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Audio Output
"Peak"
LED
Power
25C1020
(EA)
Channel B: This follows an identical scheme: noise signals from U2 are ad-
justed by squelch level control R41 before being amplified by U108C, filtered by
U108A, rectified by D107, and smoothed by C140.
Comparator Circuits
U106C and U106D compare the dc noise signals from the two channels.
When these signals are comparable, U106C and U106D direct analog switches
U103C (Channel A) and U103B (Channel B) to allow both channels to pass
through. The signals from each channel are correlated while the noise is uncor-
related, which makes possible a theoretical signal-to-noise improvement of 3 dB.
When the dc noise signals are not comparable, the channel with less noise is
chosen.
U106A imposes an additional criterion on Channel A, and U106B does the
same for Channel B. Each channel's dc noise voltage is compared to a threshold
set by user-adjustable R112 (Squelch). A channel whose noise exceeds this
threshold is squelched. If both channels exceed their thresholds, the audio out-
put of the receiver is squelched. Squelching is accomplished by tying together
the outputs of U106A and U106C (Channel A) or U106B and U106D (Channel
B). These outputs also control the yellow "Diversity" status indicators. Compara-
tor U102 provides additional attenuation in the squelched state by shutting off
compandor U3.
The audio signals from the two channels pass through analog switches
U103C and U103D, are buffered by U109A, and enter U3, which provides a 2:1
logarithmic expansion. An additional
with U3 to provide a lower noise floor. The output of this stage passes through
the Volume control (R126) to the "Output" connector (J101). A bridging amplifier
formed by U109B and U109C provides the "Balanced Low Z" output.
This red indicator warns the user when the transmitter deviation is approach-
ing the limit of 15 kHz. This function is implemented by a window comparator
(U102C and U102D) and a pulse stretcher (U102B) that makes short transients
more visually apparent.
The green "Power"
battery pack) is supplying power to the receiver. The circuitry operates normally
with an input of 12–18 Vdc. D10 provides reverse polarity protection, and U101
supplies voltage regulation. The "Dc Input," audio "Output," and "Balanced
Low Z" output connectors are filtered to prevent local oscillator radiation from the
cables.
Shure T4 Diversity Receiver
amplifier (U109D) operates in conjunction
IC
indicates when the external power converter (or a
LED
3
Characteristics
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