Shure LX2 Hand-Held Transmitter
Characteristics3
25D1006 (CG)
The base-emitter circuit of Q201 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 with changes in battery voltage,
regulated 5 Vdc bias is applied to the varactor diode and to the base
of Q201. Temperature compensation is provided by C206 and C207.
The collector circuit of Q201 is tuned to the third harmonic of the
oscillator frequency (approximately 60 MHz) by means of L202 and
C208. C210 couples rf energy to a second tuned circuit consisting
of L203, C214, and C215, which also forms a capacitively-tapped
voltage divider for matching into the base of Q202, operating as a
buffer amplifier.
Regulated dc bias is again employed to minimize changes in loading
on the oscillator stage and stabilize drive levels. The collector circuit of
Q202 is again tuned to the third harmonic of the oscillator frequency by
L204 and C216. C219 couples rf energy to another tuned circuit consist-
ing of L205, C222, and C224, which also forms a capacitively-tapped
voltage divider for matching into the base of Q203.
Frequency Tripler
The third stage (Q203) is operated as a frequency tripler with the
collector circuit tuned to the output frequency (for example, 180 MHz).
In this case L206, C226, C229, L207, C230, and C232 perform tuning
and impedance 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.
The final output stage (Q204) is operated as a tuned amplifier.
Resistive loading on the input enhances stability. The output circuit
consists of a tuned tank circuit (L209, C237, and C239) which also
provides impedance matching to the output low-pass filter (C239,
L210, and C241). L211 and C242 match the antenna to the filter.
Stable Operation
To promote stable operation, the LX2 is designed to minimize the
production and radiation of spurious emissions and harmonic energy.
In addition to the features described previously, the collector of each rf
stage is separately decoupled from the 9 Vdc supply using ferrite chokes,
resistors, and bypass capacitors of the appropriate value. The base cir-
cuits are similarly decoupled except that they use resistor–capacitor
(R–C) networks that are more appropriate, due to the higher impedance
level. The use of surface mount components results in better perfor-
mance because of a reduction in stray inductances and unwanted
radiation from component leads. Double-sided printed circuit board
construction improves ground–plane performance.
