Listen Technologies Portable FM 863 MHz, 863.050, 863.400, 863.5, 863.200 Design Manual

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905 22 Oberasbach, Germany
[phone]
[phone]

Tel

Fax

www.listentech.de
[e-mail]

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Best regards,

Russell Gentner and the Listen Team

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Assistive Listening • Language Interpretation • Soundfi eld • Tour Group • Conferencing

Portable FM Table of Contents

Design Guide
FM Technology Overview 5
System Overview 6
Key Concepts in Designing an FM System 8
Notes 10

LT-700-863 Portable FM Transmitter
Specifi cations 17
Block Diagram 18
Quick Reference 19
Setup Instructions 20
Operating Instructions 23
Programming Instructions 25
Accessories 28
Notes 29

LR-400/500-863 FM Receivers
Specifi cations 37
Block Diagram 39
Quick Reference 40
Setup Instructions 42
Operating Instructions 45
Programming Instructions (LR-500 only) 51
Accessories 52
Notes 53

Design Guide

LT-700

Supplementary Information
Frequency Chart 61
Battery Charging Information 62
Troubleshooting 63
Frequently Asked Questions 65
Compliance, Warranty and Contact Information 67
Notes 68

LR-400/500

Supplementary

Portable FM Design Guide Table of Contents

FM Technology Overview 5
System Overview 6
Key Concepts in Designing an FM System 8
Notes 10

Design Guide

3

Frequency Modulation (FM) Technology Overview

Frequency modulation (FM) is a means of transmitting audio using electromagnetic waves.
This same technology is used by local FM radio stations to broadcast voice and music. FM signals can travel
through most barriers – walls, fl oors, and ceilings. The distance a signal travels has many different variables
such as Radio Frequency (RF) output power, the type and placement of the antenna and the broadcasted
frequency. This section of the manual will help you design a system that will get the best range with the least
amount of interference.

Microphone

Design Guide

FM Transmitter

When designing an FM system it is important to keep the following in mind:

Receiver

• Multipath

Multipath interference is a form of RF interference that occurs when radio signals have more than one path
between the receiver and the transmitter causing the two RF signals to add out of phase. This could occur in areas

with RF-refl ective surfaces, such as furniture, walls, or fl oors. The result of multipath is the receiver audio coming in
and out of squelch when the receiver is moved. Portable applications are much more susceptible to multipath
than those that are stationary. Multipath distortion is often worse as the distance between transmitter and receiver
increases (RF power decreases). Multipath can decrease the audio quality of the transmission (refer to page 9 for

strategies on eliminating multipath).

• Antenna Positioning

For full RF functionality of the portable system, the antenna on both the LT-700 transmitter and receiver (LR-400/500)

must be in the upright position (shown below).

5

System Overview

Design Guide

Behind-the-Head Microphone

There are three main parts to a Portable FM System – Input source, transmitter, and receiver.

LA-278

(other microphones available)

LT-700-863

Portable Display FM

Transmitter (863 MHz)

LR-400-863

Portable Display FM

Receiver (863 MHz)

LR-500-863

Portable Programmable

Display FM Receiver (863 MHz)

Input Source

A microphone is the most common input source for the LT-700 transmitter, although, the LT-700 can transmit
two audio inputs at once on one transmission channel. With the use of the LA-263 both the inputs will be mixed.
Thus, a person can use a line input source and mic source.

Mic input

3.5mm Phone Wiring

Unbalanced

+ Tip

- Sleeve

+ Ring

Line input

-

6

System Overview

Transmitter

The LT-700 modulates the audio on an FM carrier and transmits the signal via an anten­na on the unit.

Design Guide

LT-700-863

Portable Display FM

Transmitter (863 MHz)

Receivers

Listen offers two different portable receivers.

LR-400-863

Portable Display FM

Receiver (863 MHz)

LR-500-863

Portable Programmable

Display FM Receiver (863 MHz)

7

Key concepts in designing a portable FM system

Channel Selection

Design Guide

Use this section of the guide to choose the channel settings for the transmitter and receivers.

• The goal is to fi nd a transmission channel(s) that is free from interference. Interference originates from
other transmitters and from other equipment (such as a computer).

• Listen products offer 17 different channels to choose from. This increases the chance you’ll fi nd an

interference free channel.

• The best way to check for interference is to use the “SEEK” button on the receiver. Be sure all Listen

transmitters have been turned off if there is interference, the receiver will fi nd these channels.
If the receiver fi nds a frequency in use, do not use this channel.

• In general, most of the channels will be free from interference.

• Interference can be overcome by maximizing the transmission power and by using the receivers close to
the transmitting antenna.

• If more than one channel is in use, space the channels further from each other. Use the Listen frequency
chart on page 61 to determine the appropriate channels.

EXAMPLE: 2 channel system use channels 01 & 11
3 channel system use channels 01, 06 & 11

Range

The range of a portable transmission will vary from one application to another. There are many different
obstructions that could minimize the range. In most applications, the range will be approximately as shown
below (P= Power Level - refer to page 26 for programming power levels of the LT-700).

P1 – 30 meters (100 ft)
P2 – 46 meters (150 ft)
P3 – 61 meters (200 ft)

Range varies depending on environment.

Maximizing Range
Here are some tips to maximize the transmission range:

• Eliminate or minimize obstructions between the transmitter and receivers.

• Minimize the distance between the transmitter and receivers.

• Move transmitter and receivers away from metal objects.

• Orient both transmitting and receiving antennas vertically.

8

Key concepts in designing a portable FM system

Eliminating Multipath

As mentioned on page 5, multipath can occur with portable RF systems. The audio quality of the
transmission can decrease with the occurrence of multipath. Here are a few potential improvements that
can help if multipath takes place:

Squelch

Squelching the receiver will mute the audio output when the signal from the transmitter is turned off or
the signal is too weak to be received. Without squelch you would hear radio noise in your earphone with
no RF signal present. “Tightening” the squelch will turn the receiver off if it is chattering because of
multipath (refer to page 48 for programming squelch). Keep in mind, “tightening” squelch will limit the
range between transmitter and Receiver.

Transmitter power level

The RF power level can be adjusted on the LT-700 (refer to page 26 for programming LT-700 power levels).
The lower the power setting, the less range the transmitter will broadcast. Decreasing the broadcast range
will force the receivers to remain closer to the transmitter. To maintain an RF signal, the higher power
settings will increase range between transmitter and receiver thus introducing a higher probability of
multipath.

Multiple Portable Systems

In some applications, there will be more than one RF transmission in the same general vicinity.
Intermodulation interference can occur if the transmission frequencies are too close together. Here are
some ways to eliminate the chance of RF interference:

Design Guide

Squelch

Much like a multipath situation, “tightening” the squelch on the receiver will mute the audio output when
the signal is too weak to be received. Tightening the squelch will help with force the receiver to squelch
when signal becomes too weak. (refer to page 48 for programming squelch).

Transmitter power level

As mentioned above, the RF power level can be adjusted on the LT-700 (refer to page 26 for programming
LT-700 power levels). The lower the power setting, the less range the transmitter will broadcast. Lowering the
power could help decrease the amount of interference if there are many frequencies being used in the
vicinity.

Spacing of channels

Use a Listen frequency chart (on page 61) to determine the channels frequency and space the number of
channels further from each other.

DX/Local

The receiver can be adjusted to receive only the strongest signals (refer to page 50 for DX/Local
information). A DX signal will be less sensitive because distance is the ultimate goal. For applications where
you want to minimize the distance, local mode will only search for strong signals. Local mode is much
more sensitive than DX.

9

Notes

Design Guide

10

Notes

Design Guide

11

800

LT-700 User’s Manual Table of Contents

Specifi cations 17
Block Diagram 18
Quick Reference 19
Setup Instructions 20
Operating Instructions 23
Programming Instructions 25
Accessories 28
Notes 29

LT-700 Package Contents

• LT-700-863

• Quick Reference Card

• Warranty Card

• FM Manual Card

LT-800LT-

LT-700

Listen Confi gurations

• LT-700-863

LT-700-863

Portable Display FM

Transmitter (863 MHz)

15

800

LT-700-863 Specifi cations

Architectural Specifi cation

The LT-700-863 portable FM Transmitter shall be capable of broadcasting on 17 channels. Channel tuning shall be capable of being
locked. The transmitter shall have a SNR of 70 db or greater. The output power shall be adjustable to quarter, half or full. The device
shall have an audio frequency response of 50 Hz to 15 kHz, +/-3 dB. The device shall incorporate a microphone sensitivity switch.
The device shall incorporate a mute switch. The unit shall utilize two 2 AA batteries. The battery door shall be capable of being
mechanically locked. The device shall incorporate an LCD display that indicates battery level, channel, channel lock, low battery,
battery charging, programming, and RF signal strength. The portable transmitter shall incorporate automatic battery charging
circuitry for recharging of NiMH batteries. The Listen LT-700-863 is specifi ed.

Specifi cations LT-700-863

RF Frequency Range 863.050 - 864.950 MHz
Number of Channels 17 wide band
Frequency Accuracy ± .005% stability 32 to 122º (0 to 50º C)
Transmitter Stability 50 PPM

RF

Transmission Range 0 ft. (0 m) to 200 ft. (60.9 m)
Output Power 10 mW maximum (adjustable)
Antenna Uses microphone cable
Antenna Connector 3.5 mm connector
Compliance CE, ETSI, RoHS

LT-800LT-

LT-700

Audio

Controls

Indicators

Power

Physical

System Frequency
Response

System Signal to Noise
Ratio

System Distortion <2% total harmonic distortion (THD) at 80% deviation
Microphone Input
Microphone Sensitivity Three position switch: high, middle, and low; 6 dB increments
Line Input
Microphone Power 3 VDC Bias

User Controls Power, mute, channel UP/DOWN
Set-up Controls (battery

compartment)
Programming Channel Lock out, Channel lock

LED

Display

Battery Type Two AA batteries, alkaline or NiMH
Battery Life

(Listen batteries)
Battery Charging

(NiMH only)

Dimensions (H x W x D) 3.00 x 1.00 x 5.00 in. (7.6 x 2.5 x 13.0 cm)
Color Dark Grey with white silk screening
Unit Weight 3.9 oz (111 g)
Unit Weight with batteries 5.8 oz (164 g)
Shipping Weight 1.0 lbs. (0.45 kg)

Door

50 Hz - 15 kHz (± 3 dB)

70 dB (A-Weighted)

Unbalanced, tip of 3.5mm connector, -20 dBu nominal, -30 dBu
maximum, impedance 21 ohms

Unbalanced, ring of 3.5mm connector, -10 dBu nominal input
level, -3 dBu maximum, impedance 10K ohms

Mic sensitivity, NiMH/alkaline battery, SQ enable/disable

Red, illuminated when unit is on. Flashes when batteries are
low, or to indicate charging. Flashes when muted

Channel Designation, lock status, signal strength indication,
battery life, RF Power

15 hours alkaline (LA-361), 8 hours NiMH rechargeable (LA-

362)
Fully automatic, 13 hours

Manually Lockable. Up, down, and power buttons through
door. Other controls behind door (see controls)

Environmental

Temperature - Operation 14 to 104º F (-10 to 40º C)
Temperature - Storage -4 to 122° F (-20 to 50° C)
Humidity 0 to 95% relative humidity, non-condensing

Specifi cations are subject to change without notifi cation

17

00

LT-700 Block Diagram

LT-800

LT-700

115/230VAC

50/60 Hz

LCD Display

Universal

Power Supply

(not included)

ANTENNA

7.5 VDC

Power Supply

CPU Module

Alkaline

(2) AA Batteries

ALkaline or NiMH

NiMH

Pre-emphasisRF Modulation

3 VDC

Companding

Q-Technology

Up

Down

Power

Power charge indicator

Red LED

Mic

Tip

Sleeve

Line

Ring

18