attenuatorand matching techniques

8/8/2019 Attenuatorand Matching Techniques

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Attenuator

IntroductionAttenuators are devices used to adjust signal levels, to control impedance mismatchand to isolate circuit stages. Attenuators weaken orattenuate the high level output of a

signal generator, for example, to provide a lower level signal for something like theantenna input of a sensitive radio receiver. The attenuator could be built into the signagenerator, or be a stand-alone device. An attenuator section can also provide isolationbetween a source and a troublesome load.

In the case of a stand-alone attenuator, it must be placed in series between the signalsource and the load by breaking open the signal path. In addition, it must match boththe source impedance ZI and the load impedance ZO, while providing a specifiedamount of attenuation.

Passive designPassive attenuators consist of two types pi and Tee attenuators.

(1) AttenuatorThe circuit for the Pi attenuator is


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Q.1 What resistor values would be required for both the attenuators for 10 dB ofattenuation matching a 50 source and load?

Sol The 10 dB corresponds to a voltage attenuation ratio of K=3.16 of the abovetable. Transfer the resistor values in that line to the resistors on the schematic diagram


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(2) Tee AttenuatorThe circuit for the T attenuator is

(3) Bridged T AttenuatorThe circuit of the bridge T attenuator is


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This circuit is commonly used with PIN diodes to form an electronic attenuator as only two

variable resistors are required (instead of 3 for the Pi or T attenuators).

RF attenuators

For RF, the individual sections must be mounted in shielded compartments to thwart

capacitive coupling if lower signal levels are to be achieved at the highest frequencies.The individual sections of the switched attenuators in the previous section are mountedin shielded sections. Additional measures may be taken to extend the frequency rangeto beyond 1000 Mhz. This involves construction from special shaped lead-less resistiveelements.

Coaxial T-attenuator for RF Coaxial -attenuator for RF


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RF connectors (not shown) are attached to the ends of the above T and attenuators.

The connectors allow individual attenuators to be cascaded, in addition to connecting

between a source and load. For example, a 10 dB attenuator may be placed between

a troublesome signal source and an expensive spectrum analyzer input. Even though

we may not need the attenuation, the expensive test equipment is protected from thesource by attenuating any overvoltage.

Cascaded sections

Attenuator sections can be cascaded for more attenuation than may be available froma single section. For example two 10 dB attenuators may be cascaded to provide 20dB of attenuation, the dB values being additive. The voltage attenuation ratio K or

VI/VO for a 10 dB attenuator section is 3.16. The voltage attenuation ratio for the twocascaded sections is the product of the two Ks or 3.16x3.16=10 for the two cascadedsections.


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Example - 1. A 50, / 4 line with ZL= 50 + j100 . Find Zin, L, VSWR.

Example- 2. A 75 line with ZL = 150 + j150 . Find L, VSWR at 0.375 from

load, shorted distance to give a resistiveZin and its value.

Example 3 . A 100 line with YL= 0.0025 - j0.0025 S, find Zin for 3.15 from

load and VSWR.


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Example.4 . A 100 line with ZL has VSWR=2 and xmin = 50cm. Minimum

location is shifted toward load by 15cm as load becomes a short circuit, findZL.


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Impedance matching concept: For given ZL, design a matching network to have

in = 0 or selected value


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Example 1. ZL=50-j75, Z0=100, find ds, shunt short stub length ls

Example 2. ZL=100/(2+j3.732), Zo=100, find shortest ds, shunt short stub

length ls


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Example.3. L=0.4 - 30, in=0.2 45, Z0 = 50, f = 4 GHz, find ds, shunt C or series L


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Example 4. ZL=100 + j 50 , Zo = 50, l= / 8, d= / 8, find l1, l2

attenuatorand matching techniques

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