Lectrosonics H187 - Manual (Page 4)

H187
LECTROSONICS, INC. 4
The H187 transmitter is comprised of four major
functional subsystems: the mic preamp/gain control, the
compandor, the compressor/limiter and the RF transmit-
ter (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-empha-
sis is implemented in the transmitter to provide another
10dB signal-to-noise ratio improvement. 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
RF
OUT
INPUT
LIMIT LED
LEVEL LED
COMPANDOR
45MHZ 90MHZ
180
MHZ
+9V IN
LIMITER
35 dB
MIC
PREAMP
BATTERY
SENSOR
CIRCUIT
POWER
LED
15MHZ XTAL
OSCILLATOR
(VCO)
FREQ.
TRIPLER
FREQ.
DOUBLER
FREQ.
DOUBLER
VOLTAGE
REGULATOR
FEEDBACK
REGULATOR
MIC LEVEL
XLR
Introduction
Thank you for selecting Lectrosonics wireless. This
VHF transmitter is part of a family of narrowband
wireless components that have served the needs of
television broadcast and motion picture production
markets since 1988.
The H187 plug-on transmitter replaces earlier versions,
updating the design with surface mount PCB construc-
tion and the latest electronic components. The input
coupler has evolved since the first versions were
introduced, making it more rugged and better able to
adapt to the wide variety of different XLR connectors
found on hand held and shotgun microphones.
The housing and external mechanical parts are con-
structed of machined aluminum to provide the rugged-
ness needed in field production. The battery door
remains attached to the housing when opened, and the
control panel is recessed to prevent damage from rough
handling. These and other features have been added
or modified since the first versions were introduced.
The H187 remains a workhorse in broadcast ENG,
continuing the Lectrosonics commitment to quality.
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 the crystal stabilized
main oscillator followed by a frequency tripler and two
frequency doublers. All three stages are double tuned.
Double tuning provides maximum attenuation of spuri-
ous 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
source. 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 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 on request.
H187 Block Diagram
General Technical Description