### Frequency errors

#### Frequency errors within a communications link are caused by two mechanisms – inaccuracy of the frequency sources used in the modulation and demodulation process, and Doppler shift caused by the relative motion between source and receiver or from moving reflectors. For modulation formats that use frequency or phase to represent different symbols, such as M-ary PSK or M-ary QAM, frequency errors can lead to significant performance degradation for the modem.