microwave filters filters allow some frequencies to go through while block the remaining in...
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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
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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
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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
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Filter Types:
Attenuation (dB)
Frequency
60
4
0
20
0
LP HP BP BS
fc fc f1 f2
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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
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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, …
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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
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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)
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(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
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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
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LP to HP LP to BP LP to BS
Transformations:
Transformations:
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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
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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
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Kuroda’s Identities
Separate T.L stubs
Transform series stubs into shunt stubs and vice versaChange impractical Z to practical ones
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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
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Stepped Impedance LPF
Microstrip form* Low cost (simple in fabrication)* Inferior performance characteristics* Spurious response tends to occur at
lower frequencies* Frequency ~< 20 GHz
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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
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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
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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
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Planar FiltersHairpin filter structure
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Inductive Waveguide Filter
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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
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A diplexer also can be placed at the output of the mixer stage where it functions as absorptive filter
IF outputRF
Input
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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
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Microstrip diplexer
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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
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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