03-08-2012, 05:02 PM
Principles of SAWR-stabilized oscillators and transmitters
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Principles of SAW-stabilized oscillators and transmitters
Physical principle:
An oscillator is a network comprising an active stage (amplifier) and a frequencysensitive
feedback stage (resonator, filter). Part of the output signal is fed to the input
of the amplifier passing the feedback. The insertion loss and the phase of the
feedback depend on the frequency.
For oscillation, the insertion loss of the feedback is at a minimum and the total phase
in the oscillator loop must be n*360°. In this case, there is a positive feedback and an
oscillation can be generated.
The total gain G in the loop is greater than 1. So the gain of the amplifier must be
greater than the attenuation of feedback and power dissipation in the RF load.
The total phase in the circuit must be n*360° to achieve positive feedback.
The phase slope of the resonator is very steep in the area of the resonance
frequency. This steep phase slope is necessary for good frequency stability.
To start up an oscillator, the thermal noise is amplified in the active stage. This very
low broadband noise continuum is fed to the amplifier's input by the feedback loop.
In this continuum, there are frequency components at the desired oscillating
frequency. These frequency components pass the frequency-sensitive feedback
stage without strong attenuation. If the phase condition is met, the feedback signal is
added to the input signal. This procedure is repeated cycle by cycle, and the output
signal increases. The maximum oscillating level is limited by nonlinear effects of the
amplifier. When the signal amplitude and the oscillation frequency are fixed, the
oscillator is in the steady-state mode.
Typical oscillation circuits
Oscillators with one-port resonators
Oscillators with one-port resonatorThe common-base Colpitts oscillator is a preferred
circuit for one-port resonators. This type of circuit is used in many applications and
ideal for modulation with amplitude shift keying (ASK) and on-off keying (OOK).
The bill of material for a Colpitts oscillator is very short. The transmitter antenna can
easily be integrated on the PCB.
The Colpitts oscillator also works with a two-port SAW resonator in a one-port
configuration, but a real one-port SAW resonator has many advantages
This provides merely initial values for further optimization. The circuit is much more
complicated, because there are numerous parasitic capacitances (e.g. output
capacitance of the transistor, PCB capacitance, etc.).
R1, R2 and R3 are used for DC biasing. C3 matches the high-impedance collector
output to the load impedance. The feedback of the oscillator is set by the ratio of C1
to C2, which also determines the transient response and other specific oscillator
properties.