QUESTIONS:

In which way do the different material properties affect the electrical performance of my circuit?

ANSWER:

Selection of proper material for RF/microwave applications depends on many factors like dielectric constant, loss tangent, thickness and cost. Besides selecting a material with the right properties, the issue of tolerances also needs to be addressed. Normal variations in a material can cause a design to have poor yields if not taken into account. It is important to understand how each property can affect circuit performance.

The use of advanced CAD tools enables designers to fully understand the behavior of a circuit before spending time and money making the first prototypes. These tools have shortened the time to market of many commercial and military products. There are three properties that are quite important to a designer, including the control of these:

• dielectric constant
• loss tangent
• dielectric thickness

To understand how a design is affected, we will analyze a circuit and vary each property. Let’s take an edge coupled microstrip band pass filter, 1GHz with a 10% bandwidth on RT/duroid 6010LM 0.050” thick with ½ oz (17 micron) copper. The analysis was performed using HP-Eesof Touchstone circuit simulator. The material properties for this material are given in table 1 while figure 1 provides the analysis results.

Each property is then varied between the tolerance values while keeping the other two properties at nominal. It is important to note that this is worst case analysis since these properties are varying in a Gaussian distribution with an average and standard deviation. With this analysis we can understand how a band pass filter is affected by lot to lot and within sheet variations. Dielectric constant, loss tangent and thickness each affect the performance of the filter in different ways and the cause/effect analysis will assist when dealing with potential problems that may arise.

Figure 1.

Figure 2.

The first property to alter is the dielectric constant. Variations in dielectric constant affect the center frequency of the band pass filter by shifting it up for lower values of dielectric constant. The reason is that by shifting the dielectric constant of the material, one alters the electrical length of each of the segments. Figure 2 illustrates how the center frequency is shifted by 20 MHz when the dielectric constant changes by 0.5. Temperature variations can also change the dielectric constant of a material, some times as much as the overall tolerance itself when going from -55°C to 125°C. Selection of a material with low temperature coefficient of dielectric constant (TCK') is critical for stability in temperature varying environments. Materials with values less than 60 ppm/°C are considered to be stable.

Loss tangent changes affect the performance of the filter independent of frequency. As the loss tangent increases, so does the loss of the pass band. Figure 3 illustrates a drop of 0.2 dB when the loss tangent changes from 0.0020 to 0.0028. This type of change at times can also be attributed to either absorbed chemicals that were not removed properly during circuit board processing, to absorbed moisture due to environmental conditions when not using a low moisture material or to increased loss caused by the use of a solder mask over the entire surface of the circuit board or. Typically solder masks also serve the purpose of acting as a conformal coat for epoxy/glass boards due to their high water absorption. In the case of RT/duroid 6010LM (LM is Low Moisture), water absorption is quite low.

Figure 3.

Figure 4.

Thickness variation is sometimes overlooked when going through yield analysis. Thickness control manifests itself in a band pass filter as an overall change in the bandwidth. For thinner substrate, the bandwidth decreases. Figure 4 provides the result when the thickness is varied from 0.048” to 0.052”, a decrease of almost 25 MHz is noted in the bandwidth of the filter.

An evaluation of the different material properties and their variation is important when performing yield analysis of RF/microwave circuitry. It has been shown how the dielectric constant affects the center frequency, loss tangent the insertion loss of the pass band and the thickness the bandwidth of a microstrip filter. Use of CAD tools enable designers to go more rapid through the initial prototype stages by enabling sensitivity analysis to be done on a computer while giving better insight as to sources of circuit