Oscillators for microwave systems

Updated July 4, 2011

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As many amplifier designers will tell you, designing an oscillator is easy, there are infinite combinations of components that will oscillate when you don't want them to. But designing a useful oscillator with s stable and controllable output is another story indeed.

The conditions for oscillation are simple, the loop gain of a network has to be unity or more, and the total phase shift through the feedback loop must be an integer of 360 degrees. This type of feedback is known as positive, or regenerative feedback. In practice the feedback network usually has a resonance, which determines the output frequency.

Oscillators are a fundamental building block of almost every electronic system, analog or digital. Oscillators provide the signal source for all microwave systems, including both transmitters and receivers.

Here are some oscillator definitions:

Local oscillator (LO)
The signal source in used in a receiver to downconvert the received signal to an intermediate frequency. One of these days we will start a chapter on receivers!

Stable local oscillator (STALO)

Voltage controlled oscillator (VCO)
An oscillator that provides a signal whose frequency is controllable using an analog voltage signal.

Crystal oscillator (XO)
This topic already has it's own page!

Dielectric resonator oscillator (DRO)

Sweep oscillator
Generally a piece of test equipment that is able to sweep a CW signal across a specific band, either in a a frequency modulation mode. A good sweep oscillator uses a phase-locked loop to ensure that the signal frequency is accurate and stable. Such a device is often called a "synthesizer".

Here are some specifications you need to consider when procuring different types of oscillators:

Frequency accuracy
Expressed in parts per million, this tells you the worst-case error you can expect when you command your oscillator to a specific frequency. The 10 cent crystal in a cheap digital watch is accurate to 23 ppm if it loses one minute per month.

Stability
This is a measure of the frequency drift over time. As increasing number of users crowd the electromagnetic spectrum, the need for frequency stability becomes greater every year.

Aging

Short term stability

Warm-up time
Since many oscillators are so temperature dependent for accuracy often ovens are used to stay at a fixed temperature, some equipment that uses oscillators has a lengthy warm-up time. For example, it may take overnight for a network analyzer to stabilize after you plug it in, and this is the fault of the sweep oscillator.

Turn-on power
Especially with an oven-stabilized oscillator, the dissipation when you first throw the switch can be a lot higher than the

Harmonic signals
A harmonic frequency is any multiple of the intended frequency. A ten GHz oscillator may have 20 GHz, 30 GHz, 40 GHz... output signals at low power levels; these are the second, third and fourth harmonic.

Subharmonic signals
Subharmonic frequencies are signals that occur at 1/2, 1/3, 1/4... the intended signal.

Spurious signals
Spurious signals are seemingly not related to the fundamental output frequency, and can be either below or above it.