GENERAL TECHNICAL DESCRIPTION
The H185 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 input compressor is a low distortion shunt FET compressor situated before the mic preamp.
Control signals for the compressor are derived after the mic preamp 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 20dB.
The mic preamp is an ultra-low noise NE5534 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 improvement. Matching de-emphasis is
provided in all receivers.
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.
Figure 1 - H185 Block Diagram
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