rjr@mgweed.UUCP (Bob Roehrig) (01/19/85)
The problem with 567's as TU decoders is that with proper lowpass filtering, they do not respond fast enough. Rather than use tone decoder IC's like the 567, you should use either a 565 with a slicer after it (a sample circuit is in the Signetics data book), or better yet, one of the PLL chips that is designed for that purpose like the Exar XR-210. In any case, a PLL demod is lousy on the HF bands without channel filtering ahead of it, or it will lock onto the strongest signal within its capture range. They should be preceeded by either toroidal or active filters tuned to mark and space tone frequencies. Also, the VCO in the device should be tuned (with no signal input) to midway between mark and space. Bob K9EUI