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12-9.
A bandpass filter following the shift register output removes the higher frequency components of the
digital signal. The filtered DPL signal is then applied to the select switch. For the DPL off code (133Hz tone),
the processor switches the INT MOD line to the PL output so that a 133 Hz sinewave is output.
12-10. 1 kHz Tone.
A filtered 1 kHz square wave provides the fixed 1 kHz modulation source. The SYNTH
1 kHz signal from the RF Synthesizer is filtered to less than 1% distortion by a bandpass filter. Thefilteroutput
is the 1 kHz signal source.
12-11. External Modulation. A microphone and a front panel jack are the external modulation inputs. An
lnstaneous Deviation Control (IDC) circuit amplifies and limits the microphone signal (MIC IN) before
sumation with the signal (EXT MOD IN) from the front panel jack. The sumation signal is the EXT MOD source.
12-12. Modulation Control.
Level control of the three modulation sources is by either the front panel
controls or the IEEE interface module. The level adjusted sources are then returned to the Audio Synthesizer
module where they are summed together to form the composite modulation audio. The composite signal is
then routed to the Scope/DVM Control module (MOD CAL AUDIO) for modulation determination, to a buffer
amp which drives the front panel modulation output (MOD TO FP), and to a Modulation Select Switch which
routes the signal to the desired modulator.
12-13. Modulation audio is switched to the speaker (VOL CNTL AUDIO) for any generate mode, to the
DSBSC modulator (DSBSC MOD) for sideband modulation, to the RF Synthesizer for frequency modulation
(FM MOD), to the offset oscillator for frequency modulation of the duplex output (OFFSET MOD), and to the
RF output leveling loop for amplitude modulation. The signal for amplitude modulation is summed with a 5
VDC level and then routed to the variable RF level control on the front panel (5 VDC +AM MOD). At the RF level
control the signal is attenuated according to the level setting to give the DC plus AM reference signal for the
output leveling loop.
12-14. Module Control.
Processor control of the Audio Synthesizer is via the AF control bus. The four bit
address bus (AF ADD BUS 0-3) is decoded by the Address Decoder to determine which control latch is to be
accessed. Control data is transferred to the accessed latch on the four bit data bus (AF DATA BUS 0-3).
Synchronization of the data transfere is the function of the AF BUS EN1 signal line.
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