Lecture #8 - Oscillators

Question 1

What is an oscillator?

Answer 1

An electronic circuit that converts a direct current (DC) into a periodically oscillating waveform (AC).

O/p waveform is often a sine/square/triangular wave.

Question 2

What makes an LO ‘local’?

Answer 2

Implies that the oscillation stays within the system, doesn’t travel through air.

RF only travels through air.

Question 3

How do we divide oscillators?

Answer 3

1. Sinusoidal or harmonic oscillators; LC or RC-tuned circuit that produce a sin waveform of constant amplitude and frequency.
2. Relaxation oscllator; produce square/triangle/sawtooth

Question 4

What forms the basis of many RF sinusoidal oscillators

Answer 4

A LC tank circuit

Question 5

In order to maintain a steady-state oscillation, the energy lost in each cycle should be compensated. How do we do this?

Answer 5

An active device e.g. BJT, FET is used to compensate the energy.

i.e. use an amplifier

Question 6

Why do we need a feedback mechanism.

Answer 6

We want the constellation amplitude to be maintained at a constant level by using a feedback mechanism.

The compensated energy should neither be too small nor too large, since it would result in damping or clamping of the oscillation, respectively.

Question 7

What are the two Barkhausen stability criterion:

Answer 7

1. The absolute value of loop gain is unity |AB|=1
2. The phase shift around the loop is 2npi, n is an element of {0,1,2, …}

Question 8

What is frequency stability?

Answer 8

The measure of an oscillator to maintain a constant frequency of oscillation over an extended period of time.

Question 9

What is Allan variance?

Answer 9

Used to measure the oscillator’s stability in time domain

Question 10

Name and briefly explain the important considerations for oscillators used in RF and microwave systems:

Answer 10

1. Tuning range
2. Frequency stability
3. AM and FM noise
4. Harmonics

Question 11

When talking about an Analog Variable Frequency Oscillator, what can we change to change the oscillation frequency

Answer 11

Changing the value of these passive components i.e. usually a variable C

Question 12

The Clapp VFO uses a varying capacity. What is the disadvantage of this?

Answer 12

Can only be used for very small frequencies.

Question 13

What is a voltage-controlled oscillator (VCO)

Answer 13

VCO is a type of oscillator, whose oscillation frequency is varied by varying the DC input voltage.

Question 14

What is the tuning range

Answer 14

The difference between the maximum and minimum frequency at which a VCO can be tuned.

Question 15

What is the tuning sensitivity

Answer 15

The change in tuning frequency of a VCO in the control voltage changes by 1V.

Question 16

What is supply pushing

Answer 16

Changes in the supply voltage of the VCO cause a corresponding change in its output frequency.

Question 17

Is supply pushing a positive or negative parameter.

Answer 17

VCO sensitivity and supply pushing are expressed in the same units (Hz/V).

Therefore, we don’t want supply pushing to happen as we change the sensitivity.

Question 18

How can pushing be minimised?

Answer 18

1. Regulating the supply voltage by using a zener diode
2. Using a high Q resonator.

Question 19

How can pushing be minimised?

Answer 19

1. Regulating the supply voltage by using a zener diode
2. Using a high Q resonator.

Question 20

What is load pulling

Answer 20

The VCO frequency is specified with an assumption that the load is perfectly matched

Question 21

How does heat effect the output power of VCO

Question 22

What is phase noise?

Question 23

Describe the PSD of phase noise

Question 24

Explain jitter

Question 25

Explain the difference between random jitter and deterministic jitter

Question 26

What is a PLL?

Answer 26

A frequency synthesis technique that can generate a wide range of frequencies with high stability and relatively low phase noise.

We’re trying to make a perfect sinusoidal noise.