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.
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The prototype, directly connected to the serial port
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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.
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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.
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PCB design |
Component layout |

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