Oscillators provide the timing pulse for measurement instrumentation, military weaponry, telecommunication systems and a vast array of consumer electronics. Unfortunately, like many electronic devices, oscillators may be adversely affected by noise on the power supply line. The resulting frequency instability may render a system incapable of meeting desired performance parameters. Consequently, the ability of an oscillator to reject supply noise is an important consideration during the oscillator and system design phase.

In most instances, noise is unwanted energy that interferes with the proper operation of a system. The voltage applied to an oscillator's input pins is typically DC, but has a combination of noise from the power supply source as well as superimposed interference from the power distribution lines. The offending noise is classified as either broadband or narrowband and stochastic or deterministic. All combinations of these may be present in an oscillator's operating environment. Narrowband noise sources include switching power supplies; rectification (60 Hz and 120 Hz components); CRT monitors; and even other nearby oscillators. Linear power supplies produce a combination of random noise and line supply-related spurious signals.

It is not uncommon to find a phase noise or spurious signal level requirement on an oscillator source control drawing. On the other hand, it is a bit unusual to find any mention of the anticipated supply line ripple the end use system will generate, so it may very well be an often overlooked parameter.

Equations 1 and 2 represent a first-order approximation commonly used to analyze the voltage ripple sensitivity of an oscillator. Remember that these equations are valid only when the small angle condition is met, i.e.: a small modulation index.

Sensitivity (Hz/ volt):

Sideband (dBc):

Where:

• L(f_m) is the spur level of the modulated signal in dBc
• S is the sensitivity in Hz/volt
• f_m is the modulation frequency in Hz
• Vpeak is the peak amplitude in volts of the modulating signal

Unless a regulator is used to power the internal oscillator circuitry, expect to see healthy sidebands show up during ripple measurements. Even with internal regulation, substantial sidebands can be present. For instance a 10 MHz oscillator with a sensitivity of 1 ppb/volt will output a -108 dBc sideband when a 50 mV peak, 60 Hz sinusoidal signal is impressed on the supply line.

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