This includes coverage of OSCILATOR introduction and ring oscillator performance and effects

1. OSCILATORS

2. INTRODUCTION
• An oscillator is an electronic circuit that generates a periodic
waveform on its output without an external signal source. It is
used to convert dc to ac
• An oscillator is a circuit that produces a repetitive signal from a
dc voltage
• The feedback oscillator relies on a positive feedback of the
output to maintain the oscillations
• The relaxation oscillator uses RC timing circuit to generate a
non sinusoidal signal such as square wave

3. • When switch at the amplifier input is open, no oscillation occurs.
• Consider V₁, results in V0=AV₁ (after amplifier stage) and V0=
B(AV)(after feedback stage).
• Feedback voltage V₁= B(AV) where BA is called loop gain.
• In order to maintain Vf= Vi*BA must be in the correct magnitude and
phase.
• When the switch is closed and V, is removed, the circuit will continue
operating since the feedback voltage is sufficient to drive the amplifier
and feedback circuit, resulting in proper input voltage to sustain the
loop oscillations

4. BARKHAUSEN CRITERIA
• A*B Is called as LOOP GAIN
• At a specific frequency at f which A*B=1 which means the loop
gain will be infinite, the circuit will have finite output for zero input
signal
• Thus the frequency of oscillation is solely determined by the phase
characteristic of the feedback loop – the loop oscillates at the
frequency for which the phase is zero.

5. PERFORMANCE PARAMETERS
1.Frequency Range
An RF oscillator must be designed such that its frequency can
be varied across a certain range. This range includes two components:
(1) the system specification; for example, a 900-MHz GSM direct
conversion receiver may tune the LO from 935MHz to 960 MHz
(2) additional margin to cover process and temperature variations and
errors due to modeling inaccuracies.
2.Output Voltage Swing
we must produce sufficiently large output swings to ensure
nearly complete switching of the transistors in the subsequent stages.
excessively low output swings exacerbate the effect of the internal noise
of the oscillator.

6. 3. Drive Capability
The LO must drive the input Capacitance of at least one mixer and
one divider[frequency synthesizer].
4.Phase Noise
The spectrum of an oscillator in practice deviates from an impulse
and is “broadened” by the noise of its constituent devices. Called “phase noise,”
this phenomenon has a profound effect on RF receivers and transmitters. Since
the phase noise of LC oscillators is inversely proportional to the Q of their tank,
we will pay particular attention to factors that degrade the Q.
5.Power Dissipation
The power drained by the LO and its buffers proves critical in some
applications as it trades with the phase noise and tuning range. Thus, many
techniques have been introduced that lower the phase noise for a given power
dissipation

7. RING OSCILLATOR
• A ring oscillator is a closed loop comprising of the odd number of
stages of identical inverters which forms the feedback circuit
• This feedback from its last output to the input causes the oscillations
• Even number of transistors in a circular chain feedback loop cannot
be used in the oscillator because the last output of the inverter is same
as the input
• Even stage design can be considered as the memory element and acts
as building block of SRAM

9. • The most important factor in ring oscillator is gate delay because in devices
fabricated with MOSFET, gate cannot switch immediately.
• The gate capacitance needs to be charged before current flows between drain
and source so that every inverter takes time to give output.
• Therefore increase in the number of stages of ring oscillator increase the gate
delay.
• Odd number of inverter stages used to give the effect of single inverter
amplifier with a negative feedback gain of greater than 1 so that the output will
be in opposite direction to the input and it will be amplified with an amount
more than the input.

10. JITTER AND PHASE NOISE
• Phase noise is the representation of random fluctuations infrequency
domain which is caused by the jitter.
• It may be caused due to the electromagnetic interference(EMI) and
crosstalk with other signals.
• Period jitter:
It is the difference between any one clock period and an ideal clock
period. It tends to be important in synchronous circuits.
• Cycle to cycle jitter:
It is the difference between the duration of two adjacent clock periods.
It can be important for clock generation circuits

11. THANKING YOU