General Technical Description - Lectrosonics CR187 Operating Instructions And Troubleshooting Manual

Compact receiver

Hide thumbs Also See for CR187:

Instruction manual (12 pages)

Table Of Contents

Table of Contents

The CR187 receiver is comprised of six major functional subsystems: the RF front-end amplifier, the double bal-

anced 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 approxi-

mately 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 clock-

wise 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.

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.

2 SECTION

HELICAL

RESONATOR

ANT

JFET

RF AMP