ENE324. Microwave experiments

Transcription

1 ENE324 Microwave experiments Gunn diodes are fabricated from a single piece of n-type semiconductor. The most common materials are gallium Arsenide, GaAs and Indium Phosphide,InP. However other materials including Ge, CdTe, InAs, InSb, ZnSe and others have been used. The device is simply an n-type bar with n+ contacts. It is necessary to use n-type material because the transferred electron effect is only applicable to electrons and not holes found in a p-type material. Within the device there are three main areas, which can be roughly termed the top, middle and bottom areas.

2 Gunn diode oscillator Gunn oscillator When a voltage is placed across the device, most of the voltage appears across the inner active region. As this is particularly thin this means that the voltage gradient that exists in this region is exceedingly high. It is found that when the voltage across the active region reaches a certain point a current is initiated and travels across the active region. During the time when the current pulse is moving across the active region the potential gradient falls preventing any further pulses from forming. Only when the pulse has reached the far side of the active region will the potential gradient rise, allowing the next pulse to be created. Experiments Measurement of the Gunn oscillator

3 Movable-Vane (Flap) Attenuator a waveguide variable, dissipative attenuator. The card enters the waveguide through the slot in the broad wall, thereby intercepting and absorbing a portion of the TE10 wave. The card penetration, and hence the attenuation, is controlled by means of the hinge arrangement to obtain variable attenuation. The ratings are typically 30 db and are widely used in microwave equipment. However, the attenuation is frequency sensitive and the phase of the output signal is a function of card penetration and hence attenuation. Slide screw tuner Slide Screw Tuner are used for matching purpose by penetration and position of a screw in the slot, provided in the center of waveguide section. These consist of a section of waveguide, flanged on both ends and a thin slot is provided in the broad wall of waveguide. POSTS and SCREWS made from conductive material can be used for impedance-changing devices in waveguide.views A and B illustrate two basic methods of using posts and screws. A post or screw that only partially penetrates into the waveguide acts as a shunt capacitive reactance. When the post or screw extends completely through the waveguide, making contact with the top and bottom walls, it acts as an inductive reactance. Note that when screws are used, the amount of reactance can be varied. Single stub tuner The waveguide equivalent of a single stub co-axial tuner is the slide-screw tuner which consists of a post that is inserted into the waveguide, through a slot, which is cut along the top surface. In the design of the single stub tuner, a susceptance is introduced in parallel at a loca>on where the real part of the input admi?ance equals the characteris>c admi?ance. The axial loca>on, l, of the post is adjustable. The depth of inser>on, d, yields the desired susceptance so that the design can be implemented.

4 PIN Modulator Waveguide terminator The PIN Diode Modulator Circuit is actually a PIN Diode attenuator circuit in which the PIN diode is forward biased by the signal wave while the RF Carrier Wave is also present in the PIN diode. The forward biased Resistance of the PIN diode is (relatively) slowly and continuously varied by the information signal waveform producing a Continuous Amplitude-Modulated RF wave. RF carrier frequency retains its sinusoidal wave-form while the amplitude envelope varies at the modulation frequency. A method for terminating a waveguide is the use of a wedge of highly resistive material, as shown. The plane of the wedge is placed perpendicular to the magnetic lines of force. When the H lines cut through the wedge, current flows in the wedge and causes a power loss. As with the other methods, this loss is in the form of heat. Since very little energy reaches the end of the waveguide, reflections are minimum. Frequency meter Using the frequency meter A wave meter consists of an adjustable resonant circuit calibrated in frequency, with a meter or other means to measure the voltage or current in the circuit. When adjusted to resonance with the unknown frequency, the resonant circuit absorbs energy, which is indicated by a dip on the meter. Then the frequency can be read from the dial.

5 Wavelength measurement