Lecture #8 - Oscillators Flashcards
What is an oscillator?
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.
What makes an LO ‘local’?
Implies that the oscillation stays within the system, doesn’t travel through air.
RF only travels through air.
How do we divide oscillators?
- Sinusoidal or harmonic oscillators; LC or RC-tuned circuit that produce a sin waveform of constant amplitude and frequency.
- Relaxation oscllator; produce square/triangle/sawtooth
What forms the basis of many RF sinusoidal oscillators
A LC tank circuit
In order to maintain a steady-state oscillation, the energy lost in each cycle should be compensated. How do we do this?
An active device e.g. BJT, FET is used to compensate the energy.
i.e. use an amplifier
Why do we need a feedback mechanism.
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.
What are the two Barkhausen stability criterion:
- The absolute value of loop gain is unity |AB|=1
- The phase shift around the loop is 2npi, n is an element of {0,1,2, …}
What is frequency stability?
The measure of an oscillator to maintain a constant frequency of oscillation over an extended period of time.
What is Allan variance?
Used to measure the oscillator’s stability in time domain
Name and briefly explain the important considerations for oscillators used in RF and microwave systems:
- Tuning range
- Frequency stability
- AM and FM noise
- Harmonics
When talking about an Analog Variable Frequency Oscillator, what can we change to change the oscillation frequency
Changing the value of these passive components i.e. usually a variable C
The Clapp VFO uses a varying capacity. What is the disadvantage of this?
Can only be used for very small frequencies.
What is a voltage-controlled oscillator (VCO)
VCO is a type of oscillator, whose oscillation frequency is varied by varying the DC input voltage.
What is the tuning range
The difference between the maximum and minimum frequency at which a VCO can be tuned.
What is the tuning sensitivity
The change in tuning frequency of a VCO in the control voltage changes by 1V.
What is supply pushing
Changes in the supply voltage of the VCO cause a corresponding change in its output frequency.
Is supply pushing a positive or negative parameter.
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.
How can pushing be minimised?
- Regulating the supply voltage by using a zener diode
- Using a high Q resonator.
How can pushing be minimised?
- Regulating the supply voltage by using a zener diode
- Using a high Q resonator.
What is load pulling
The VCO frequency is specified with an assumption that the load is perfectly matched
How does heat effect the output power of VCO
What is phase noise?
Describe the PSD of phase noise
Explain jitter
Explain the difference between random jitter and deterministic jitter
What is a PLL?
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.
