Development of a Digital Potentiometer Circuit for Digital Compensation of Frequency and Temperature Variations of Kvco to Provide Reprogramming of the Transmitter RF Center Frequency in the Field

Abstract

Cobham Electronic Systems, Inc. has developed a digital potentiometer circuit to allow for digital compensation of frequency and temperature variations in the VCO/PLL frequency control loop of a telemetry transmitter. The ability to reprogram the RF center frequency of a telemetry transmitter is a useful feature and is required on many telemetry programs. When setting the frequency modulation deviation (FM Modulation Index) of a telemetry transmitter, the exact setting will change with RF center frequency due to the variation of the transfer function of the VCO (Kvco). Typically, a resistor divider is used to set the frequency modulation deviation level by setting the output data signal amplitude. However, since Kvco varies with respect to RF center frequency, a method of adjusting frequency modulation deviation for each frequency setting is required. The shunt resistor in the resistor divider is replaced with a digital potentiometer to provide the necessary adjustment, using the on-board microprocessor to store a look-up table of settings versus frequency. A key feature of the digital potentiometer circuit is a method to increase the frequency bandwidth of the potentiometer. Digital potentiometers typically have frequency bandwidths measured in kiloHertz to MegaHertz, which limits their use in setting the frequency modulation deviation of high data rate telemetry transmitters. The circuit consists of a 256 position digital potentiometer and several resistors that are used to adjust the slope of the resistance vs. digital code curve and to translate the curve up and down along the Y-Axis. Adding external resistors to the digital potentiometer helps to increase the frequency bandwidth of the digital potentiometer. The selection of the maximum resistance range of the digital potentiometer is also important, as the potentiometer bandwidth is greater when a small portion of the total resistance is used. This paper will explore various methods of increasing the effective bandwidth of a digital potentiometer, with the goal of making them suitable for use in dynamically setting the frequency modulation deviation via digital control.