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Thermopad | temperature variable attenuator

This application note covers RF thermopad basics used as temperature variable attenuator.It mentions thermopad manufacturers and RF thermopad application used in RF frequency converter design.

The term thermopad is used for the passive device which is used to have variable resistance based on temperature change in the circuit design. It is mainly available as surface mount device due to the need in surface mountable PCB designs in RF and microwave domain.

Unlike voltage variable and digitally controlled attenuators, thermopads do not require any control inputs. The resistance varies automatically based on temperature change.

RF thermopad-temperature variable attenuator

Figure-1 depicts one such thermopad layout(bottom, top, side). The 1 and 2 positions are connected on input and output sides of the place where in required value of attenuation is needed. The GND position will be grounded.

Following technical specifications need to consider while selecting the RF thermopad:
• Frequency Range
• Size
• Impedance (50 Ohm)
• Tolerance in dB/dB/degreeC
• Power rating
• Operating temperature
• Substrate used
• Resistive and terminal materials used

RF thermopad application note

thermopad based RF frequency converter

EMC Technology's RF thermopads are used as compensation of amplifier gain variation due to temperature. It does not require any control inputs for the variation to occur. It senses the temperature and accordingly resistance will vary. Both positive and negative temperature coefficient thermopads are available. Thermopads upto 36GHz of frequency range are available from EMC-RFLabs.

Let us take example of RF frequency converter module used in C band transceiver design. Let us assume that it has been designed for 25dB of nominal gain. Due to GaAs amplifier devices and other devices gain varies and will not provide constant gain of 25dB from -50 degreeC to +50 degreeC temperature range.

RF thermopad temperature vs gain curve result

Gain variation over temperature is recorded in the environment labs. After the results have been obtained suitable value of thermopads(3dB or 6dB) are used in the design line up as shown in figure-2.

Many companies who have developed the RF converter using normal chip resistor based pi networks earlier have replaced the pi networks with thermopads. This is because thermopads are easy to solder without changing the layout provided spacing was kept to the minimal.

Figure-3 depicts the gain variation results without compensation and as obtained after compensation using thermopads. The results have been used in the application note for explaination purpose. The actual results might vary.

RF Thermopad Manufacturers

Following are the popular thermopad manufacturers:
• EMC Technology/Florida RF Labs (www.emc-rflabs.com)
• RFMW Ltd. (www.rfmw.com)

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