Rules of thumb I have known and loved: while no substitute for good design practices, in certain situations they can be useful.

A RULE OF thumb is a simple relationship that allows us to effortlessly "put in the numbers" at the starting point of an estimate. What we gain in simplicity, we lose in accuracy. Rules of thumb can help feed our intuition, but they can't replace good design work; to wit, one should never use a rule of thumb to sign off on a design.

In my new book, Signal Integrity: Simplified, I have collected 100 useful rules of thumb. Here is a short sampling of rules of thumb, in no particular order.

1. The rise time of a signal is ~ 10% the clock period and ~ 1/10 x [1/F.sub.clock]. For example, a 100 MHz clock has a rise time of about 1 nsec.

2. The bandwidth, BW, and rise time, RT, of a signal are related by BW = 0.35/RT. For example, if the rise time is 1 nsec, the bandwidth is 350 MHz. If the bandwidth of an interconnect is 3 GHz, the shortest rise time it can transmit is about 0.1 nsec.

3. The ESL (equivalent series inductance) of an axial lead resistor is about 8 nH. The ESL of an SMT resistor is about 1.5 nH.

4. The sheet resistance of 1 oz. copper is about 0.5 milliohms/square. For example, a trace 0.005" wide and 1" long has 200 squares and would have a series resistance of 200 x 0.5 = 100 milliohms = 0.1[ohm]

5. The capacitance available in the power and ground planes of a typical FR-4 circuit board, when the separation is 0.010", is 100 pF/sq. in., and this scales inversely with the dielectric thickness. For example, the total board area available for decoupling an ASIC may be only 4 [in.sup.2]. If the plane-to-plane separation is 0.010v, the capacitance in the planes available for decoupling would be 0.4 nF. For a 1 W chip, it would provide decoupling for about 0.2 nsec.

6. The loop self inductance of a round loop 1" in diameter, made from 0.010" thick wire, is 85 nH.

7. The speed of a signal in air is about 12"/nsec. The speed of a signal in most polymer materials is about 6"/nsec.

8. The wiring delay, 1/v, in most laminates is about 170 psec/in.

9. A 50[ohms] microstrip in FR-4 has a dielectric thickness of about half the line width of the trace. For example, if the line width is 0.010", the dielectric thickness will be 0.005".

10. If the width of the return path in a 50[ohms] microstrip is at least three times the line width, the characteristic impedance is within 1% of the characteristic impedance when the return path is infinitely wide.

11. An axial lead resistor with a loop inductance of about 10 nH may contribute too much reflection noise for rise times of less than 1 nsec. In this case, switch to surface mount resistors.

12 For lines 0.003" or wider, the low loss regime is all frequencies above about 10 MHz. In the low loss regime, the characteristic impedance and signal speed are independent of loss and frequency. There is no dispersion due to loss in typical board level interconnects.

13 The shortest rise time that can be propagated by an FR-4 interconnect is 10 psec/in, x Len. For example, a signal propagating down a 10" length of 50[ohms] line in FR-4 will have a rise time of at least 100 psec.

14. The near-end crosstalk, for 50[ohms] microstrip traces with line width equal to the spacing, is 5%.

15. In a 50[ohms] bus, stripline or microstrip, to keep the worst-case near-end noise below 5%, keep the spacing between the lines more than twice the line width.

16. To keep the switching noise below an acceptable level, keep the mutual inductance in nH below 2.5 times the rise time in nsec. For example, if the rise time is 0.5 nsec, the mutual inductance should be less than 1.3 nH for acceptable switching noise crosstalk, due to coupling between the only two signal-return path pairs.

17. In the tightest coupled differential microstrip, with a spacing equal to the line width, the differential impedance drops only about 10% from the differential impedance when the traces are far apart with no coupling.

DR. ERIC BOGATIN is vice president and CTO of GigaTest Labs (gigatest.com). His new hook, Signal Integrity--Simplified, is available now. Bogatin can be reached at [e-mail].