Lectrosonics M187 - Manual (Page 3)

Belt-Pack Transmitter
GENERAL TECHNICAL DESCRIPTION
The M187 transmitter is comprised of four major functional subsystems: the input compressor, the mic preamp/gain
control, the compandor, and the RF transmitter (see block diagram below).
The mic preamp is an ultra-low noise type op-amp. Aside from gain control, this stage also adds high frequency pre-
emphasis to the audio signal. This insures highest signal-to-noise ratios under varying signal conditions. Gain control
is semi-logarithmic to provide smooth gain control action.
The compandor is a high quality audio device that processes the input signal such that large dynamic ranges of input
signal can be transmitted to the receiver without overload or noise. A complementary system in the receiver recovers
the original dynamics of the signal for full audio quality. Compression and expansion ratios are complementary at 2:1.
High frequency pre-emphasis is implemented in the transmitter to provide another 10dB signal-to-noise ratio improve­
ment. Matching de-emphasis is provided in all receivers.
The compressor/limiter is a low distortion shunt FET compressor. Control signals for the compressor are derived
after the compandor to eliminate any possibility of overload under any signal condition up to a maximum of 3 Volts
input. The range of limiting action before gross distortion occurs is 35dB.
The compandor in the 187 series transmitters is much tighter than the 185 series. The H187 compandor will hold the
transmitter deviation to no more than ±15kHz under all circumstances. The 185 compandor would allow some
excursions beyond 15kHz during strong audio “spikes.”
The RF transmitter is composed of a crystal stabilized main oscillator followed by a frequency tripler and two fre­
quency doublers. All three stages are double tuned. Double tuning provides high attenuation of spurious signals,
which in turn minimizes the possibility that a transmitter would interfere with another transmitter/receiver system on
another frequency.
All RF stages are biased from a regulated internal power supply. The output stage has a separate feedback regulator
which not only stabilizes its operating point, but also minimizes AM distortion. These regulators keep the RF and
audio performance consistent from the beginning (9 Volts) to the end (6.5 Volts) of battery life.
Schematics and alignment instructions will be provided to qualified repair personnel upon request.
MIC PREAMP
LIMIT
LED
LEVEL
LED
COMPANDOR (VCO)
+9V IN
45MHZ 90MHZ 180MHZ
1
2
3
4
5
15 MHZ XTAL
OSCILLATOR
BATTERY
SENSOR
CIRCUIT
POWER
LED
FREQ.
TRIPLER
FREQ.
DOUBLER
VOLTAGE
REGULATOR
FEEDBACK
REGULATOR
FREQ.
DOUBLER
RF
OUT
LIMITER
35 dB
M187 Block Diagram
Rio Rancho, NM – USA 3