Issue link: https://iconnect007.uberflip.com/i/701250
July 2016 • The PCB Magazine 33 ROUND ROBIN OF HIGH-FREQUENCY TEST METHODS BY IPC-D24C TASK GROUP (PART 1) material's ɛ r . Unlike other transmission line structures, half of the microstrip is exposed to free space with ɛ r equal to that of air while the other half is in the circuit board material. The following equation is used to calculate ɛ r using the effective dielectric constant when the closed form analytical expression fol- lows; where and The result is a calculated normal ɛ r for the circuit board material at all frequencies of inter- est [8] . Unfortunately, this method for measuring normal permittivity does not directly yield a value for tan δ. Microstrip Differential Phase Length ɛ r The microstrip differential phase length method is straight forward and (as the name im- plies) is the measurement of electrical phase dif- ferences for two microstrip transmission lines [9] . This method provides normal ɛ r but does not directly provide normal tan δ. The measure- ment method requires only a VNA and a series of 50 Ohm microstrips of variable length. Two lengths of microstrip were used for the measurements in this paper, one of six inches and the other of two inches; both were etched in close proximity to each other on the same panel to minimize geometric variations. Figure 5 illustrates the microstrip design and lengths used for this test. In general, one of the microstrip lines should be measured using the VNA at a specific fre- quency. The phase angle of the energy traveling through the microstrip should be recorded. The other microstrip of a different physical length should then be measured and the phase angle of the energy should also be recorded. Calcula- tion of the effective dielectric constant is simple using the following equations: where and Once the effective dielectric constant is cal- culated, ɛ r is found by substituting the effective dielectric constant back into the equations un- der the group delay method, or by using a com- mercial transmission line calculator. Figure 5: Microstrip design for differential phase length measurement.