Lectrosonics CR185 - Manual (Page 3)

GENERAL TECHNICAL DESCRIPTION
The CR185 receiver is comprised of six major functional subsystems: the RF front-end amplifier, the
double balanced mixer/local oscillator, the first IF filter, the second IF filter and audio demodulator, the
compandor, and the balanced microphone level output circuit.
The RF front-end amplifier consists of three cascaded pairs of helical resonators for high selectivity.
Between the first and second helical resonators, and the second and third helical resonators are low
noise grounded gate JFET 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 cascaded helical resonators results in no overloading, even on
extremely strong signals. Rejection of out of band signals is maximized, and intermodulation products
are suppressed.
The mixer stage consists of a high level double balanced diode mixer. The oscillator is biased from a
/regulated supply, yielding stable performance over the entire life of the battery. The local oscillator
crystal operates at approximately 16mHz, and can be adjusted above and below the nominal frequency
in order to place the 21.4mHz 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 output 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. The filter ensures that supersonic
noise will not cause the compandor to increase gain incorrectly. This filter also drives the -20dB
modulation LED. 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 signal 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.
The balanced microphone level output circuit generates a truly balanced signal at the XLR output
connector. The volume control is a balanced attenuator, and gives a gain range from -20dBV (at full
modulation) in the fully clockwise position to -50dBV in the fully counter-clockwise position. In addition,
the headphone circuit is driven from the microphone circuit. This means that the headphone output
level is dependent not only on the headphone volume control, but also the microphone level control.
JFET
RF AMP
ANT
LC FILTERS
2:1
SQUELCH
NOISE
AUDIO
0.5uV
2 SECTION
HELICAL
RESONATOR
2 SECTION
HELICAL
RESONATOR
JFET
RF AMP
2 SECTION
HELICAL
RESONATOR
1ST
LOCAL
OSCILLATOR
DBL BAL
DIODE
MIXER
1ST
IF AMP
6 POLE
HIGH FREQ
CRYSTAL
FILTER
DBL BAL
ACTIVE MIXER
2ND
LOCAL
OSCILLATOR
2ND
IF AMP
ACTIVE FILTERS SEPARATE
NOISE AND AUDIO
DBL TUNED
DISCRIMINATOR
COMPLEMENTARY
EXPANDER
BALANCED
OUTPUT STAGE
HEADPHONE
AMP
Figure 1 - CR185 Receiver Block Diagram
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