microwave test bench

Experiment -1

Aim: To study the microwave test bench with its components.
Equipment Required: Power supply for the Klystron microwave source, Isolator, A detector mount, A VSWR meter (which monitors the detector output),Variable Attenuator, CRO and BNC cable, Small table fan.
THEORY:
Klystron Tube- It makes use of velocity modulation to transfer a continuous beam into microwave power. Electrons emitted from the cathode are accelerated and passed through the positive resonator towards negative refractor, which retards and finally reflects the electrons; and the electrons turn back through the resonator.
Klystron Power Supply- It basically generates the necessary frequency which is to be transmitted through waveguide. It provides the beam voltage (anode) and repel voltage (cathode) to the klystron mount.
The mod switch is kept to CW position.
Keep the beam voltage control knob to fully anticlockwise and reflector voltage control knob to clockwise direction. The ranges for beam voltage 240v- 400v and repel voltage 25v-247v.
Isolator- The function of isolator is to remove present signal which are not required from output section/ detector mount to reach klystron mount. It allows sirgnal to pass through waveguide only in one direction, Klystron supply to detector mount.
Frequency meter- It helps to read the frequency of signal passing through the waveguide setup. It is read at the intersection of horizontal and vertical red lines.
Variable attenuator- It sets the amount of attenuation to be provided to the signal passing through the waveguide. It can be adjusted through screw gauge filled over it.
Slotted section- in this section the waveguide is slotted by a gap and it is done so as to adjust the amount or clock cycles of signal so that wavelength with respect to which it can be measured using vernier scale.
Detector mount- The signals transmitted by the waveguide is forwarded through the BNC to VSWR or CRO so as to measure its characteristics.


Block diagram of Microwave Test Bench Setup

VSWR- Voltage Standing Wave Ratio Meter: When the characteristic impedance and load impedance does not match maximum power transfer does not take place and only those waves are preferred which are standing in nature.
So,
VSWR = Vmax / Vmain
CSWR = Imin / Imax
Range of VSWR is >1 i.e. 1∞
Relation with reflection coefficient: To calculate the amount of reflected wave reflection wave reflection coefficient determines it.
ZR = (ZL – Z0) / (ZL + Z0)

ZL= Load impedance
Z0 = Characteristic Impedance.
Range of ZR is -1to 1.

>> It is proved with two scales i.e. of SWR and dB and each scale of SWR has two modes –
Normal
Expand
In the SWR scale the normal mode has 1-4 then scale expand mode it is 3-10 for dB scale. Normal mode has 0-10 and dB scale expanded dB has 0-2.
Result: Hence the study of microwave test bench has been done successfully.