FREQUENCY DISCRIMINATOR CIRCUITS

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