Abstract: By studying the operation principle and optical characteristic of TN type liquid crystal, an experi- mental metho~t of liquid crystal electrooptic modulation derived by square wave voltage signal is proposed. Using liquid crystal electrooptie effect experimental apparatus,the electrooptic curve and threshold voltage of twisted ne- matic liquid crystal （TN-LC） excited by the voltages with different frequency and wave types, are measured. The influence of the frequency and wave types on the electrooptic curve, threshold and gradient,is analyzed. On the ba- sis of these,the sine wave modulation is realized by adding 3 000 Hz square wave to TN type liquid crystal, and the result of the electrooptic modulation is investigated. This method of electrooptic modulation can be used to the audio -visual display of electric signal response property of liquid crystal materials.

Key words: twisted nematic liquid crystal （TN-LC）, electrooptic effect, threshold voltage, electrooptic amplitude modulation