low noise oscillator design

Posted on Feb 6, 2014

The overall performance of any rf circuit in regards of it`s phase noise and spectral purity depends significantly on the frequency generating circuitry, commonly an oscillator. Low noise oscillators with high spectral purity are neither witchcraft nor black magic. Many people think the design of oscillators is black magic as such circuits usually

low noise oscillator design
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either work or not. Especially beginners are glad when they finally get a circuit to oscillate and attributes such as signal quality are usually secondary and often neglected. This article is going to show two easy tricks that can be implemented in almost any oscillator design. To be precise, the oscillator is a Miller crystal oscillator. In case you are wondering where the LC tank circuit in the collector path of T1 is, look closer! The inductor L1 has an unavoidable capacitance known as parasitic capacitance` or stray capacitance`. I personally prefer the term parasitic capacitance because the term stresses that the capacitance is a (usually) unwanted phenomena. Parasitic capacitance is the reason why inductors have a self-resonance frequency (SRF) mentioned in their datasheet. The first step is to improve the power supply. Usually circuits in the amateur radio community use some sort of linear voltage regulator to stabilize the supply voltage to a fixed voltage. Linear voltage regulators have the unfortunate habit of introducing a significant amount of noise into the system. [ 1 ] It should be apparent that a high noise floor in the supply voltage has a direct influence on the output signal of an oscillator. The noise directly modulates the oscillator circuit and produces plenty of phase noise. Luckily, there`s an easy remedy for this problem; a low pass filter. A simple RC low pass filter, consisting of R1 and C3, on...

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