Patent ID: 7792209

Claim:
An aircraft transponder comprising: an antenna for receiving a radio frequency (RF) signal including a carrier wave modulated with a data component using differential phase-shift-keying (DPSK) modulation; a downconverter coupled to the antenna for downconverting the RF signal to an intermediate frequency (IF) signal; an analog-to-digital converter coupled to the downconverter to undersample the IF signal at ⅘ of a frequency of the IF signal to produce a digital signal; a first plurality of delays coupled to an output of the analog-to-digital converter, the first plurality of delays configured to generate a delayed output signal based upon the digital signal received from the analog-to-digital converter; a multiplier configured to receive the output of the analog-to-digital converter and to receive the delayed output signal from the first plurality of delays, and configured to output a multiplied output signal corresponding to a product of the output of the analog-to-digital converter and the delay output signal from the first plurality of delays; a second plurality of delays configured to receive the multiplied output signal from the multiplier, wherein each of the second plurality of delays are connected in series; an averager configured to receive an output from each of the second plurality of delays, and configured to output a filtered output signal, wherein the filtered output signal is defined by one of a positive sign and a negative sign; and a sign converter configured to receive the filtered output signal from the averager, and configured to output a demodulated DPSK signal defined by one of a one and a zero based on the sign of the filtered output signal, wherein the demodulated signal is used to recover the data component.