Shure LX2 Hand-Held Transmitter
Characteristics5
25D1006 (CG)
Final Output
Transmitter: The transmitter can deliver up to +11.5 dBm (14 mW)
to the antenna. No user adjustments permit this value to be exceeded.
Testpoint TP6 provides a termination point for the 50 Ω output. C243 and
L211 act as a series-resonating network for the battery, which acts as the
antenna. The unit should be powered exclusively by a 9 Vdc dry battery.
An alkaline type such as the Duracell MN1604 is recommended.
Voltage Measurements
With 9 Vdc applied to the unit, the following voltages should appear
at the terminals of the output transistor:
S Vc = 8.83 Vdc
S Vb = .097 Vdc
S Ve = .32 Vdc
S Base current = 0.27 µAdc
S Emitter current = 9.67 mAdc
S Collector current = 9.67 mAdc
S Power input = 87 mW
The output power is +9.5 dBm (8.9 mW) into a 50 Ω load, at a
frequency of 169.445 MHz. At the lowest acceptable battery voltage
of 6 Vdc, the final collector current drops to 8.84 mAdc and the output
power to +7.3 dBm (5.4 mW).
Spurious Emissions
To promote stable operation and to minimize the production and
radiation of spurious emissions and harmonic energy, the collector of
each rf stage is separately decoupled from the 9 Vdc supply by ferrite
chokes, resistors, and bypass capacitors. The base circuits are similarly
decoupled, except for using resistor-capacitor (R-C) networks whose
higher-impedance levels are more appropriate.
Power Section
The transmitter delivers a maximum of +3 dBm (2 mW) ERP, to
comply with ETSI Class 1 or 2 limits. There are no user adjustments
which would permit this value to be exceeded. The low effective radiated
power is due to the low radiation resistance of the battery antenna, which
is only a small fraction of a wavelength at this frequency.
The transmitter is intended to be powered exclusively by a 9 Vdc dry
battery (alkaline type recommended) such as a Duracell MN1604, which
also serves as the antenna. The ground plane of the circuit board serves
as an untuned counterpoise.
