Circuit for spread spectrum transmission and method thereof

A spread spectrum transmission circuit comprising:

• a phase locked loop, which is composed of a filter and the filter is a capacitor, wherein the phase locked loop generates a series of incremental control signals and decreasing control signals based on the frequency difference and phase difference between a reference clock signal and a feedback signal, charges and discharge the incremental control signals and the decreasing control signals to generate a control voltage and generate a clock signal based on variations of the control voltage;
• a frequency locked loop to receive the reference clock signal and output frequency locked output signal and a ready signal;
• an amplitude locked loop connected to outputs of the frequency locked loop and the phase locked loop and receiving the ready signal to activate the amplitude locked loop and receiving the reference clock signal and the clock signal, the amplitude locked loop includes a locked amplitude loop control voltage to output an amplitude locked output signal;
• a digital-analog converter connected to the frequency locked loop to receive the frequency locked output signal;
• an injection current source to modulate amplitude of the locked amplitude loop control voltage and output an injection current;
• an extraction current source to adjust the amplitude of the locked amplitude loop control voltage of the locked amplitude loop and output an extraction current;
• a multiplexer connected to the locked phase loop, the locked amplitude loop, the digital-analog converter, the injection current source and the extraction current source and to receive the control voltage, the control current, the locked amplitude output signal, the injection current and the extraction current, wherein the control voltage is outputted to the locked phase loop;
• and
• a rail-to-rail digital signal generator having an input connected to the multiplexer and an output connected to inputs of the locked frequency loop and the locked amplitude loop, wherein the rail-to-rail digital signal generator converts the small signal of the control voltage to rail-to-rail digital signal and transmits the converted control voltage to the locked frequency loop and the locked amplitude loop to allow the locked frequency loop to estimate frequency of the control voltage.