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Shure LX2 Hand-Held Transmitter
2 Characteristics 25D1006 (CG)
Circuit Description
The Shure LX2 Hand-Held Transmitter contains three circuit boards;
an audio section, an rf section, and a microphone head interface board.
It is intended for use with the matching LX3 and LX4 receivers.
Audio Section
Audio signals from the microphone head enter the transmitter via a
printed circuit board (pcb) with spring contacts, which also interconnects
the audio and rf printed circuits.
The audio signal next enters a preamplifier stage consisting of
one section of operational amplifier (U102C). The voltage gain of this
stage may be adjusted over a 40 dB range by means of an externally-
accessible potentiometer (R125) to enable the user to compensate for
variations in sound level at the microphone.
The preamplified audio signal is then passed through a passive
pre-emphasis network consisting of R142, C110, C111, R112, and R115,
that has a pole at 63 microseconds and a zero at 12 microseconds.
Audio Signal Compression
The signal then enters the NE571D integrated circuit compander
(U101A). This provides 2:1 logarithmic compression of the audio signal.
A lower noise floor is achieved by U102A. An internal potentiometer
(R130) is provided for nulling system audio distortion. Operational
amplifier U102B operates as a two-pole active low-pass filter to restrict
the bandwidth of the system to audio frequencies.
Reverse Battery Protection and Low Battery Warning
The NE571D also contains an identical second channel (U101B),
which in this case is used to supply regulated, low-noise 5 Vdc bias to
various audio and rf circuit points. Transistor Q105 provides reverse
battery protection to the circuit. Q110 drives a green LED (D103) that
serves as a power ON indicator. Q107 drives an amber LED (D102)
that provides a low battery warning signal to the user. Q106 and Q108
drive a red LED (D101) that serves as a final low battery indicator.
Rf: Domestic (FCC/IC)
Processed audio enters R201, an internal potentiometer that is
adjusted for 15 kHz deviation (100% modulation) with a -“2.2 dBV, 1 kHz
tone at the output of the audio section. The audio is then fed to varactor
diode D201, which is part of the modulated oscillator-tripler stage (Q201).
