An 8-bit FAX demodulator for AM and FM ====================================== Please print this file with 12cpi. The files AM_FM1.GIF to AM_FM6.GIF show the circuit diagram of an 8 bit AM/FM FAX demodulator. You can use JVFAX to make printouts of these files. Circuit description: ==================== 1) power supply section: This circuit derives the stabilized voltages +5V, +8V and -8V (-7V) from a single 13-15V source. The NE555 serves as a chopper to generate the negative voltage. Of course, this circuit could be replaced by any one other that is capable to deliver the above mentioned voltages. 2) AM section: The audio frequency signal applied to JP4 passes the band pass filter U7c/U7d that has a center frequency of 2400 Hz. It is then rectified by U7a/D13/D14. Pot P6 is to adjust the symmetry of the rectifer. D13 and D14 should be paired germanium diodes. (other than indicated in the circuit diagram. P7 controls the black-threshold of the demodulated signal, while the correct leveling is done by variing the amplitude of the AF input signal. After rectification the signal passes a the pass filter U7b/U6c/U6d that filters out the remaining carrier frequency components. The audio frequency signal is fed to the PLL NE567, too. It locks to the 2400Hz carrier and generates the clock signal for the A/D converter. By this the whole circuit acts as a synchroneous demodulator. 3) FM section: The AF signal applied to JP3 is amplified in U4c and then shaped to a rectangular waveform by Schmitt trigger U4d. P98 is to adjust the symmetry of the rectangular signal. C11/R9 differentiates the rectangular signal, it is then fed to comparator U4a via the diode network D11/D12. At the comparator output appear short pulses with twice the input frequency. The pulses derived from both the rising and falling edges of the input signal can be adjusted to equal width with P99. These pulses are integrated in U4b and do also serve as a clock signal for the ADC. P1 is to adjust the offset voltage of the integrator. P2 to P4 and P9 serve for the adjustment of several deviations. Finally the signal passes the low pass filter U6a/U6b. 4) ADC section: Via the analogue switches U9 either the demodulates FM signal and clock or the demodulated AM sdignal and clock is fed to the ADC. The LTC 1099 has a built-in sample and hold and an output latch which makes interfacing to it very easy. A/D converter output is fed to the muptiplexer U11 via U99. U11 muptiplexes the 8 bit of output data to 2x4 Bit depending on the RTS signal delivered by the computer. U99 keeps the data to be multiplexed stable while the multiplexing process takes place. 5) display section: A simple tuning indicator (that could be omitted) and a level shifter from TTL to RS232. Alignment procedure =================== 1.: FM section Connect the demodulator to the computer and run the JVFAX program. Switch to FAX reception and hit the "C" key to invoke the calibration help. (If the JVFAX program has not yet been configurated for this demodulator - 8 bit, serial port, bit 0 is SSTV sync: no - then do this first. Apply 1900 Hz to the FM input (JP3) and connect an oscilloscope to pin 14, U4d. Set the input amplitude just high enough to obtain a well shaped rectangular waveform on the oscilloscope. Use P98 to adjust the positive and negative half waves to equal widths. Now connect the scope to pin 1, U4a. Adjust P99 until the short positive pulses have equal widths. Set the mode switch to FM150, continue to apply 1900Hz to the FM input. Turn pots P2,P3,P4,P9 fully to the grounded side. Turn pot P5 until the calibration help of JVFAX 5.0 shows the value 127. Now turn P2 to the full level. Adjust pot P1 to re-obtain the 127 display of the calibration help routine. Now change AF to 2050 Hz and adjust pot P2 to get a 255 display. Check if display shows 0 when the AF is set to 1750 Hz. Eventually, fine tune P2 and P1 until the display shows 0 for 1750 Hz, 127 for 1900 Hz and 255 for 2050 Hz. Minor deviations from these frequencies are not critical. For the further steps don't change the P1 setting any more. Set mode switch to FM150 (ATC) and repeat the above adjustment with P3 and the edge frequencies 2200 Hz and 1600 Hz. Now mode switch to FM400 und repeat adjustment with P4, 1500 Hz and 2300 Hz. Finally set the mode switch to FM400 (ATC) and adjust with P9, 1100 Hz and 2700 Hz. This completes the alignment of the FM part. (SSTV reception must be performed witt mode switch setting FM400 (ATC). 2.: AM-section If not yet done: Connect the demodulator to the computer and run the JVFAX program. Switch to FAX reception and hit the "C" key to invoke the calibration help. (If the JVFAX program has not yet been configurated for this demodulator - 8 bit, serial port, bit 0 is SSTV sync: no - then do this first. Set the mode switch to the AM position; don't apply any AF. Set P7 until the calibration help routine shows a value slightly above 0. (2..5) Now apply 2400 Hz to the AM input (JP4) via a volume control. Connect the scope to pin 6, U7b. Set the AF amplitude to a value that doesn't overdrive U7c/U7d/U7a. Now adjust P6 for minimum ripple of the rectified voltage. (rectified half waves must have equal amplitudes) Vary input amplitude and set P6 to a compromise value, if necessary. Now turn P8 until the AM sync LED lights. Slightly change the input frequency and fine tune P8 so that the LED turns off on frequencies symmetrical to 2400 Hz. Disconnect the AF source from the input. Re-adjust P7 until the calibration help routine just reaches 0. This completes the alignment procedure.