Data Interface Circuits for PSK Modulators for Data Transmission from Space to Ground

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  • Author: Jolie. R and D.V. Ramana
A PSK modulator is used to modulate a high frequency carrier. The carrier frequency should be in the band allotted for data transmission (20 MHz in S-band and 375MHz in X-band), from space to ground. The PSK modulator used is either BPSK or QPSK, depending on the data throughput. In future, it is also envisaged to use an 8-PSK modulator for higher data rate transmission. The payload data from an on-board satellite camera is processed and fed to the modulator. Various logic interfaces such as TTL, RS422 and LVDS can be used. The payload data needs to be converted from the chosen logic level to bipolar levels required to switch the diodes in the modulator. The data interface circuit for the modulator needs to be designed carefully since the data levels control all the characteristics of the modulator. The main characteristics of a modulator include carrier suppression, phase imbalance and amplitude imbalance. Each characteristic of the modulator is dependent on its drive levels, i.e., the level of data input. The interface circuit can be tuned to get optimized modulator performance. The number of streams of data depends on the PSK modulation scheme used. In this paper, various modulator data interface circuits such as TTL, RS-422 and LVDS, which are used in Indian Remote Sensing Satellites, are discussed.