Lectrosonics DR185 User Manual download pdf (Page 3)

Ratio Diversity Receiver

GENERAL TECHNICAL DESCRIPTION

The DR185 receiver consists of two independent receivers operating with common first and second oscillators. The

audio outputs of the two receivers are mixed via a comparator circuit in a ratio controlled by sampling and comparing

the signal to noise ratios in both receivers. High frequency noise is compared between the receivers and an elec-

tronic “panning” circuit mixes in a greater percentage of audio from the quieter receiver. The panning action is

damped to prevent ill effects caused by transients in the signal. The purpose of this approach is to utilize the maxi-

mum amount of useable audio from both receivers simultaneously. By sampling and mixing the audio rather than

using RF level as reference, several major benefits are enjoyed, including:

a) The diversity action occurs irrespective of RF level.

b) No audible effects of “hard-switching” are produced.

c) A 3dB improvement in signal/noise ratio occurs with a strong RF signal at both receivers.

Each 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 audio demodulator and the compandor. The receivers share a common

comparator circuit and audio section.

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. Rejec-

tion 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 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 LED drive are provided by one mono-

lithic 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 in-

creases 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 pro-

duces 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.

DOUBLE BALANCED

ACTIVE FILTERS

ANT

LC FILTER

IF AMP

2nd

RECEIVER "A"

RECEIVER "B"

6 SECTION

HELICAL RESONATOR

DIODE MIXER

1st

OSCILLATORS

6 POLE HIGH

FREQUENCY

CRYSTAL FILTER

ACTIVE MIXER

DOUBLE TUNED

DISCRIMINATOR

SEPARATING

NOISE AND AUDIO

OPTICALLY COUPLED

PANNING CIRCUIT

COMPLIMENTARY

EXPANDER

BALANCED

OUTPUT STAGE

DR185 Block Diagram

Rio Rancho, NM – USA

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

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Lectrosonics DR185 User Manual Page 3

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