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The local oscillator must produce a stable frequency with low harmonics. [ 1 ] Stability must take into account temperature, voltage, and mechanical drift as factors. The oscillator must produce enough output power to effectively drive subsequent stages of circuitry, such as mixers or frequency multipliers.
In optical interferometry, homodyne signifies that the reference radiation (i.e. the local oscillator) is derived from the same source as the signal before the modulating process. For example, in a laser scattering measurement, the laser beam is split into two parts. One is the local oscillator and the other is sent to the system to be probed.
A carrier recovery system is a circuit used to estimate and compensate for frequency and phase differences between a received signal's carrier wave and the receiver's local oscillator for the purpose of coherent demodulation.
An example is Yaesu's FRG-7 communications receiver, [5] which uses the system to remove local oscillator drift. The Racal RA17 and Realistic DX-302 [6] also used the Wadley Loop in their design. An optical implementation of a Wadley Loop has recently been proposed. This allows a compact relatively unstable laser to be used as a local oscillator.
Fessenden's receiver did not see much application because of its local oscillator's stability problem. A stable yet inexpensive local oscillator was not available until Lee de Forest invented the triode vacuum tube oscillator. [8] In a 1905 patent, Fessenden stated that the frequency stability of his local oscillator was one part per thousand. [9]
A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. [1] [2] [3] The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers.
The Dick effect (hereinafter; "the effect") is an important limitation to frequency stability for modern atomic clocks such as atomic fountains and optical lattice clocks.It is an aliasing effect: High frequency noise in a required local oscillator (LO) is aliased (heterodyned) to near zero frequency by a periodic interrogation process that locks the frequency of the LO to that of the atoms.
Quartz crystal resonators are many orders of magnitude more stable than LC circuits and when used to control the frequency of the local oscillator offer adequate stability to keep a receiver in tune. However the resonant frequency of a crystal is determined by its dimensions and cannot be varied to tune the receiver to different frequencies.