Lectrosonics R185 - Manual (Page 4)

GENERAL TECHNICAL DESCRIPTION
The R185 receiver is comprised of five functional subsystems: the RF front-end, the double balanced mixer/local
oscillator, the first IF filter, the second IF filter and demodulator, the compandor and audio output section.
6 SECTION ACTIVE FILTERS SEPARATE
ANT
LC
FILTERS
IF AMP
2:1
SQUELCH
NOISE
AUDIO
0.5uV
HELICAL
RESONATOR
DBL BAL
DIODE
MIXER
6 POLE
HIGH FREQ
CRYSTAL
FILTER FIRST
OSCILLATOR
DBL BAL
ACTIVE MIXER
SECOND
OSCILLATOR
DBL TUNED
DISCRIMINATOR
NOISE AND AUDIO
COMPLEMENTARY
EXPANDER
BALANCED
OUTPUT STAGE
Figure 1 - R185 Block Diagram
The RF front-end consists of three cascaded pairs of helical resonators for high selectivity. Between the first and
second pair, and the second and third pair of helical resonators are low noise, grounded gate J-FET amplifiers.
These amplifiers are designed to provide only enough gain to make up for the loss through the helical resonators.
This combination of low front-end gain, coupled with the extremely high selectivity of the helical resonators prevents
overload in the mixer and provides exceptionally high image rejection, even with extremely strong RF signals.
Rejection of out-of-band signals is maximized, and intermodulation products are suppressed.
The first mixer stage consists of a high level, double balanced diode mixer. The oscillator is biased from a
regulated supply, yielding stable performance. The local oscillator crystal operates at approximately 16 MHz, and
can be adjusted above and below the nominal frequency in order to place the 21.4 MHz IF in the center of the
crystal filter’s narrow passband. The high selectivity of the crystal filter stage further minimizes the possibility of
interference from signals on adjacent frequencies.
The second IF filter and the audio demodulator, as well as the squelch and RF LED drive are provided by one
monolithic integrated circuit. The second IF filter is centered on 1MHz, and drives a double tuned quadrature type
FM demodulator. The squelch circuit is a supersonic noise detector type and is factory set for a -20dB SINAD level
(about .5uV). The squelch level is regulated and temperature compensated to maintain a consistent squelch level
under all conditions.
The compandor is driven by a multiple pole active low-pass filter. This filter ensures that supersonic noise will not
cause the compandor to increase gain incorrectly. The compandor senses the signal level, and dynamically
increases the gain for loud signals or decreases the gain for soft signals. In this way, the original dynamic range of
the transmitted signal is restored, while preserving a high signal-to-noise ratio. The expansion ratio is 2:1, which
produces a 2dB change in output level for a 1dB change in input level. In addition, the compandor has a de-
emphasis network that is complementary to the pre-emphasis network in the transmitter.
2:1 1:2
COMPRESSION EXPANSION
RF LINK
TRANSMITTER RECEIVER
0dB
+20dB
-80dB
+20dB
0dB
-80dB
-40dB -40dB
+10dB +10dB
50dB dynamic range 100dB dynamic range
AUDIO
SOURCE
TO AUDIO
SYSTEM
100dB dynamic range
Figure 2 - Compandor Action
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