Digitally Controlled Frequency Synthesizers Interview Questions and Answers:

1. What is PLL?

Ans. PLL is basically an electronic circuit that consists of a phase detector, a low-pass filter, a dc amplifier and a voltage controlled oscillator connected,.

2. What are the basic building blocks of a PLL?

Ans. The basic building blocks of a PLL are phase detector (or comparator), low-pass filter, dc amplifier and a VCO.

3. What is the need for a low-pass filter in a PLL?

Ans. While the loop is trying to achieve lock, the output of the phase detector contains frequency components at the sum and difference of the signals compared. A low-pass filter is provided in a PLL to pass only the low-frequency component of the signal so that the loop can obtain lock between input and VCO signals. Low-pass filter also removes high frequency noise.

4. What is the function of a phase detector in a PLL?

Ans. A phase detector is basically a comparator that compares the input frequency f_in with feedback frequency f_out. The phase detector receives two signals, one from the input, the other fed back from the output. The output of phase detector is proportional to the phase difference between f_in and f_out. It means that the function of a phase detector is to compare input frequency and the frequency of the VCO and produce a dc voltage that is proportional to the phase difference between the two frequencies.

5. What is the role served by the VCO in a PLL chip?

Ans. The role of the VCO in a PLL is to provide an oscillating output signal (typically of square or triangular waveform), whose frequency can be adjusted over a range controlled by a dc voltage.

6. In a PLL system define the locked state.

Ans. Before the application of the input, the PLL circuit is in the free running state. When the input is applied, the VCO output frequency tries to capture the input frequency, so, it is in the capture mode. Once the two frequencies (input signal frequency and VCO output frequency) have matched, the PLL is in the phase-locked state.

7. Define the lock range of a PLL system.

Ans. The lock range of a PLL system is the frequency range over which it can maintain lock with the input signal frequency after lock has occurred.

8. Define capture range of a PLL.

Ans. The range of frequencies over which the output signal frequency of the VCO will be able to lock with the input signal frequency is called the capture range. The capture range is always smaller than lock range.

9. A PLL frequency translator has a centre frequency f_o and the input frequency is f_i. What will be the output frequency?

Ans. Output frequency, f_out = f_i ≈ f_o.

10. What is a low-pass filter?

Ans. A low-pass filter is a circuit that passes all frequencies from zero to cut-off frequency.

11. Why do we use higher order filters?

Ans. Higher order filters are used to achieve faster filter response or characteristic closer to the ideal characteristic.

12. What is a VCO?

Ans. A VCO is a circuit that provides an oscillating output signal (typically of square or triangular waveform), whose frequency can be controlled by means of an input voltage, called the control voltage.

13. Name any three applications, making use of a PLL.

Ans. Three common applications of a PLL are frequency multiplication, frequency demodulation, FSK demodulation.

14. Give the advantages and limitations of PLL synthesizer.

Ans. The PLL synthesizer solves several of the problems associated with purely analog techniques. It is accurate, easily controlled digitally and exhibits good long-term stability.

However, it suffers from limitations of its analog components. The loop must remain locked for reliable performance. Short-term drift is caused by variation in filter and VCO component values. We are to wait for the loop to stabilize when changing frequencies. Frequency is determined by an analog voltage or current. Any noise injected into this point in the circuit will cause jitter.

15. List applications of IC 555 and IC 565.

Ans. The 555 is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillation. Like general-purpose op-amps, it is very much reliable, easy to use and cheaper in cost. It has a variety of applications including monostable and astable multivibrators, dc-dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control devices, voltage regulators etc. The timer basically operates in one of the two modes either as a monostable (one-shot) multivibrator or as an astable (free-running) multivibrator.

With the rapid development of IC technology, the phase-locked loop (PLL) has emerged as one of the fundamental building blocks in electronic technology. Common applications of a PLL include (i) frequency synthesizers that provide multiples of a reference signal frequency; (ii) FM demodulation networks for FM operation with excellent linearity between the input signal frequency and the PLL output voltage; (iii) demodulation of the two data transmission or carrier frequencies in digital-data transmission employed in frequency-shift keying (FSK) operation and (iv) a wide variety of areas including telemetry receivers and transmitters, tone decoders, AM detectors, tracking filters and motor speed controls.

16. What is direct digital synthesis (DSS)?

Ans. DDS is the most recently developed frequency synthesis technique and uses logic and memory to digitally construct the desired output signal, and a data conversion device to convert it from the digital to the analog domain. It is a unique technique because it is digitally deterministic; the signal it generates is synthesized from a digital definition of the desired result.

17. How is a totally digital frequency synthesizer superior over other types?

Ans. A totally digital frequency synthesizer removes the effects of analog component variation, providing the accuracy of a crystal oscillator, and easy, immediate digit control.

The other advantages are:

1. Micro-hertz tuning resolution of the output frequency and sub-degree phase tuning capability, all under complete digital control, can be achieved.
2. Extremely fast hopping speed in tuning output frequency (or phase), phase-continuous frequency hops with no over/ under shoot or analog-related loop settling time anomalies.
3. It eliminates the requirement for the manual system tuning and tweaking associated with component aging and temperature drift in analog synthesis.
4. It facilitates an environment where systems can be remotely controlled, and minutely optimized, under processor control.

18. What are the limitations of the DDS ?

Ans. Being a sampled data system, the DDS technique is subject to inherent limitations (like quantisation noise, aliasing and spurious components) of such systems.