Summary: AE_Lecture11_Part2 describes the Radio-Frequency Oscillators.
AE_LECTURE 11_Part2.
Part 2 deals with RF Oscillators.
Section 1. Class of RF OSCILLATORS
(100kHz- 1000kHz:Medium Wave RF & 1MHz-30MHz:Short Wave RF;
30MHz-70MHz: Amateur Band for HAM Radio practitioners;
70MHz-300MHz: Very High Frequency (VHF) Band for FM and TV transmissions;
300MHz-800MHz: Ultra High Frequency(UHF) Band for Police Communication;
1GHz-100GHz: Microwaves for satellite communication;
Terra Hz(0.4µm-100µm): Optical Fiber Communication)
LC oscillators are RF oscillators.
Figure 1. Parallel Resonance Circuit or Tank Circuit.
Figure 2. Circuit Diagram of a Tank Circuit driven by a constant current source.
Parallel resonance circuit is known as tank Circuit.
Resonance frequency=ω_{o}=
Figure 3. Magnitude of the reactance of a tank circuit vs frequency.
At ω=
When quality factor Q of the tank circuit =∞, the circuit is purely reactive and there is no dissipation.
When quality factor Q is finite say 1000 then an equivalent R_{P} comes in parallel with the tank circuit. R_{P }accounts for the losses
For a finite quality factor tank circuit, the effective impedance is a pure resistance R_{P} at resonance frequency.
Hence
Frequency response of the tank circuit is :
Figure4. The peak response of a tank circuit for various quality factors.
At Q=∞, spike response.
At Q=finite, peak response.
As Q falls, sharpness of the peak response is lost.
Section 1.1. GENERALIZED LC OSCILLATOR.
Figure 5. Block Diagram of generalized LC Oscillator
Where
Therefore:
Examining the feedback network in Figure 8:
Feed back voltage
If loop gain=1∟0˚
This identity can be true only if
Subs(4) & (5) in (3)
Starting condition is
This means
Table 1. Different possible configurations of a generalized LC Oscillator.
Z 1 | Z 2 | Z 3 | Type of Oscillator | |
1 | L | L | C | Hartley Oscillator |
2 | C | C | L | Colpitts Oscillator |