Polyphase Sliding Goertzel Demodulator for Continuous Phase Frequency Modulated Signals

We describe the design of a new demodulator for continuous phase frequency modulated signals such as link-16 messages. A link-16 message consists of short RF pulses containing 32 ldquochipsrdquo each one having duration of 200 ns. One of the main difficulties in demodulation of such signals is the very short ldquochiprdquo period as compared to the frequency difference between the carrier and its´ modulation. For example in link-16, the difference in frequency between the carrier and its´ modulated signal is only 1.25 MHz. This makes traditional demodulation approaches, i.e., mixing the incoming signal with the carrier frequency and then low-pass filtering ineffective. We present a new method of demodulating such signals by digitizing the incoming signal in a superconducting delta-sigma modulator driving a 1 to 64 deserializer, then digitally filtering with a polyphase implementation of a sliding Goertzel filter. We have designed and fabricated a bandpass analog-to-digital converter (ADC) chip with an on chip deserializer, an additional room-temperature deserializer, and an FPGA polyphase implementation of sliding Goertzel filter. The results are presented for our experimental evaluation of link-16 message demodulation.