Today’s design engineers must deal with a variety of complexities.  Mixed technologies, analog systems, digital circuits, all integrated together.  Take these technologies design them to work harmoniously in a smaller  space, using smaller components and higher levels of integration, then you  have a real challenge, one reason for increased Starbuck sales.

With higher circuit complexities, more mixed signals, in closer proximities,  the signal to noise ratio becomes critical.  The higher the frequencies, the more critical it is to maintain low noise levels.  Only by correct bypassing, adequate shielding, and  decoupling where required, will circuits, analog and digital function properly.  “Bypassing” is the only way to insure your power distribution is providing a clean power source and maintain a critical low impedance in the power source.  The stray inductances must be eliminated as much as possible.  The easiest way to do this is by using capacitors as bypassing elements.

Oscillators are a unique component and perform a critical function in today’s circuits.  The primary function of an oscillator is to provide a stable “clean” signal.  Anytime, however you generate a signal, you are also generating noise.  Oscillators, like most other active components require properly designed bypassing in order to function correctly in a circuit. 

Circuit startup is a critical operation, correct startup of an oscillator is typically not an issue for the design where the oscillator is just one of the hundreds of components which must start and function correctly. Oscillator startup however requires several physical and electrical properties to take place in order to start on the correct frequency.  If the oscillator is not protected from spikes and circuit noise typically found during the startup, the oscillators reliability can be compromised.  In order for the oscillator to start  correctly, the oscillator generates it own “noise” or frequency to be able to start the quarts crystal in the oscillator.  The lack of bypassing on the oscillator’s power line  can affect the oscillators starting process. 

Bypass capacitor selection and placement of the bypass capacitor(s) requires knowledge of the complete circuit’s operation, the number of power supplies, length of circuit traces, noise sources, the highest frequencies generated in the circuits, and the dielectric of the bypass capacitor are some of the factors which must be taken into account.  The value of the bypass capacitor is not as critical as the series inductance value the capacitor might add to the circuit.  Also selecting a capacitor too large in value can add not only unwanted inductance to the circuit, but also have a negative affect on the total system impedance.

Unless most of the above parameters are forwarded to MMD’s application engineers, MMD cannot define the exact bypass capacitor required for every oscillator application.  We can state however, as with other integrated circuits, oscillators must be provided with bypassing on the oscillators power line to function per published specifications.

Lothar Baum
V.P. Engineering / Quality
MMD / Monitor Products