Lectrosonics CR185 User Manual download pdf (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

1 2 3 4 5 6 7 8 ... 11 12

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Lectrosonics CR185 User Manual Page 3

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