If your oscillator has poor power supply immunity or if it must work inside a noisy system, give it some extra power supply filtering. The amount of filtering required depends on how much a reduction in jitter you must achieve. Determining a precise value for required jitter reduction is almost impossible because all the parameters vary:
Nevertheless, you need to do something. Most manufacturers of clock sources recommend a circuit somewhat like the one in Figure 12.60, often without the resistor. Using the values and parasitic assumptions listed in Table 12.5 and the layout in Figure 12.60, this filter achieves better than to 20 dB of power supply noise reduction in the frequency band from 10 MHz to 1 GHz (10 9 Hz). Cascading two sections roughly doubles the attenuation. The purpose of the resistor is to preclude resonance at the frequency f C where the filter begins to function. That frequency f C , called the lowpass cutoff, is given by
Equation 12.19
Figure 12.60. This filter provides reasonable attenuation of power supply noise from 10 MHz to 1000 MHz.
Without the resistor R1 installed, the cutofffrequency resonance appears in Figure 12.61 at a frequency of 500 KHz. A reasonable value of R 1 that provides the right amount of damping to prevent such a resonance at f C is
Equation 12.20
Figure 12.61. The frequency response of a realistic filter is an amalgamation of compromises.
With this resistance installed, the loss slope of the filter is “20 dB per decade. One decade above f C you can expect an attenuation of roughly 20 dB.
Above 100 MHz the parasitic shunt capacitance of the inductor ( C L,SHUNT ) begins to short out the inductor. At the same frequency the parasitic series inductance of the capacitor ( L C,SERIES ) substantially degrades its performance. This is a general example of the principle that at high frequencies, components do the opposite of what you want (inductors turn into capacitors and vice versa). At 1 GHz the parasitic effects render the filter totally ineffective .
The frequency at which a onestage filter becomes totally degraded depends on the values of the parasitic elements. It is usually somewhere in the vicinity of this value:
Equation 12.21
where 
C L,SHUNT is the parasitic shunt capacitance of the inductor, F, and 
L C,SERIES is the parasitic series inductance of the network of capacitors, H. 
For the component values listed in Table 12.5 this resonance should appear at 1 GHz. As expected, a debilitating resonance appears at that frequency in Figure 12.61. The solution to this problem is to include a short trace on the downstream side of inductor L 1 . Provided the trace is laid out with its input and output well separated, it exhibits almost no endtoend shunt capacitance. This trick increases the series impedance of the overall L 1 structure, disrupting the parasitic resonance at 1 GHz.
Table 12.5. Parasitic Models for Components in Power Filter
Resistance 
Inductance 
Capacitance 
Circuit model 

L 1 
.1 
1 uH 
100 pF 

R 2 
2.2 
1 nH 
1 pF 

C 3 , C 4 
.1 
0.5 nH 
.047 uF 
Filters designed for wideband operation are built as a cascade of multiple sections, each section scaled to provide coverage in successively higher frequency bands, which is what has been done here. Before you attempt any type of advanced filter design, it is crucial that you measure and understand the parasitic parameters of your components. For inductors, the main parasitic elements are a shunt capacitance and a series resistance; for capacitors the main parasitic elements are a series inductance and a series resistance. These values are an integral part of any filter you build and must be included in your calculations.
When laying out any highfrequency filter, take care to keep the input and output well separated. All capacitors should preferably connect directly to a solid reference plane with large vias, at least 500 m m (20 mil) diameter. Keep all extraneous circuit traces as short as possible (less than 2.5 mm, or 0.1 in.), unless included intentionally as part of the circuit. Surfacemounted components, in the smallest size practicable, work best.
POINT TO REMEMBER
12.12.1 Healthy Power
POINT TO REMEMBER
12.12.2 Clean Power
POINT TO REMEMBER
Fundamentals
Transmission Line Parameters
Performance Regions
FrequencyDomain Modeling
Pcb (printedcircuit board) Traces
Differential Signaling
Generic BuildingCabling Standards
100Ohm Balanced TwistedPair Cabling
150Ohm STPA Cabling
Coaxial Cabling
FiberOptic Cabling
Clock Distribution
TimeDomain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. TwoPort Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )
Notes