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4-level FSK-interface



To 'read along' with ERMES, 4 level FLEX and other paging networks, I built a 4-level interface. My interface is based on the design of Libor Ulcak, who was inspired by PA3EIK in his turn.

The disadvantage of Libor's print is that it uses screw terminals for audio and DC. Connecting inverse DC polarity will swiftly kill the ICs...

I made small adaptations to the circuit diagram and the PCB design. It now contains a terminal for a 3,5 mm-plug, and a DC-terminal for a cheap universal DC-adaptor. The circuit is protected by a diode, while the input voltage of 10 - 15 Volt is stablilised at 8 V.

Prototype

The prototype, directly connected to the serial port



4 level interface

The data slicer is connected to the discriminator output of your scanner or receiver. DC power is 12 Volt, which is stabilised to 8 Volt. An inverter IC generates -8 V.

The interface works hassle-free. Alignment of the variable resistor is not critical. The reference voltage for the window comparator, available between the cathode of D1 and the anode of D2 (orange circles, also see circuit diagram) is aligned to about 3.5V at the presence of a signal.

Top view of the adapted interface, with test points



Below you can find the PCB design and component layout. It is largely inspired by Libor's design. Click on the pictures for a higher resolution. The resolution of the PCB design is 600 dpi. The circuit diagram and PCB design are available in Eagle-format.



PCB Design

Component layout

PCB design

Component layout

PDW Window

Screen dump of ERMES with PDW. Lots of news and automatic status messages.



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