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RECEIVER MODULE MIXER, MIXER INJECTION AMPLIFIER AND NSK, FSK FILTER SWITCHING
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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FREQUENCY DISCRIMINATOR AND SWITCHING, RECEIVER MODULE A4
TM 11-5805-387-34-2
Tty Channel 2 . Channel 2 nsk tones are applied through pin 15 of connector A4J1 (fig. FO-5) amplified by
amplifier A4A2Q2, limited by limiter amplifier A4A2Q5, and applied to the nsk only discriminator through discriminator
driver A4A1Q6. Because the input tone is centered at 2,805 Hz, no mixing is necessary before the input signal is fed to
the discriminator.
1-56. FREQUENCY DISCRIMINATOR CIRCUITS
The two frequency discriminator circuits demodulate fsk and nsk mark and space tones, and provide differential
voltages which are a function of the input frequency (mark or space tone frequency). Each frequency discriminator
develops a differential voltage of one polarity for a mark tone frequency input and one of the opposite polarity for a space
tone input frequency. The differential voltages are applied to mark-space generation circuitry (para 1-57), which converts
the differential voltages into mark and space pulses. The two discriminator circuits are identical, except that the pass
band of fsk/nsk frequency discriminator A4A1Q9, Q10 can be changed to demodulate either fsk tones (mark and space
signals 850 Hz apart) or nsk tones (mark and space signals 85 Hz apart), depending on the mode of operation. In the
850-Hz mode, energized relay A4A1K1 switches the connections to fsk/nsk frequency discriminator A4A1Q9, Q10 so that
the pass band is widened for fsk operation.  in this mode, relay A4A1K1 also reverses the phase-sensing circuit
connections of the discriminator so that the higher tone input will eventually produce a mark pulse and the lower tone
input will produce a space pulse.  This reversal corrects the tone reversal caused by subtractive mixing In mixer
A4A1CR1, CR2, T1 (para 1-55). Since fsk/nsk frequency discriminator A4A1Q9, Q10 and nsk frequency discriminator
A4A2Q7, Q8 are similar, only the operation of the fsk/nsk discriminator will be discussed.
The discriminator circuit is resonant at 2,805 Hz (the center frequency of the applied signal) and produces a
differential voltage for an input frequency above or below resonance.  This discriminator circuit differs from the
conventional type in that the phase-sensing circuit (A4A1L4, C22, C23, C24) is link-coupled to the secondary of
transformer A4A1T2. With this arrangement, the voltage developed across parallel-connected capacitors A4A1C22, C23
at resonance differs 900 in phase from the voltage across the secondary of transformer A4A1T2.  Therefore, at
resonance, transistors A4A1Q9 and A4A1Q10 will conduct equally, producing equal but opposite voltages so that the
sum of their outputs is zero. When the frequency of the incoming signal is above or below resonance, the phase shift
across capacitors A4A1C22, C23 is greater or less than. 900, which causes one transistor to conduct more than the
other. This results in a differential voltage output which is a function of the phase shift. The output of the frequency
discriminator is balanced (ungrounded). Resistor A4A1R29 prevents waveform distortion and causes a drastic reduction
in "Q" when the transistors conduct. Because transistors act as switches in only one direction, diodes A4A1CR6 and
A4A1CR7 provide a dc return for the collector current of their associated transistors. Resistors A4A1R50 and A4A1R51,
in series with the base of each transistor, isolate sensing circuit A4A1L4, C24, C23, C22 from the loading effect of
A4A1Q9, Q10.
1-44

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