Simple CMOS FSK modem


Dilshan Jayakody writes:

This project is about 4046 PLL base simple FSK modem. Under given configuration this modem send digital signals over two frequencies and which is switching around 378kHz and 226kHz. In this system, modulator uses VCO of 4046, and demodulator is based on close loop mode of 4046 phase comparator and external LPF assembly.
This FSK modem is specifically design to transfer digital signals over mixed signal lines and it can directly use with any slow digital interfaces like UARTs. This system operates reliably under 10kHz range and we test it with standard TTL inputs.

Join the Conversation


  1. This was being used back in the 80’s, checkout EA and ETI magazines and I believe Brian Moffat did some work with this also.

  2. The 4046 is such a versatile beast – this modem is just one of the things that this chip does well. It’s been re-done in other logic families e.g. 74HC and got some upgrades in terms of VCO range and linearity, and along the way it picked up a third phase-freq detector. I used to reach for the 4046 quite often when I was doing pure analog design work. There was some oddball 74C9xx part that was a 300bps FSK modem in a package, too, if I recall correctly.

Leave a comment

Your email address will not be published. Required fields are marked *

Notify me of followup comments via e-mail. You can also subscribe without commenting.