30-06-2012, 02:07 PM
Study of Bridged T and L- type attenuators.
Study of Bridged T and L- type attenuators..docx (Size: 36.61 KB / Downloads: 37)
APPARATUS REQUIRED:
Trainer kit, CRO, AF Signal Generator, BNC- Crocodile lead and connecting leads, multimeter.
theory:
An attenuator is a two-port resistive network and is used to reduce the signal level from a given amount. In a number of applications it is necessary to introduce a specified loss between source and a matched load without altering the impedance relationship. Attenuators may be used for this purpose. Attenuators may symmetrical or asymmetrical and can be either fixed or variable. A fixed attenuator with constant attenuation is called a pad. Variable attenuators are used as volume controls in radio broadcasting sections. Attenuators are also used in laboratory to obtain small value of voltage or current for testing circuits.
The increase or decrease in power due to insertion or substitution of a new element in a network can be conveniently expressed in decibels (dB), or in nepers. Accordingly, the attenuation offered by a network in decibels is:
Attenuation in dB = 10log10(P1/P2),
where, P1=input Power, P2=output power .
For a properly matched network, both terminal pairs are matched to the characteristic resistance, Ro of the attenuator.
Hence, P1/P2 =I12Ro / I22Ro = I12/I22,
where, I1= input current, I2 = output current leaving the port.
Or, P1/P2 = V12/V22,
where, V1=voltage at port 1, V2= voltage at port 2
Hence, attenuation in dB = 20log10(V1/V2) = 20log10(I1/I2) ,
Also, V1/ V2= I1/ I2 = N, then P1/ P2=N2,
And, dB = 20log10N or N= antilog (dB/20)
BRIDGED-T ATTENUATOR:
The values of the arms of the network can be
specified in terms of characteristic impedance, Zo ,and propagation constant, γ, of the network. This network is a symmetrical resistive circuit; hence Zo=Ro and γ = α.
The bridged-T network may be designed to have any characteristic resistance Ro and desired attenuation by making RARB=Ro2 , where RA & RB are variable resistances and all other resistances are equal to the characteristic resistance Ro of the network.
We can obtain the design equations of the bridged-T attenuator by bisection theorem. A bisected half section is shown below:
According to the bisection theorem, a network having mirror image symmetry can be reduced to an equivalent lattice structure. The series arm of the equivalent lattice is found by bisecting the given network into two parts, short circuiting all the cut wires and equating the series impedance of the lattice to the input impedance of the bisected network; the diagonal arm is equal to the bisected network when cut wires are open circuited.