Harmonic response

16. Kuroda’s Identities Separate T.L stubs Transform series stubs into shunt stubs and vice versa Change impractical Z to practical ones

17. Series inductors Series stubs Shunt capacitors Shunt stubs Add /8 lines of Zo = 1 at input and output Apply Kuroda identity for series inductors to obtain equivalent with shunt open stubs with λ/8 lines between them LPF using Kuroda’s identities

18. Stepped Impedance LPF Microstrip form * Low cost (simple in fabrication) * Inferior performance characteristics * Spurious response tends to occur at lower frequencies * Frequency ~< 20 GHz

19. Wave-guide technology Expensive to manufacture & avoided where possible, Offer satisfactory performance at higher microwave frequencies. Wide band & large size High-attenuation stop bands which can be made to be free of the spurious responses for all modes. High power rating Operating frequency : > 10 GHz Waffle iron filter

20. Coaxial Low Pass Filter type Frequency Range (f c ): few hundreds of MHz up to 10 GHz Spurious response: appears when the high impedance lines are roughly half wavelength long ~ 5 f c

21. 2- Edge coupled Band Pass Filters Usually used in narrow band applications Shielding is necessary to avoid radiation Long structure at lower microwave frequencies 3- Combline Band Pass Filters Employ lumped and distributed elements Commonly used for narrow band applications at the lower microwave frequencies 4- Folded Edge-Coupled Band Pass Filters Similar to the edge coupled filter, but it is considerably shorter Commonly used for narrow band applications 5- Interdigital Band Pass Filters Commonly used at the lower microwave frequencies for narrow band applications

23. Planar Filters Hairpin filter structure

24. Inductive Waveguide Filter

25. Diplexer Filters Diplexers are two or more combined filters combined in a single package that are adopted to separate two or more different frequencies. The diplexer is required to connect the high power output, or transmit (Tx) stage and the very low power input, or receive stage, of a radio to a single dual band antenna. PA LNA Diplexer 0 dB -70 dB

26. A diplexer also can be placed at the output of the mixer stage where it functions as absorptive filter IF output RF Input

27. Diplexer Parameters and Requirements Passband attenuation (IL) and reflection (  in or S 11 ) (Tx efficiency & Rx noise figure) LNA isolation from the Tx power generated in the Rx range (-80 dBm which is greater than the minimum expected signal of the receiver) Harmonic rejection Low pass filters are sometimes required to provide sufficient spurious and harmonic attenuation PA LNA Diplexer

28. Microstrip diplexer

29. Multiplexer and Demultiplexer This topology may cause some degradation in the performance due to the interaction that might be occur due to the reflection filters used. Channel 1 Channel 2 Channel n Multiplexers split a wide frequency band into a number of signals of different frequency ranges. The separation of the desired frequency band is commonly achieved by using bandpass filters combined at a common input as shown in the figure

30. Channel 1 Channel 2 Channel n Matched load One way to overcome the above problem is the use of circulators as shown in the following Figure