Lectrosonics DR185 User Manual

DR185

RATIO DIVERSITY RECEIVER

OPERATING INSTRUCTIONS

and trouble-shooting guide

LECTROSONICS, INC.

Rio Rancho, NM

INTRODUCTION

Thank you for selecting the Lectrosonics system.

The DR185 represents over 70 years of combined experience in the design of RF
and audio devices and sets new standards for RF performance and flexibility. This
receiver utilizes a “maximal ratio combining” technique, providing a very effective
type of diversity reception. This is the first high performance receiver on the market
to utilize this technique to combine the outputs of two separate receivers without
hard-switching. The result is seamless audio reproduction without dropouts and
superb, noise-free operation, even in weak overall signal conditions.

This is a “true diversity” receiver in the purest sense. The audio outputs of two
separate receivers are mixed via a non-switching circuit that blends the two audio
signals together in an optimum ratio. The ratio of this mixture is controlled by a
circuit that monitors the comparative signal to noise ratios in both receivers and then
“pans” back and forth to regulate the mixture. The receiver with the least high
frequency noise is favored in the resultant blend.

The DR185 receiver was designed for professional users who demand outstanding
performance and flexibility. It is compatible with all Lectrosonics high band
transmitters.

TABLE OF CONTENTS

INTRODUCTION ................................................................................................... 2

GENERAL TECHNICAL DESCRIPTION ............................................................. 3

FRONT PANEL CONTROLS AND FUNCTIONS ................................................. 4

REAR PANEL CONTROLS AND FUNCTIONS ................................................... 5

ANTENNA USE AND PLACEMENT .................................................................... 6

OPERATING INSTRUCTIONS ............................................................................. 7

INDICATOR QUICK REFERENCE ....................................................................... 7

TROUBLESHOOTING ........................................................................................... 8

REPLACEMENT PARTS and ACCESSORIES.................................................... 8

SPECIFICATIONS AND FEATURES.................................................................... 9

SERVICE AND REPAIR ...................................................................................... 10

RETURNING UNITS FOR REPAIR .................................................................... 10

WARRANTY ........................................................................................... Back cover

2

GENERAL TECHNICAL DESCRIPTION

Ratio Diversity Receiver

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 elec­tronic “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 maxi­mum 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. Rejec­tion 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 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. This filter ensures that supersonic noise will not
cause the compandor to increase gain incorrectly. 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 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.

ANT

RECEIVER "A"

6 SECTION

HELICAL RESONATOR

ANT

RECEIVER "B"

DOUBLE BALANCED

DIODE MIXER

1st

CRYSTAL FILTER

OSCILLATORS

6 POLE HIGH
FREQUENCY

DOUBLE BALANCED

ACTIVE MIXER

2nd

LC FILTER

IF AMP

DOUBLE TUNED
DISCRIMINATOR

ACTIVE FILTERS

SEPARATING

NOISE AND AUDIO

N

A

N

A

COMPLIMENTARY

EXPANDER

OPTICALLY COUPLED

PANNING CIRCUIT

BALANCED

OUTPUT STAGE

DR185 Block Diagram

Rio Rancho, NM – USA

3

FRONT PANEL CONTROLS AND FUNCTIONS

MODULATION INDICATORS

Nine (9) LEDs indicate the audio level (modulation) of the incoming signal, which are typically used for proper adjust­ment of the transmitter’s “MIC LEVEL” or “GAIN”. The leftmost LED (red) functions as a pilot lamp to indicate that the
receiver is powered up. The seven green LEDs indicate the modulation level of the signal. The rightmost yellow LED
indicates maximum modulation and that the transmitter audio input may be “limiting.”

OFF/MUTE/ON

This switch turns the receiver power off and on and selects either an “audio mute” or “audio on” operation. The “audio
mute” position is normally used to cut the audio output from the receiver while observing the modulation LEDs to
adjust the transmitter gain.

RF INDICATOR

Two LEDs are provided to indicate the status of each receiver in the DR185. These LEDs glow when the transmitter
is turned on and the receiver has a good RF signal. When the carrier signal from the transmitter is too weak to
produce a clean audio signal, the lamp will go out.

OPTI-BLEND LEDs

The audio outputs of two separate receivers are mixed together in a “ratio” that is regulated by comparing the signal
to noise ratios in the two receivers. The mixture ratio of the two audio signals is indicated on the front panel by two
LEDs that vary in brightness. As a build-up of noise occurs in one receiver, the corresponding LED for that receiver
dims or goes out as the circuitry pans to the other receiver. Since the mixture ratio of the two audio signals is deter­mined by a comparison of their signal to noise ratios, the “blending” action occurs regardless of how strong (or weak)
the incoming RF signals are. This is the distinct advantage of this receiver design over any other type of diversity
system available.

OUTPUT ATTENUATOR

This control knob adjusts the output levels of the two balanced outputs on the rear panel (XLR and 1/4"). In a fully
clock-wise position, the output at the XLR jack will be 100mV when the signal is a full modulation. The “-3,” “-12dB”
and “-20” markings indicate the knob position that will lower the audio output by the “dB” amount on the markings
around the knob.

LECTROSONICS

DIVERSITY RECEIVER

DR185

-24 -20 -15 -12 -7 -3 0 +3

dB AUDIO LEVEL

RF

A B

OFF ON

AUDIO

MUTE

POWER

-12dB

-20

-50

0dB

OUTPUT

ATTENUATOR

N

-3

O

R

M

A

L

OPTI

BLEND

A B

DR185 Front Panel

4