g45. RTTY-, Hell- and CW-demodulator techniques


The slide-back detector for better zero crossing for RTTY and CW detectors. Basic circuit diagram, see also ATC
It is particularly suitable for single frequency signals (ASK - amplitude shift keying) which was used for telex on telephone lines.

ATC (Automatic threshold control) for better zero transition for RTTY and CW detectors (time-constants for RTTY)..
First saw it mentioned for Mainline TT/L Terminal unit, this was described by OZ9JB Jörgen Badstue.
While the slideback-detector has audio input, ATC has DC-signal from the detector/lowpassfilter.

CW demodulator with MC1496P. A double balanced modulator used to give a variable
DC voltage out dependent of audio input level. Will also work for higher frequencies.
Some RF decouple capacitors should be connected across the 3300W resistors from pin
6 and 12 to +12V (or to ground).

CW demodulator with S042P (sensitive AF/LF/HF-level detector)
For some experiments I needed a simple and efficient detector. Had good experience with
MC1496P, but only few left and many Siemens S042P available. The resistor RA decides
the gain. In spite of small bypass-capacitors it is flat over the range 0.2-20MHz (RA=100W).
A comparable circuit diagram for use as product detector is shown on page-c13

RF input level DC level at pin 2 DC level at pin 3
-50dB 6.91 6.95
-40dB (8mV) 6.89 6.97
-30 6.73 7.12
-26 6.48 7.36
-23 6.17 7.66
-20dB (80mV) 5.77 8.04
-17 5.35 8.44
-15 5.11 8.6
-13 5.01 8.75
-10 (250mV) 4.75 8.82V

Suggested ATC circuit for CW-demodulator using OP-amps instead of germanium diode bridge
The circuit is based on +4V as reference voltage (depends on preceeding stages)