AN2336 - PSoC® 1 - Simplified FSK Detection

AN2336 presents the standard correlator method for frequency shift keying (FSK) demodulation using an efficient almost all-hardware implementation in PSoC® 1. This signal processing technique is readily applicable to caller ID and several modem standards. The associated project makes extensive use of switched-capacitor block features unique to PSoC 1 and that is not available in PSoC 3 and PSoC 5. For PSoC 3 and PSoC 5, see AN60594.

Frequency shift keying (FSK) is the modulation cornerstone of a number of digital data transmission systems. The transmit signal is fairly easy to generate, AN67391 discuss the implementation in detail. Receive processing is tolerant of a wide range of signals and relatively immune to a large class of interfering signals.

This Application Note outlines a simple, almost all-hardware method for detecting FSK signals using the analog signal processing capabilities of the PSoC 1 device. It describes circuits and signal processing techniques for demodulation. This signal processing technique is readily applicable to caller ID and several modem standards but it is not a complete modem; it does not include the phone line or other interfaces; it also does not include the data processing code.

A separate project is included and outlined in the Appendix A to simplify testing of the demodulator.

In order to be able to understand the implementation, following pre-readings are suggested:

AN2041: Understanding switched capacitor blocks – Explains the operation of switched capacitor blocks, and provides practical examples for their use.

AN2168: Understanding switched capacitor filters - Discusses how low pass, band pass and notch filters can be implemented using switched capacitor blocks.

KB Article: Implementation of a full wave rectifier and low pass filtering using all hardware technique.