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Pasternack BlogA local oscillator (LO) is an electronic oscillator that generates a very narrow bandwidth signal, or tone, with high purity and stability for use in frequency conversion circuits and timing. As the performance of frequency conversion circuits and timing systems are often dependent on the LO performance, the quality of the oscillator and LO electronic design is often of high priority. LOs differ from typical RF oscillators as there is often some form of stability control and sometimes additional tuning circuits that enables a LO to reliably shift frequency output in order to change the output frequency of an upconverter or downconverter.
A LO may also be implemented using much more complex frequency synthesizers, which exhibit greater control and signal generation capability than a basic oscillator at the cost of complexity and size. LOs are often realized with additional circuitry other than an oscillator, as the resonator or other oscillator components are intrinsically unstable and compensation circuitry is generally necessary to achieve the desired signal quality. Temperature compensation is commonly used as most resonators and oscillator circuitry is susceptible to temperature dependent performance. Moreover, aging, shock, vibration, power supply variation, load variation, and other environmental factors also induce changes in resonator and oscillator performance and many LO designs include compensation and protection from these factors.
The goal for an LO is for it to deliver a pure sinusoidal waveform free of amplitude (noise) or phase fluctuations (phase noise). This means that the noise and phase noise parameters of an LO are of critical importance. Especially in modern wireless communications with extremely deep modulation schemes that are sensitive to phase noise, this parameter is becoming increasingly significant.
In Superheterodyne receivers, the noise sideband signal energy from the LO can mix with incoming signals outside of the IF passband and generate noise in the If passband. If this noise energy from an LO can then result in an increased noise floor of the receiver, and may also be high enough to drown out the desired receive signals. Hence, a superheterodyne receiver sensitive may be dependent on a LO’s noise performance. A similar case occurs with moving target indicator (MTI) radar, where the moving targets produce spectral components shifted away from the signal frequency of the strong clutter echos. Low velocity targets may be indistinguishable if the transmitter or receiver LOs are too noise, thus reducing the subclutter visibility of the MTI radar.
Given that an RF mixer requires an input signal at a certain power level for optimal operation, the output signal strength of an LO is also significant. In some cases the LO power level may not be high enough and a low noise amplifier (LNA) may be needed to gain the LO signal to reach acceptable LO input power levels for a given mixer.
Amplification of the LO signal may also be needed if a LO output filter is used to enhance the LO signal purity. As very high Q filters tend to exhibit high insertion loss, amplification of the LO signal may be needed to offset this loss. Unfortunately, introduction of a filter and amplifier in the LO output results in added noise, phase noise, and nonlinearities from the active elements of the amplifier which result in LO performance tradeoffs.
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