The local oscillator produces a signal which is injected into the mixer along with the signal from the antenna in order to effectively change the antenna signal to a frequency which can be handled by the IF amplifier. Many radiotelescopes use a quartz crystal derived local oscillator signal. Quartz crystal oscillators are quite stable and drift little in frequency. Because most radiotelescopes are quite broadband in nature, a small amount of frequency drift in the local oscillator may be tolerable. One must be careful, however, that the drift is not so great that received signal frequency begins to infringe upon the bandpass of the antenna or any front end filters that precede the mixer. Also, it is possible to drift into an interfering signals frequency. When either of these things happen, it can result in a change in the output of the telescope which could be mis-interpreted as a real change in received cosmic noise.
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