TABLE OF CONTENTS Communications receivers are seldom single-channel devices, but more often cover wide frequency ranges. In the superheterodyne receiver, this is accomplished by mixing the input signal with an LO signal. The LO source must meet a variety of requirements:
It must have high spectral purity.
It must be agile so that it can move rapidly (jump) between frequencies in a time frame that can be as short as a few microseconds.
The increments in which frequencies can be selected must be small.
Frequency resolution between 1 and 100 Hz is generally adequate below 30 MHz; however, there are a number of systems that provide 0.001-Hz steps. At higher frequencies, resolution is generally greater than 1 kHz.
In most modern receivers, such a frequency source is typically a synthesizer that generates all individual frequencies as needed over the required frequency band. The modern synthesizer provides stable phase-coherent outputs. The frequencies are derived from a master standard, which can be a high-precision crystal oscillator, a secondary atomic standard (such as a rubidium gas cell), or a primary standard using a cesium atomic beam. The following characteristics must be specified for the synthesizer:
Frequency range
Frequency resolution
Frequency indication
Maximum frequency error
Settling time
Reference frequency
Output power
Harmonic distortion
SSB phase noise
Discrete spurs (spurious frequencies)
Wide-band noise
Control interface
Power consumption
Mechanical size
Environmental conditions
Free-running tunable oscillators, once used in radio...
