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TRANSMITTER MODULE A3
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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DIVIDE-BY-10 WAVEFORMS, TRANSMITTER MODULE A3
TM 11-5805-387-34-2
Space oscillator. Oscillator A3A1Q21, Q22 provides the fundamental frequency used to generate the space
tone. The circuit is identical with the 221.0-kHz oscillator described in (1) above, except for the frequency-
determining elements and reference designators. The 227.8-kHz output of the oscillator is applied to shaper
amplifier A3A1Q4 through isolation diode A3A1CR4.
1-44. MARK AND SPACE OSCILLATOR OUTPUT SWITCHING
(fig. FO-10)
The oscillator output switching circuits consists of phase reverse switch A3A1Q1, 221.0-kHz switch A3A1Q2, 227.8-kHz
switch A3A1Q3, isolation diodes A3A1CR3 and A3A1CR4, and shaper amplifier A3A1Q4. The output of either the mark
or space oscillator will be applied to shaper amplifier A3A1Q4, depending on whether a mark or space tone is to be
generated.
A positive dc pulse is generated by the send circuit of loop battery module A5 (para 1-41) each time a mark tone is to
be transmitted. This positive pulse is applied through connector A3J3 pin 14 and rf noise filter A3A1C33, L2 to contact 2
of transmit REV/NORM switch A3A1S1. With the switch set at NORM, the pulse is connected to the base of phase
reverse switch A3A1Q1, causing the transistor to conduct. The resulting low collector voltage of A3A1Q1 biases 221.0-
kHz switch A3A1Q2 to the nonconducting state. With switch A3A1Q2 off, no voltage drop occurs across collector load
resistor A3A1R4. This allows the +20 volt dc supply voltage to be applied through A3A1R4 to forward bias isolation
diode A3A1CR3, turning on 227.8-kHz switch A3A1Q3. The 221.0-kHz mark oscillator frequency (coupled to diode
A3A1CR3) is coupled through the forward biased diode to shaper amplifier A3A1Q4. The 227.8-kHz space oscillator
frequency applied to isolating diode A3A1CR4 is blocked, because the diode is reversed biased by the low collector
voltage of conducting switch A3A1Q3.
When a space tone is to be generated, no pulse is present at the input to phase reverse switch A3A1Q1. The circuits
are biased so that with no input, transistor A3A1Q1 is nonconducting and transistor A3A1Q2 is conducting, holding
A3A1Q3 off. Consequently, the 221.0-kHz mark oscillator signal is blocked and the 227.8-kHz space oscillator signal is
fed through biased diode A3A1CR4 to shaper amplifier A3A1Q4. Shaper amplifier A3A1Q4 amplifies the oscillator input
signal and shapes it to the proper waveform to trigger the bistable multivibrators of the divideby-8 circuit (para 1-45).
When transmit REV/NORM switch A3A1S1 is set at REV, phase reverse switch A3A10Q1 is bypassed, and the base of
221.0-kHz switch A3A1Q2 is grounded through resistor A3A1R2 and contacts 4 and 5 of the switch. Thus, switch
A3A1Q2 is held nonconducting and a mark frequency (221.0 kHz) is fed to shaper amplifier A3A10Q4 until a send pulse
is received.
1-45. MARK AND SPACE OSCILLATOR OUTPUT DIVIDER CIRCUITS
(fig. FO-10)
The mark and space oscillator output divider circuits consist of a divide-by-8 circuit and a divide-by-10 circuit. The
mark or space oscillator output is divided by 8 for the fsk (850-Hz) modes and by 10 for the nsk (85-Hz) modes.
Divide-by-8 is accomplished by a circuit composed of three series-connected divide-by-2 bistable multivibrators
A3A1Q5, Q6; A3A1Q7, Q8; and A3A1Q9, Q10. The circuit is used during all modes of operation. A 221.0-kHz mark or
227.8-kHz space oscillator signal is applied to multivibrator A3A1Q5, 06 from shaper amplifier A3A1Q4. The divide-by-8
chain of multivibrators divides the signal, and the resulting 27,625 or 28,475 Hz (28,050 425 Hz) signal from
multivibrator A3A1Q9, Q10 is fed to both the divide-by-10 circuit and, through coupling resistor A3A1 R24, to the sine
wave pass filter (para 1-30).
1-33

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