schematics

 

After the justification and the description of the various functions, here are the schematics. The realization can be very fast, by simply soldering the components onto a Veroboard circuit.

 

The pseudo-random generator and the building of the biphase signal

 

click to enlarge

Two inverters among the 6 of the 74HCT04 U1 are connected at the output of a 4096 kHz crystal oscillator and feed the D flip flop U2 cabled out of divider by 2. We obtain on the Q output of U2 a square signal 50/50 at the clock frequency of clock which is precisely 2048 kHz. An adjustable capacitor makes it possible to adjust this frequency.

The IC U3 and U4, 74HCT164, are circuits containing 8 shift registers each one. They are chained and the 14th and 15th output are connected to an exclusive OR U6 74HCT86 whose output is returned at the entry of the register U3. The counter U5 is incremented by each new zero from the pseudo-random generator and is set up at zero on each arrival of one. In the event of blocking of the pseudo-random generator, the counter U5 sees arriving fifteen zero successive which causes the setting with one of its carry out output RCO and force the introduction of a one in input of the registers. The restarting is thus automatic.

The circuit U7 forms a multiplexer 2 to 1. It makes it possible to choose the signal which will be sent on the formation of biphase by an exclusive OR connected on the output. When the input " input select " is left open, the signal present at the output is the signal from the pseudo-random generator. If the input " input select " is put at zero, the signal present will be that coming from the auxiliary input " local data memory input ". This auxiliary input is planned for the data which will be stored in the EPROM of 1 Mbit (message of identification, image and software to be downloaded).

Test Points (TP) make it possible to control the main signals. The clock test point is very useful to synchronize an oscilloscope during the examination of the eye pattern.

The Nyquist filter emission and the video amplifier

 

 

The biphase signal coming from the logic card is connected to the input of the Nyquist filter through two resistances to match the impedances. The Nyquist filter is also matched to the output. A three stages amplifier makes it possible to provide a 1 V peak to peak level in an impedance of 75 ohms. A potentiometer makes it possible to adjust this level.

 

Local data memory: forthcoming publication

 

The modem (receive part): forthcoming publication

B5+ et 73 de Jean-François Fourcadier, F4DAY

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