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RF MODULE

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EIGENFREQUENCY VS. TEMPERATURE<br />

By repeating the structural and electromagnetic analyses for a number of operating<br />

temperatures, an eigenfrequency-versus-temperature curve is obtained. The results for<br />

two different designs appear in Figure 3-13.<br />

Figure 3-13: Eigenfrequency (Hz) versus temperature (K) for two different designs.<br />

For the first design, the entire filter is made of brass whereas for the second design the<br />

post is made of steel. It is obvious that the combination of steel and brass is superior<br />

to a design using brass alone. The reason is the reduced capacitive coupling between<br />

the top of the post and the nearby face of the box, which results from the different<br />

coefficients of thermal expansion for the two materials. This coupling has a strong<br />

influence on the resonant frequency and, when reduced, it counteracts the effect of an<br />

overall increase in cavity size.<br />

Model Library path: <strong>RF</strong>_Module/<strong>RF</strong>_and_Microwave_Engineering/mw_filter<br />

THERMAL DRIFT IN A MICROWAVE FILTER | 99

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