Antennas and Microwave Engineering MCQs | Page - 17

 the series resonant frequency of a crystal oscillator is given by 1/√lc. substituting the given values of l and c in the expression, the series resonant frequency is 0.25 mhz.

 parallel resonant frequency of an oscillator is given by√(lcₒc/(cₒ+c)).

 the parallel resonant frequency of a crystal oscillator is given by 1/

 in microwave oscillator, for a current to flow in the circuit the negative impedance of the device must be matched with positive impedance. this results in current being non-zero and generates oscillation.

 a positive resistance implies energy dissipation while a negative resistance implies an energy source. the negative resistance device used in the microwave oscillator, thus acts as a source. the condition xin+ xl=0 controls the frequency of oscillation. xin is the impedance of the negative resistance device.

 the condition for steady state oscillation in a microwave oscillator is zin=- zl. since this condition is satisfied in the above case, steady state oscillation is achieved.

 the condition for steady state oscillation to be achieved in terms of reflection coefficient is Гin=1/Гl. here Гin is the reflection coefficient towards the reflection coefficient device and Гl is the reflection coefficient towards the load.

 the condition zin + zl=0 is only a necessary condition for stable oscillation and not sufficient. stability requires that any perturbation in current or frequency is damped out, allowing the oscillator to return to its original state.

 in a transistor oscillator, a negative resistance one port network is created by terminating a potentially unstable transistor with impedance designed to drive the device in an unstable region.

 oscillators require a device that has high instability. to achieve this condition, transistors are used with a positive feedback to increase instability.