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 withinassigned spectral limits and so improve the use of electromagnetic

spectrum

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

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

Filter Types:

Attenuation (dB)

Frequency

60

4

0

20

0

LP HP BP BS

fc fc f1 f2

Response:

PL

R

Maximally flat

Equal ripple

/c

PLR 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

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, …

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

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)

(3) LP prototype:

g1 g3 g5 g7

g2 g4 g6

g0 = 1

gn + 1= g8 = 1

g1

g2

g3

g4

g5

g6

g7

g0 = 1

gn + 1

Impedance Scaling: L’ = RoL

C’ = C/Ro

R’s = Ro

R’L = Ro RL

Frequency Scaling of LP prototype:

Replacing by /c

L L/c

C C/c

LP to HP LP to BP LP to BS

Transformations:

Transformations:

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

Microwave Filters

Richard’s Transformation

Lumped elements Transmission line stubs

S.C

/8 at c

L=ZoL

O.CC=1/ ZoC

Lumped elements filter can be implemented as sections of transmission lines

Harmonic response

Kuroda’s Identities

Separate T.L stubs

Transform series stubs into shunt stubs and vice versaChange impractical Z to practical ones

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

Stepped Impedance LPF

Microstrip form* Low cost (simple in fabrication)* Inferior performance characteristics* Spurious response tends to occur at

lower frequencies* Frequency ~< 20 GHz

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

Coaxial Low Pass Filter type

Frequency Range (fc): few hundreds of MHz up to 10 GHz

Spurious response: appears when the high impedance lines are roughly

half wavelength long ~ 5 fc

2- Edge coupled Band Pass FiltersUsually used in narrow band applications Shielding is necessary to avoid radiationLong structure at lower microwave frequencies

3- Combline Band Pass FiltersEmploy lumped and distributed elementsCommonly used for narrow band applications at the lower microwave frequencies

4- Folded Edge-Coupled Band Pass FiltersSimilar to the edge coupled filter, but it is considerably shorterCommonly used for narrow band applications

5- Interdigital Band Pass Filters Commonly used at the lower microwave frequencies for narrow band applications

Planar FiltersHairpin filter structure

Inductive Waveguide Filter

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

A diplexer also can be placed at the output of the mixer stage where it functions as absorptive filter

IF outputRF

Input

Diplexer Parameters and Requirements

Passband attenuation (IL) and reflection (in or S11) (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

LNADiplexer

Microstrip diplexer

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

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