Shure LX2 Hand-Held Transmitter
4
Characteristics
25D1006 (CG)
Rf: Euro (ETSI)
Audio Input
Processed audio enters R201, an internal potentiometer that is
adjusted for 15 kHz deviation (100% modulation) when the audio
section provides a –2.2 dBV, 1 kHz tone.
Oscillation
The audio then goes to varactor diode D201, which is part of the
modulated oscillator-tripler stage (Q201). The latter’s base-emitter circuit
operates as a crystal-controlled Colpitts oscillator in the 20 MHz region.
Fundamental-mode crystal Y201 is tuned 10 kHz below series resonance
by the series combination of frequency-netting coil L201, diode D201,
capacitor C203, and capacitor divider C206 and C207.
Frequency Stability
To ensure frequency stability despite changes in the battery voltage,
regulated 5 Vdc bias is applied to the varactor diode and to the base of
Q201. C203, C206, and C207 provide temperature compensation.
Tuned Circuits
Stage 1: The collector circuit of Q201 is tuned to the third harmonic
of the oscillator frequency (approximately 60 MHz) by L202, C208, C244,
C210, L203, C214, C213, and C215. The latter components also form a
capacitively-tapped voltage divider for matching into the base of Q202.
The output is double-tuned to provide high spectral purity. Regulated dc
bias is again employed to minimize changes in loading on the oscillator
stage and to stabilize the drive levels.
Stage 2: Q202 operates as a buffer with the collector circuit tuned to
the output frequency (for example, 60 MHz). In this case, L204, C216,
C245, C218, L205, C222, C221, and C224 perform tuning and imped-
ance-matching functions. As in the preceding stage, regulated dc bias
is applied to the base circuit to stabilize the drive level, and the output
is double-tuned to provide spectral purity.
Stage 3: Q203 operates as a frequency tripler, with the collector
circuit tuned to the output frequency (for example, 180 MHz). In this
case, L206, C226, C227, C229, L207, C230, and C232 perform tuning
and impedance-matching.
Stage 4: Q204 operates as a tuned amplifier. Resistive loading on
the input provides stability. The output circuit consists of a resonant tank
circuit (L203 and C213) capacitively coupled to a low-pass filter (C219,
L206, and C218). C213 and C219 provide a capacitively-tapped voltage
divider for matching into the low-pass filter.
