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July 2016 • The PCB Magazine 31 ness (H1) and microstrip line widths (W1) were measured using a pneumatic gauge and an opti- cal comparator respectively. Additionally, after the impedance measurements were made, each line was cut and a cross section was taken to ver- ify the dielectric thickness (H1), microstrip line thickness (T1), and microstrip line width on top (W2) and bottom (W1) of the microstrip. This provided the actual values needed to accurately calculate ɛ r for the circuit board materials. Utilizing a TDR, impedances were measured for all six samples from both directions for each material. This yielded 12 impedance measure- ments for three line widths and two lengths. Figure 3 illustrates an example (Sample A) of the TDR impedance output versus propagation time. This figure demonstrates one of the nuances to using this impedance extraction method. The TDR provides impedance measurements versus time. Subsequently, the impedance of test ca- bles and connectors must be considered when choosing at which point in time to measure impedance. As time directly relates to distance, a time should be chosen where the impedance being measured is somewhere within the trans- mission line and away from the connectors. Ad- ditionally, this point in time must be identical for all lines measured. The 12 impedances for each circuit board material were measured and plotted versus measured line width. A linear regression func- tion was developed to allow for calculation of a microstrip line with 50 Ohm characteristic impedance. This set of tests yielded impedance, dielectric thickness, microstrip line thickness, and microstrip line widths for each test sample. The measured characteristics were then used in a commercial field solver in order to back-calcu- late the ɛ r . There are many calculators and field solvers available and most are suitable for this calculation. Group Delay Extraction of ɛ r from Phase of Microstrips Microstrip transmission lines are a type of quasi transverse electromagnetic (TEM) struc- ture [6] . Since the electromagnetic field propa- gates in media with different relative permittiv- ity below and above the signal, the structure is inherently dispersive. The rate at which a pulse of energy traverses a transmission line is called group velocity (ν g ). For dispersive transmission Figure 3: Time domain reflectometry (TDR), impedance versus time. ROUND ROBIN OF HIGH-FREQUENCY TEST METHODS BY IPC-D24C TASK GROUP (PART 1)