Lectrosonics CR187 - Manual (Page 4)

CR187
General Technical Description
The block diagram of the receiver provides a basic
overview of the major circuit sections.
The RF front-end amplifier consists of three cascaded
pairs of helical resonators for high selectivity. Between
the three resonators are two low noise grounded gate
JFET amplifiers. These amplifiers are designed to pro­
vide 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 prod­
ucts are suppressed.
The mixer stage consists of a high level double bal­
anced diode mixer. The oscillator is biased from a regu­
lated supply, yielding stable performance over the entire
life of the battery. 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
pass band. The high selectivity of the crystal filter stage
further minimizes the possibility of interference from
signals on adjacent frequencies.
One monolithic integrated circuit filters the second
IF, demodulates the audio, provides squelch control
and drives the RF output LED. The second IF filter is
centered on 1 MHz and drives a double tuned quadra­
ture type FM demodulator. The squelch circuit is a
supersonic noise detector type and is factory set for a
-20 dB SINAD level (about .5 uV). The squelch level is
regulated and temperature compensated to maintain a
consistent squelch level under all conditions.
CR187 Block Diagram
The overall wireless system utilizes “compandor”
noise reduction, which consists of a compressor in the
transmitter and expander in the receiver. The receiver
decodes (expands) the compressed signal coming from
the transmitter in a 2:1. The process senses the signal
level, and dynamically increases the gain for loud sig­
nals or decreases the gain for soft signals. In this way,
the original dynamic range of the transmitted signal
is restored and the signal-to-noise ratio is increased
significantly. De-emphasis (HF roll-off) is also applied
to reverse the pre-emphasis (HF boost) applied in the
transmitter as an additional noise reduction technique.
The expander circuit is driven by a multiple pole active
low-pass filter. The filter ensures that supersonic noise
will not cause the expander to increase gain incorrectly.
This filter also drives the -20 dB modulation LED.
The output of the receiver is a balanced microphone
level signal delivered on an XLR connector. The output
level control is actually a balanced attenuator to adjust
the signal from -20 dBV in the fully clockwise position
to -50dBV in the fully counter-clockwise position. This
preserves the signal to noise ratio regardless of what
output level is set.
A separate output is provided to drive headphones
separately from the main XLR output. The level is af­
fected by both the main output control and a secondary
trimpot on the side panel.
Warning: The CR187 is a negative ground
device. Do not connect this receiver to a
positive ground device through the audio
cabling. Damage to either device could result.
LECTROSONICS, INC. 4