GENERAL DESCRIPTION
The H175DC is a “plug-on” transmitter that converts any microphone with an XLR connector to wireless operation.
The 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 digital code system provides much more reliable control over the receiver audio output than a conventional
squelch can provide. The transmitter generates a 9-bit code at turn on, and another code at turn off, to open and
close the audio output on the receiver. The squelch on the receiver cannot be opened by anything other than the
specific code it is programmed to receive. This superior squelch system is very necessary when the receiver is
connected into an automatic mixing system.
The input jack provides a phantom 5 Volts DC positive bias for all Lectrosonics’ electret lavalier microphones. Many
other electret microphones may be adapted to the H175DC Series by making wiring changes in the microphone
connector.
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 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 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 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.
RF
OUT
INPUT
COMPANDOR
45MHZ
90MHZ
180MHZ
+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
D.C.S.
4
(11 pages)
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