General Technical Description - Lectrosonics H187 Operating Instructions And Troubleshooting Manual

Plug-on transmitter

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Instruction manual (10 pages)

Quick start manual (8 pages)

Table Of Contents

Table of Contents

The H187 transmitter is comprised of four major functional subsystems: the mic preamp/gain control, the compandor,

the compressor/limiter 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 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 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 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.

MIC LEVEL

XLR

LIMITER

35 dB

INPUT