用DSM调制1-bit输出，经简单模拟滤波电路，产生VGA控制电压。对于内部频率高、但VGA变化速度较慢的场合，可以达到很高的控制精度，从而省却了内部高精度DAC的模块-DSM modulation with 1-bit output, through a simple analog filter circuit, producing VGA control voltage. in the circumstance of high internal frequency, but need a slower　changing VGA control, this program can achieve high control precision, and thus save the internal high-accuracy DAC module