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CLIPPER AND LOW-PASS FILTER (U7A)
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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SQUELCH CIRCUIT - TM-11-5820-815-140077


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
VR3-VR4. Pad R65-R66 provides a 10-kilohm load to
ready lamp (remote operation) or chassis A10 ready
the receive data output line. Zener diodes VR3 and VR4
lamp (local operation). Figure FO-16 shows ready lamp
provide protection to the audio circuit against high-
circuit. When off, key 2 inhibit transistor Q9 allows keyer
voltage noise spikes that may be present on the receive
transistor Q10 to be turned on and off by key driver
data audio output line from the radio set.
transistor Q6.  Under the above conditions, a ground
applied to the ptt input (P1-K) turns key driver transistor
4-169.
COMPRESSION
AMPLIFIER
(RECEIVE
Q6 on which, in turn, turns keyer transistors Q7 and Q10
AUDIO).
The receive audio compression amplifier
on to ground the key 1 (P1-5) and key 2 (P1-M) key
(figure FO-21) is composed of the variable attenuator
lines. When a fault is detected, the logic 0 fault signal
circuit (Q11), amplifier circuit (U5A), and detector circuit
applied to the p11 fault or rf filter fault inputs turns fault
(Q13). Functionally, the compression amplifier maintains
switch Q8 off. This ungrounds the ready lamp circuit to
a constant (within 3 dB) audio output signal (about 1.6
turn off the ready lamp. In addition, Q8 turns on key 2
volts rms at test point TP13) for voice audio input signals
inhibit transistor Q9 to turn off keyer transistor Q10. This
(receive af input, P1-7) that vary as a result of the
ungrounds the key 2 key line to unkey the power
percent  modulation  of  the  received  rf  signal.
amplifier. Key 1 key line remains grounded to mute the
Compression begins when the input signal exceeds
receiver.
about 0.22 volt rms.
4-164.  Capacitor C21, diode CR6, and resistors R46
4-170.  Compression is achieved by controlling the
and R47 provide fast attack-slow release keying to the
attenuation ratio of variable attenuator Q11 which in turn
key 1 keyer circuit.  When Q6 turns on (ptt input
controls the level of audio applied to amplifier U5A.
grounded), the fast charge of capacitor C21 through CR6
Negative audio peaks that exceed the threshold of
provides fast attack to ground key 1. When Q6 turns off,
detector Q13 (about 2.24 volts peak or 1.6 V rms) are
the slow discharge (relative to charge time) of capacitor
converted into a dc control voltage and applied to the
C21 through R46 and R47 provides slow release to
gate of variable attenuator FET Q11 to control the
unground key 1.
attenuation ratio of the variable attenuator. As the audio
input signal increases, detector Q13 decreases the
4-165. Capacitor C22 and resistors R45 and R50 pro-
control voltage applied to the gate of variable attenuator
vide slow attack-fast release keying to the key 2 keyer
Q11 to increase its attenuation ratio. This reduces and
circuit. When Q6 turns on, the charge of capacitor C22
maintains the audio level applied to the amplifier input at
through resistor R45 provides slow attack to ground key
the level necessary to hold the output of the amplifier just
2.  When Q6 turns off, the fast discharge (relative to
at the detector threshold. The compression amplifier has
charge time) of capacitor C22 through R50 provides fast
an attack time of about 4 milliseconds and a release time
release to unground key 2.
of about 150 milliseconds.
4-166.  BANDPASS FILTER/'AMPLIFIER.  The band-
4-171. Variable Attenuator. The variable attenuator is
pass filter/amplifier (U4 of figure FO-21) amplifies and
composed of resistors R67 and R68 and FET Q11 that
shapes the frequency response of the received wide
together function as a variable voltage divider. FET Q11
bandwidth data signal from receiver rf module A3
acts as a variable resistor that changes from infinite
(receive af input, P1-7). The filter is a 6-pole, active filter
resistances to some finite value as the detector-
with a voltage gain of about 4.2 V/V and cutoff
developed control voltage applied to its gate decreases
frequencies of 16 Hz and 25 kHz. The three poles that
from -12 V dc toward 0 volt.
control the low-frequency response are determined by
resistor-capacitor networks R57-C23, R56-C24, and
4-172. Amplifier. Amplifier U5A provides a voltage gain
R58-C25.
The three poles that control the high-
of about 48 V/V. Together, the variable attenuator and
frequency response are determined by resistor-capacitor
amplifier provide a variable compression amplifier gain
networks R112-C69, R113-C70, and R114-C67. In the
that varies from about 7.1 V/V to less than 0.64 V/V. The
passband, the output varies less than +1 or -3 dB when
minimum gain depends upon the minimum turn-on
referenced to the output at 1000 Hz.
resistance of FET Q11.  In addition, amplifier U5A
provides a voltage gain of about 3.7 V/V to sidetone
4-167.  Bias voltage of about +0.64 V dc developed
audio. During transmit mode, transmit audio detected at
across resistor R59 of voltage divider R60-R59 biases
the antenna (rf filter module) is fed back and inserted
the output voltage of U4 to approximately 5 volts with no
into the receive audio path at this point so that it appears
input audio signal.  This allows the output of U4 a
as sidetone in the audio output. Receive audio is muted
maximum swing of approximately 17 volts when an
by receiver rf module A3 during this time. During receive
audio input signal is applied.
mode, sidetone mute switch Q18 is turned on by the
logic 1 key 1 key line. This grounds the sidetone input to
4-168. Capacitor C32 couples the output of U4 to the
amplifier U5A.
receive data audio output (P1-15) through impedance
matching pad R65-R66 and transient protection circuit
4-31


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