LOCAL OSCILLATOR

Transmitters and Receivers Glossary

Transmitters and Receivers — Local Oscillator

 

 

 

 

 

 

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Information on Local Oscillator

When the receiver is required to tune across a frequency band, the local oscillator (LO) must be capable of tuning to be offset by the IF away any frequency in the band. The ability of a receiver to remain tuned to a particular frequency depends upon the frequency stability of its local oscillator (LO) (as well as the LO of the transmitter) because any variation in the LO frequency will cause a corresponding shift in the frequency of the output signal. In an AM broadcast receiver the requirement for frequency stability is not very stringent since the receiver is tuned by ear. An ISB/SSB receiver should be stable enough in frequency to ensure that the receiver will not drift from its nominal value by more than about 20 Hz.
     Receivers operating at one or more fixed frequencies can use a crystal oscillator with frequency changes accomplished by switching between crystals. When a receiver is to be able to be tuned over a band of frequencies, frequency synthesis is normally used.
     Ideally the local oscillator (LO) should have a stable frequency output; should provide the mixer with an output that has an amplitude greater than the input RF signal (mixing is a non-linear process that requires the mixing semiconductors to be driven into their non-linear regions—since the RF signal is too weak to do this, the local oscillator (LO) signal must have a sufficient amplitude); should provide a constant amplitude across the frequency range; and should produce minimum radiation of spurious frequencies.


Other topics in our resources on Transmitters and Receivers related to Local Oscillator include: 
 
  • crystal oscillator
    		•
  • frequency synthesis.

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