ACCURATE COMPENSATION OF INSERTION LOSS DRIFT DUE TO HUMIDITY CHANGES
Publication Date: 2011-Feb-01
The IP.com Prior Art Database
The present disclosure relates to humidity compensation for RF circuitry implemented on printed circuit boards (PCB) in electronic equipment
Brian Wade Carlson, Christopher B Jones, James B summers
The present disclosure relates to humidity compensation for RF circuitry implemented on printed circuit boards (PCB) in electronic equipment.
Changes in environmental conditions such as humidity and temperature vary the dielectric constant and dissipation factor of the substrate of a PCB. These variations in the substrate’s characteristics change insertion loss, which may alter the amount of power of an RF signal that is propagated through the circuitry and may adversely affect the operation of the system in which the PCB is included. Using a humidity sensor to measure the humidity in the environment around the substrate of the PCB to compensate for the change in power level would not be accurate, because it may not accurately represent the moisture absorbed by the substrate of the PCB, particularly when the humidity is changing and the absorption rate of moisture in the substrate of the PCB is different from the sensor. The accuracy of this approach would be further diminished by the correlation required between the humidity sensor and the insertion loss of the PCB.
An approach to mitigate these inaccuracies is shown in FIG. 1. It includes a capacitive sensor 112 that is integrated into the PCB 100. The PCB 100 includes a micro-strip RF transmission line that requires compensation. The RF transmission line includes an RF trace 102, the substrate 106 of the PCB 100 and a ground plane 108. In this example, capacitive sensor 112 includes a copper trace 104 on the same PCB substrate 106 the same ground plane 108. A capacitor 110 (shown schematically) is formed between copper trace 104 and ground plane 108, with the substrate 106 serving as the dielectric for the capacitor 110.
When the substrate 106 of the PCB 100 absorbs moisture from the environment, the dielectric constant of substrate 106 changes, affecting both the insertion loss of the RF transmission line and the capacitive sensor 112. The capacitive sensor 112 is then connected to a high resolution capacitance-to-digital converter (CDC, not shown), such as the Analog Devices AD7747. The CDC’s serial interface is connected to a microprocessor (not shown), which estimates the change in insertion loss using an estimation algorithm that may include a look-up table of theoretical and/or empirical data that could provide varying results depending on other factors including center frequency, target power and temperature, the latter also provided by a temperature sensor in the CDC. The microprocessor then either modifies the power control set point of the source or applies correction factors to the received data, whichever being applicable for the RF circuitry being compensated. The microprocessor can then re-measure the capacitance reported by the CDC on a period...