Some customers testing EM3xx device transmit functionality over temperature observe transmit issues such as signal degradation and varying frequency offsets during large changes in temperature. This happens when the customer places the device in a continuous transmit mode, for example with standalone tx, txtone or txstream test modes using the nodetest application or manufacturing library based test commands in the customer's application, and then change the temperature without exiting the continuous transmit mode. This test method, where the device resides in a continuous transmit mode, does not reflect the device’s normal transmit behavior as it would function in a real application.
The reason for the customer seeing these performance issues is the device is not able to recalibrate the channel when left in this continuous transmit mode and recalibration is not triggered when the temperature changes. When a device is powered up for the first time after programming and it selects a channel, a calibration is run automatically for proper configuration of the device. This calibration occurs the first time each channel is selected (since default channel calibration token data exists for that channel in simulated EEPROM), as well as any time there is a change in temperature (when compared with the temperature at the time of the previous calibration event) which is large enough to trigger recalibration.
Silicon Labs recommends not to remain in a continuous transmit mode while changing temperature. We recommend instead to bring the unit to the desired temperature, start the continuous transmit mode, take the desired test measurement, exit / stop the transmit mode, move on to the next temperature set point, and repeat. This allows all channel calibration checks and events to occur normally, thus allowing transmit operation to function normally at all test temperatures.
Related application note: