Temperature Sensing Terminations

Temperature Sensing Terminations

Florida RF Labs Inc.
Stuart, FL

In normal operation, most terminations never see high power. They terminate circulators and couplers, which in normal operation transmit little or no power to the termination. Whenever power is being dissipated in a termination, it is usually a sign that something in the system has failed. Abnormal reflected signals or unbalanced power are directed to the termination to be absorbed and dissipated as heat energy.

In order to perform its function, a termination is required to be coupled to a heat sink capable of conducting away all of the termination's dissipated heat for as long as operation in the failure condition continues. If operation in the failure condition is indefinite, either bulky passive heat sinks or active cooling means (such as fans) are required, or continued operation in the failed condition may result in further damage. Clearly the way to minimize cooling requirements is to discontinue operation in a failure condition. Modern system designs employ some means of detecting the failure condition and throttling back or shutting down the source of high power RF.

There are two basic approaches to detecting abnormal operation: RF and non-RF. In active components, many, but not all, failure modes will cause a detectable shift in bias currents. Non-RF detection of anomalous bias conditions has the advantage of not requiring additional components in the RF path that could negatively impact SWR and transmission loss, but has the disadvantage of not being able to detect all RF failure modes. RF detection has been relatively expensive to implement and can introduce RF losses and discontinuities. RF detection requires extra coupled or circulated ports or high power attenuators in place of terminations to obtain a low level signal sample suitable for low power detectors. Obviously, monitoring the RF signal can detect all RF failure modes.

A new method of detection is now available, which offers all of the advantages of both RF and non-RF detection. Temperature sensing terminations combine high performance RF terminations and RF sensors in one package. Electrical specifications for these terminations are listed in Table 1 . The integral sensor provides a resistance to ground, which varies proportionally to the termination's film temperature. (The termination's film temperature rises with dissipated power.) Figure 1 shows the typical change in sensor resistance vs. termination input power. By biasing the sensor with an external fixed resistor, a DC voltage output proportional to the termination's film temperature is obtained. This DC voltage monitors power for systems where a large ambient temperature change does not occur. For systems where the ambient temperature does change, an optional ambient temperature bias resistor is available, which balances out the effects of changing temperature.

Designing in a temperature sensing termination is as easy as any standard termination. Temperature sensing terminations are available in common package styles from surface-mount chip to flange mount. Similar to a standard termination, only one port is RF; the sensor output does not require RF design considerations. Because couplers or high power attenuators are not required, a temperature sensing termination offers superior SWR performance over wider bandwidths. Figure 2 shows the termination's typical RF port SWR from DC to 2 GHz for wireless applications.

The sensor time constant is 100 ms (typ). In its simplest application when biased with 5 V through a 10 kW fixed resistor, a TTL level signal is obtained directly with a transition from a 1 to a 0 occurring at 250 ms (typ) for a 0 to 100 percent power step. By detecting the change in resistance as opposed to level detection, much faster detection times can be obtained. Figure 3 shows the sensor resistance time characteristic for a 0 to 100 percent input power step.

Temperature sensing terminations are the lowest cost, highest performance and easiest method to introduce RF fault detection or power monitoring into a system design. Reduced thermal requirements offer greater mechanical design flexibility. Industry-standard packages allow for incorporation into existing designs with minimal modification. Temperature sensing terminations are currently available with a delivery time of stock to four weeks.

Florida RF Labs Inc.,
Stuart, FL
(800) 544-5594.