EMC Thermopad Attenuators: Temperature Variable RF Attenuation

This application note delves into the basics of EMC thermopad attenuators. We’ll explore how these devices function as temperature-variable RF attenuators. We’ll also touch on some of the prominent manufacturers of thermopads, including EMC.

The term “thermopad” refers to a passive device designed to exhibit variable resistance based on temperature changes within a circuit. They’re primarily available as surface-mount devices (SMD) due to the prevalence of surface-mountable PCB designs in the RF and microwave domains. Unlike voltage-variable or digitally controlled attenuators, thermopads operate without the need for external control signals. Their resistance adapts automatically to temperature fluctuations.

Figure-1: EMC thermopad attenuator

Figure 1 illustrates the layout of a typical thermopad (bottom, top, and side views). Ports 1 and 2 connect to the input and output sides of the circuit where attenuation is required. The GND port should be connected to ground.

When selecting an RF thermopad, consider the following technical specifications:

• Frequency Range
• Size
• Impedance (typically 50 Ohm)
• Tolerance in dB/dB/degreeC
• VSWR (Voltage Standing Wave Ratio)
• Power Rating
• Operating Temperature
• Substrate Material
• Resistive and Terminal Materials

Figure-2: EMC thermopad used as temperature variable attenuator in frequency converter design

EMC Thermopads in Amplifier Gain Compensation

EMC thermopads are frequently employed to compensate for variations in amplifier gain caused by temperature changes. They require no external control inputs to function. Instead, they sense the temperature and adjust their resistance accordingly. Both positive and negative temperature coefficient thermopads are commercially available. EMC-RFLabs offers thermopads with frequency ranges up to 36 GHz.

Consider, for example, an RF frequency converter module used in a C-band transceiver design. Suppose it’s designed for a nominal gain of 25dB. Due to the characteristics of GaAs amplifier devices and other components, the gain may fluctuate, deviating from the target 25dB across a temperature range of -50°C to +50°C.

Figure-3: Thermopad gain versus temperature graph

Gain variations over temperature are typically measured in environmental testing labs. Once these measurements are obtained, a suitable EMC thermopad value (e.g., 3 dB or 6 dB) can be integrated into the design as illustrated in Figure 2.

Many companies that previously used pi networks comprised of standard chip resistors in their RF converters have transitioned to using thermopads. This is because thermopads are easy to solder without significant layout modifications, provided the spacing was kept to a minimum.

Figure 3 shows example gain variation results without compensation and after compensation using RF thermopad attenuators. The results shown are for illustrative purposes only. Actual performance may vary.

RF Thermopad Manufacturers

Here are some of the well-known manufacturers of RF thermopads:

• EMC Technology/Florida RF Labs (www.emc-rflabs.com)
• RFMW Ltd. (www.rfmw.com)