Lecture 16 In this lecture we will discuss comparisons between theoretical predictions of radiation and actual measurements. The experiments consider simple configurations to test the validity of the prediction model.

Experiment #1 74LS04 10 MHz Oscillator. 50 mils #28 Gauge 1 meter of 3-wire ribbon cable + 5V This system was placed inside a semianechoic chamber to measure radiation in the frequency range of MHz. The measuring antenna was biconical. The setup of the semianechoic chamber is shown in the following. 1m 3m 1m reflected 1m ground plane direct ray antenna Ribbon cable and current probe image Figure 1 schematic of the device tested Figure 2 physical dimensions of the measurement site inverter gate

A current probe was placed at the midpoint of the cable to measure the common-mode current. The probe was connected to a spectrum analyzer and, as previously seen, the relationship between the current and the measured voltage is: (1) The electric field prediction needs to account for the ground reflection, so by introducing a correction factor we have: (2) Combining equations (1) and (2) gives: (3)

The comparison between the predicted radiated fields computed using equation (3) and the measurements is shown here: STD Figure3 Measured and predicted emissions of the device of Fig. 1 (source: Paul)

The theoretical values are shown with an X and the agreement is within 3dB except at 50MHz, 80MHz and 130MHz. If one removes the active load 74LS04 at the right of Fig. 1, the common-mode currents do not substantially change and this result is shown below: Figure 4 Measured and predicted emissions of the device of Fig. 1 (source: Paul)

The conclusion from the discussion about the previous experiment is that common-mode currents are usually the dominant radiation mechanism for long cable. Common-mode current also constitute a dominant radiation mechanism for conductor lands on a PCB. Let us consider the following configuration: 10 MHz Oscillator glass epoxy,  r =4.7 25mils 380 mils 62 mils 6 inches 14 pin DIP oscillator Figure 5 device schematic and the PCB cross-sectional dimensions This configuration is considerably small and symmetric and one way be tempted to think that common-mode currents should not exist.

The radiated emissions are compared with the theoretical predictions in the following: Figure 6 Measured and predicted emissions of the device of Fig. 5 It is apparent that the common-mode currents provide the dominant contribution to the radiated emissions. Again, if one removes the 330  load of Fig. 5 the common-mode currents are practically unchanged.

So far we have considered radiation models for wires and PCB lands. These models help us evaluate the fields that are created when these elements carry common-mode and differential-mode currents. Another important consideration, from an EMC standpoint, is the prediction of the effect of an already existing field onto a pair of wires or PCB lands. Hence, we are ready to start discussing issue of susceptibility models for wires and PCB lands. We consider the following two-conductor line problem: S Figure 7 two-conductor line model

This model applies to any two-conductor transmission line as long as the appropriate per-unit-length parameters are used. The goal is to predict the terminal voltages and once the incident plane were is known. The component of transverse to the plane of the wires induces an emf that can be viewed as an inducted voltage source of strength: The component of transverse to the wires and directed along creates an induced current source given by: (4) (5)

The incident fields nearby the line are related to their source by the Friis formula: and (6) (7) Using a distributed parameter model, We can write: (8) Fig. 8

Therefore, we obtain the transmission line equation: (10) In the special case of an electrically short line if we neglect the per-unit- length inductance and capacitance we obtain: Hence, equations of (7) simplify into: (9) Fig. 9

This simple model applies to electrically short lines where the termination impedance does not differ significantly from the characteristic impedance of the line. By introducing in this model the per-unit-length capacitance and inductance one can relax the conditions on the termination impedance, while keeping the assumption of electrically short line. (11)