McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Electrical Engineering .
| 151. |
Noise figure can be defined for any microwave network irrespective of any other constraints. |
| A. | true |
| B. | false |
| C. | topic 5.6 low noise amplifier design |
| Answer» B. false | |
| Explanation: noise figure is defined only for a matched input source and for a noise source equivalent to a matched load at a temperature t0= 290 k. noise figure and noise temperature are interchangeable noise properties. | |
| 152. |
Expression for noise of a two port network considering the noise due to transmission line and other lossy components is: |
| A. | gktb + gnadded |
| B. | gktb |
| C. | gnadded |
| D. | none of the mentioned |
| Answer» A. gktb + gnadded | |
| Explanation: expression for noise of a two port network considering the noise due to transmission line and other lossy components is gktb + gnadded. here, g is the gain of the system. nadded is the noise generated by the transmission line, as if it appeared at the input terminals of the line. | |
| 153. |
Noise equivalent temperature of a 2 stage cascade network is given by: |
| A. | te1 + te2/ g1 |
| B. | te1 + te1 |
| C. | te1 / te1 |
| D. | none of the mentioned |
| Answer» A. te1 + te2/ g1 | |
| Explanation: noise equivalent temperature of a 2 stage cascade network is given by te1 | |
| 154. |
For a Wilkinson power divider of insertion loss L and the coupler is matched to the external circuitry, and then the gain of the coupler in terms of insertion loss is: |
| A. | 2l |
| B. | 1/2l |
| C. | l |
| D. | 1/l |
| Answer» B. 1/2l | |
| Explanation: to evaluate the noise figure of the coupler, third port is terminated with known impedance. then the coupler becomes a two port device. since the coupler is matched, Гs=0 and Гout=s22=0. so the available gain is │s21│2. this is equal to 1/2l from the available data. | |
| 155. |
Noise equivalent temperature of Wilkinson coupler having a gain of 1/2L is given as: |
| A. | t (2l-1) |
| B. | t (2l+1) |
| C. | t (2l*1) |
| D. | t / (2l-1) |
| Answer» A. t (2l-1) | |
| Explanation: noise equivalent temperature of the wilkinson coupler is found using the relation | |
| 156. |
Expression for over all noise figure of a mismatched amplifier is: |
| A. | 1+ (f-1)/ (1 -│Г│2) |
| B. | 1 |
| C. | 1+ (f-1) |
| D. | (f-1)/ (1 -│Г│2) |
| Answer» A. 1+ (f-1)/ (1 -│Г│2) | |
| Explanation: the overall noise figure of a | |
| 157. |
One condition to be satisfied in an oscillator circuit so that stable oscillations are produced is: |
| A. | positive feedback is to be achieved |
| B. | negative feedback is to be achieved |
| C. | 1800 phase shift is required between the transistor input and output. |
| D. | none of the mentioned |
| Answer» C. 1800 phase shift is required between the transistor input and output. | |
| Explanation: in an oscillator a total of 3600 of phase shift is to be achieved in the entire circuit to produce oscillations. the transistor used in the oscillator circuit must produce a phase shift of 1800 to achieve stable oscillations. hence this condition has to be satisfied by the oscillator. | |
| 158. |
In an oscillator, the resonant feedback circuit must have must have a low Q in order to achieve stable oscillation. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: if the resonant feedback circuit has a high q, so that there is random phase shift with frequency, the oscillator will have good frequency stability. | |
| 159. |
Quartz crystal and tourmaline used in oscillators work on the principle of: |
| A. | photo electric effect |
| B. | piezo electric effect |
| C. | raman effect |
| D. | black body radiation |
| Answer» B. piezo electric effect | |
| Explanation: quartz crystals work on the principle of piezo electric effect. when electrical energy is applied to these crystals, they vibrate in a direction perpendicular to the application of energy producing oscillations. | |
| 160. |
A quartz crystals equivalent circuit is a series LCR circuit and has a series resonant frequency. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: a quartz crystal has an equivalent circuit such that a series lcr network is in parallel with a capacitor. a quartz crystal thus has both series and parallel resonant frequencies. | |
| 161. |
In the plot of reactance v/s frequency of a crystal oscillator, the reactance between series resonant frequency and parallel resonant frequency is: |
| A. | capacitive |
| B. | inductive |
| C. | both capacitive and inductive |
| D. | none of the mentioned |
| Answer» B. inductive | |
| Explanation: in the plot of reactance v/s frequency of a crystal oscillator, the reactance between series resonant frequency and parallel resonant frequency is inductive. in this region between the series and parallel and series resonant frequencies, the operating point of the crystal is fixed and hence can be used as part of other circuits. | |
| 162. |
In the equivalent circuit of a quartz crystal, LCR arm has an inductance of 4 mH and capacitor has a value of 4nF, then the series resonant frequency of the oscillator is: |
| A. | 0.25 mhz |
| B. | 2.5 mhz |
| C. | 25 mhz |
| D. | 5 mhz |
| Answer» A. 0.25 mhz | |
| Explanation: 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. | |
| 163. |
Parallel resonant frequency of quartz crystal is given by: |
| A. | 1/ √(lcₒc/(cₒ+c)) |
| B. | 1/√lc |
| C. | 1/√lcₒ |
| D. | 1/ √(l(cₒ+c) ) |
| Answer» A. 1/ √(lcₒc/(cₒ+c)) | |
| Explanation: parallel resonant frequency of an oscillator is given by√(lcₒc/(cₒ+c)). | |
| 164. |
The equivalent circuit of a quartz crystal has LCR arm capacitance of 12nF and inductance of 3mH and parallel arm capacitance of 4nF. Parallel resonant frequency for the circuit is: |
| A. | 3 mhz |
| B. | 0.3 mhz |
| C. | 6 mhz |
| D. | 9 mhz |
| Answer» A. 3 mhz | |
| Explanation: the parallel resonant frequency of a crystal oscillator is given by 1/ | |
| 165. |
In microwave oscillators, negative resistance transistors and diodes are used in order to generate oscillations in the circuit. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: 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. | |
| 166. |
Any device with negative impedance as its characteristic property can be called: |
| A. | energy source |
| B. | energy sink |
| C. | oscillator |
| D. | none of the mentioned |
| Answer» A. energy source | |
| Explanation: 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. | |
| 167. |
In a microwave oscillator, a load of 50+50j is connected across a negative resistance device of impedance -50-50j. Steady state oscillation is not achieved in the oscillator. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: 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. | |
| 168. |
For achieving steady state oscillation, the condition to be satisfied in terms of reflection coefficients is: |
| A. | Гin=Гl |
| B. | Гin=-Гl |
| C. | Гin=1/Гl |
| D. | none of the mentioned |
| Answer» C. Гin=1/Гl | |
| Explanation: 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. | |
| 169. |
To achieve stable oscillation, Zin + ZL=0 is the only necessary and sufficient condition to be satisfied by the microwave oscillator. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: 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. | |
| 170. |
In transistor oscillators, the requirement of a negative resistance device is satisfied using a varactor diode. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: 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. | |
| 171. |
In transistor oscillators, FET and BJT are used. Instability is achieved by: |
| A. | giving a negative feedback |
| B. | giving a positive feedback |
| C. | using a tank circuit |
| D. | none of the mentioned |
| Answer» B. giving a positive feedback | |
| Explanation: oscillators require a device that has high instability. to achieve this condition, transistors are used with a positive feedback to increase instability. | |
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