Viva Questions on Oscillators, Active Filters, PLL
1.What are the requirements for producing sustained oscillations in feedback circuits?
For sustained oscillations,
· The total phase shift around the loop must be zero
· the magnitude of the loop gain should be equal to unity
2.What are the different oscillators?
AF oscillators: RC phase shift oscillator, Wein bridge oscillator
RF oscillator: Colpitts oscillator, Hartley oscillator
3.What is a filter?
Filter is a frequency selective circuit that passes signal of specified band of frequencies and attenuates the signals of frequenciesoutside the band
4.What are the demerits of passive filters?
Passive filter max gain is unity and its roll off is less.
5.What are the advantages of active filters?
Offers high input impedance and low output impedance, thus improving theload drive capacity. Pass band gain is more than 1, roll off is more.
6.What are the various filters?
· Low pass filter
· High pass filter
· Band pass filter
· Band reject filter.
7.What is the use of All pass filter?
it passes all signals to output but provides phase difference of 0 -90 degrees.
8. What Order of the filter Represents?
Order of the filter represents No. of storage elements with in the system (circuit), as order increases Roll off increases.
9. Define cut off frequency?
The cutoff frequency is defined as the frequency at which the ratio of the (input/output) has a magnitude of 0.707. When this magnitude is changed to decibels –3dB mostly referred to as the 3dB down point.
10.Where PLL is widely used?
Radar synchronization, satellite communication systems, air borne navigational systems, FM communication systems,etc.
11.What are the basic building blocks of PLL?
Phase detector/comparator, Low pass filter, Error amplifier, Voltage controlled oscillator
12.What are the three stages through which PLL operates?
Free running, Capture, Locked/ tracking.
13.Define lock-inrange, capture range, and Pullintime of a PLL:
*The range of frequencies over which the PLL can maintain lock with the incoming signal is called the lockin range or trackingrange.
*The range of frequencies over which the PLL can acquire lock with an input signal is called the capture range.
*The total time taken by the PLL to establish lok is called pullintime.It depends on the initial phase and frequency differencebetween the two signals as well as on the overall loop gain and loop filter characteristics.
14.What is a voltage controlled oscillator?
Voltage controlled oscillator is a free running multivibrator operating at a set frequency called the free running frequency.Thisfrequency can be shifted to either side by applying a dc control voltage and the frequency deviation is proportional to the dc controlvoltage.
15.On what parameters does the free running frequency of VCO depend on?
*External timing resistor,RT *External timing capacitor,CT *The dc control voltage Vc.
16.Give the expression for the VCO free running frequency.
fo = 0.25 / Rt Ct
17.Mention some typical applications of PLL:
• Frequency multiplication/division
• Frequency translation
• AM detection
• FM demodulation
• FSK demodulation.
Viva Questions on ADC, DAC:
1.List the broad classification of ADCs.
Direct type ADC. Integrating type ADC.
2.List out the direct type ADCs.
1. Flash type converter
2. Counter type converter
3. Tracking or servo converter
4. Successive approximation type converter
3.List out some integrating type converters.
1. Charge balancing ADC
2. Dual slope ADC
4.What is integrating type converter
An ADC converter that perform conversion in an indirect manner by first changing the analog I/P signal to a linear function of time or frequency and then to a digital code is known as integrating type A/D converter.
5.Explain in brief the principle of operation of successive Approximation ADC.
The circuit of successive approximation ADC consists of a successive approximation register (SAR), to find the requiredvalue of each bit by trial & error.With the arrival of START command, SAR sets the MSB bit to 1. The O/P is converted into ananalog signal & it is compared with I/P signal. This O/P is low or High. This process continues until all bits are checked.
6.What are the main advantages of integrating type ADCs?
i.The integrating type of ADC's do not need a sample/Hold circuit at the input.
ii. It is possible to transmit frequency even in noisy environment or in an isolated form.
i.The integrating type of ADC's do not need a sample/Hold circuit at the input.
ii. It is possible to transmit frequency even in noisy environment or in an isolated form.
7.Where are the successive approximation type ADC's used?
The Successive approximation ADCs are used in applications such as data loggers & instrumentation where conversionspeed is important.
8.What is the main drawback of a dual slope ADC?
The dual slope ADC has long conversion time. This is the main drawback of dual slope ADC.
9.State the advantages of dual slope ADC:
It provides excellent noise rejection of ac signals whose periods are integral multiples of the integration time T.
10.Define conversion time.
the total time required to convert an analog signal into its digital output.
11.Define resolution of a data converter.
The resolution of an ADC is defined as the smallest change in analog input for a one bit change at the output.
12.What is meant by linearity?
The linearity of an ADC/DAC is an important measure of its accuracy & tells us how close the converter output is to itsideal transfer characteristics.
12.What is monotonic DAC?
A monotonic DAC is one whose analog output increases for an increase in digital input.
13.What is a sample and hold circuit? Where it is used?
which samples an input signal and holds on to its last sampled value until the input is sampled again. This is mainly used in analog to digital conversion
14.Explain the various types of digital to analog converters\
· Weighted resistor DAC
· R2R ladder DAC
· Inverted R2R ladder DAC
