# Another Simple MultiSim assignment

- Perform experiment in “Lab 8 – Waveguides.pdf”.

- Construct the circuit presented in the experiment with MultiSIM.

- Complete Table 21-1 and include it along with a screenshot of the Time versus Amplitude of Waveguide Output plot into a Word document

93 Experiment 21: Waveguides Purpose and Discussion The purpose of this simulation is to demonstrate the characteristics of waveguides. A waveguide is any medium which guides waves. Waveguides are used for frequencies above several giga-hertz where coaxial cable begins to exhibit skin effects and radiation attenuation and losses. The electromagnetic wave that is injected at the input by a signal launcher is bounded by the waveguide and is reflected off of the conducting walls. A wide spacing is provided which allows the transfer of hundreds of thousands of watts of power without breaking down any non-existing dielectric barriers between the conductors as is the case with coaxial cable. This results in very little loss. The signal is then received by a signal absorber. Attention to waveguide dimensions is crucial. The frequency bands of waveguides have assigned designated letters to bands of specified dimensions, frequencies and cutoff wavelengths. The cutoff frequency of a waveguide is the lowest frequency that will propagate through the conducting tube in an actual waveguide. This frequency is normally discussed in terms of wavelength as it is the length of the wave that limits its ability to propagate. Actual waveguides behave much like high pass filters providing high attenuation at frequencies below cutoff. The characteristic impedance is the impedance that would be measured at the input of an infinite length of waveguide and is given by: where and λCL = 2(broadwall dimension of the waveguide) Parts Resistors: 55 Ω Sample Waveguide AC Voltage Source Test Equipment • Oscilloscope Z fs OL CL = − 120 1 1 2 π λ λ [ ( )] ² / λfs c f = = freespace wavelength 94 Understanding RF Circuits with Multisim Formulae Characteristic Impedance Equation 21-1 Equation 21-2 λCL = 2(broadwall dimension of the waveguide) = cutoff wavelength Equation 21-3 fCL = c/λCL = cutoff frequency where c = 2.9974 x 1010 cm/s Equation 21-4 C-Band Waveguide Frequency range = 4.9 – 7.05 GHz Broadwall Dimension of waveguide in cm = 4.039 P-Band Waveguide Frequency range = 18 – 26.5 GHz Broadwall Dimension of waveguide in cm = 1.580 Z fs OL CL = − 120 1 1 2 π λ λ [ ( )] ² / λfs c f = = freespace wavelength Waveguides 95 Procedure Figure 21-1 Waveguide Example 1. Connect the circuit illustrated in Figure 21 -1. 2. Calculate the lowest acceptable propagation frequency for a C-Band waveguide. Note your results in the Data section of this experiment. Select the frequency of the AC source = 4.9 GHz. 3. Double-click the sample waveguide. Choose EDIT MODEL. Change the SPICE parameters so that LEN = 4.039 e-002. 4. Double-click the Oscilloscope to view its display. Set timebase to 0.2 ns/Div and Channel A = 1 V/Div. Run the simulation and observe the output waveform. Measure the frequency to verify that the expected signal was propagated by the simulated waveguide. Note your results. 5. Select a frequency of 7.05 GHz for the AC voltage source. Run the simulation again and note your results. Verify that the expected signal was propagated by the simulated waveguide. Run the simulation again and note your results. 6. Calculate the characteristic impedance for the C-Band waveguide. 96 Understanding RF Circuits with Multisim Expected Outcome Figure 21-3 Time versus Amplitude of Waveguide Output Data for Experiment 21 fCL = Expected (lower) Measured (lower) Expected (upper) Measured (upper) Propogation Frequency C-Band Table 21-1 ZOL = Additional Challenge Repeat all steps for a P-Band waveguide. Verify your data.

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