Lectrosonics LM400 Users Manual
LM400
FREQUENCY-AGILE UHF
BELT-PACK TRANSMITTER
OPERATING INSTRUCTIONS
and trouble-shooting guide
LECTROSONICS, INC.
Rio Rancho, NM
www.lectrosonics.com
LM400
TABLE OF CONTENTS
INTRODUCTION ..................................................................................................... 3
GENERAL TECHNICAL DESCRIPTION ............................................................... 4
CONTROLS AND FUNCTIONS ............................................................................. 6
BATTERY INSTALLATION ..................................................................................... 8
OPERATING INSTRUCTIONS............................................................................... 8
OPERATING NOTES ............................................................................................. 9
ADJUSTING THE TRANSMITTER FREQUENCY ................................................ 9
TROUBLESHOOTING .......................................................................................... 10
SPECIFICATIONS AND FEATURES .................................................................. 11
SERVICE AND REPAIR ....................................................................................... 12
RETURNING UNITS FOR REPAIR ..................................................................... 12
WARRANTY............................................................................................Back cover
The LM400 transmitter is FCC type accepted under Part 74: 536-608 MHz and 614-806 MHz
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LECTROSONICS, INC.
Frequency-Agile UHF Belt-Pack Transmitter
INTRODUCTION
Thank you for selecting the Lectrosonics LM400 frequency agile, belt-pack transmitter. The LM400 combines over 80 years of engineering experience with the very latest
components, in a design that addresses the most demanding professional applications.
The design of the LM400 was the direct result of numerous conversations with users,
staging and touring companies and dealers across the US. The specific concerns and
needs brought up in these conversations led directly to the development of the
operational features offered on the LM400. Two hundred fifty six frequencies are user
selectable in 100kHz steps to alleviate interference problems in traveling venues, and
the low frequency roll-off is user adjustable to adapt to varying acoustic environments
and preferences.
The LM400 is a rugged, machined aluminum package with a removable, spring loaded
belt clip. The input section provides 3.3 Volts of bias voltage to power electret mics.
Level indicating LEDs are provided on the control panel to make level settings quick
and accurate, without having to view the receiver. The battery compartment accepts
any 9 Volt alkaline battery and makes a positive connection via self-adjusting contacts. The antenna is a permanently attached 1/4 wavelength flexible bronze cable.
Only the LM400 transmitter is covered in this manual. Companion receivers are
covered in separate manuals. The LM400 will operate with any 400 Series Lectrosonics receiver in the same frequency group.
Rio Rancho, NM – USA
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LM400
GENERAL TECHNICAL DESCRIPTION
GENERAL
The 400 system uses 75kHz wide deviation for an extremely high signal to noise ratio. The switching power supplies
provide constant voltages to the transmitter circuits from the beginning (9.3 Volts) to the end (5.5 Volts) of battery life.
The input amplifier uses an ultra low noise op amp for quiet operation. It is gain controlled with a wide range dual
envelope input compressor which cleanly limits input signal peaks over 30dB above full modulation.
DIGITAL HYBRID TECHNOLOGY
All wireless links suffer from channel noise to some degree, and all wireless microphone systems seek to minimize
the impact of that noise on the desired signal. Conventional analog systems use compandors for enhanced dynamic
range, at the cost of subtle artifacts (known as “pumping” and “breathing”). Wholly digital systems defeat the noise by
sending the audio information in digital form, at the cost of some combination of power, bandwidth and resistance to
interference.
The Lectrosonics Digital Hybrid system overcomes channel noise in a dramatically new way, digitally encoding the
audio in the transmitter and decoding it in the receiver, yet still sending the encoded information via an analog FM
wireless link. This proprietary algorithm is not a digital implementation of an analog compandor but a technique which
can be accomplished only in the digital domain, even though the inputs and outputs are analog signals. (As of this
writing, the patent is still pending, so we cannot reveal detailed information about the algorithm at this time.)
Mic
Jack
(Fits Switchcraft
plug #850.)
Bicolor
Modulation
LEDs
+3V Bias
Supply
Hi/Lo
Pass
Filter
Shunt
Limiter
Phase Locked Loop
4MHz
Reference
Audio
Level
Input
Amp
LF
Roll Off
Freq
Switches
Audio
Microprocessor
A-D
Converter
11001001
Digital Signal Processor
D-A
Converter
11001001
Voltage
Controlled
Oscillator
Encoded
Audio +
Pilot Tone
9V
Battery
Final
Amplifier
Switching
Power
Supply
+3.3v
+1.8v
+9v
-3v
Bicolor
Power
LED
LM400 Block Diagram
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LECTROSONICS, INC.
Frequency-Agile UHF Belt-Pack Transmitter
Channel noise still has an impact on received signal quality and will eventually overwhelm the receiver. The Digital
Hybrid simply encodes the signal to use a noisy channel as efficiently and robustly as possible, yielding audio performance that rivals that of wholly digital systems, without the power and bandwidth problems inherent in digital transmission. As always, these advantages come at a cost. The Digital Hybrid system requires fairly intensive digital
processing in both the transmitter and the receiver. These processors cost money, take up space and consume power.
The Digital Hybrid system also requires that the underlying RF link be of excellent quality, with better frequency
response and distortion characteristics than that required by conventional systems.
Because it uses an analog FM link, the Digital Hybrid enjoys all the benefits of conventional FM wireless systems,
such as excellent range, efficient use of RF spectrum, and long battery life. However, unlike conventional FM systems, the Digital Hybrid has done away with the analog compandor and its artifacts.
NO PRE-EMPHASIS/DE-EMPHASIS
The signal to noise ratio of the 400 system is high enough to preclude the need for conventional pre-emphasis (HF
boost) in the transmitter and de-emphasis (HF roll off) in the receiver. Pre-emphasis and de-emphasis in an FM radio
system usually provides about a 10dB improvement in the signal to noise ratio of the system, but the high frequency
boost in the transmitter must be removed in a purely complementary manner or else the frequency response of the
original audio signal will be altered.
Pre-emphasis can also cause distortion in the receiver. As this signal is passed through the IF filters in the receiver,
distortion can be produced, most noticeably at full modulation. De-emphasis cannot be applied until the signal is
converted into audio, so there is no way around this problem short of eliminating pre-emphasis altogether. Neither of
these problems occur in the 400 system
PILOT TONE SQUELCH
The 400 system utilizes one of 256 different ultrasonic tones between 25 and 32 kHz, that modulate the carrier to
operate the receiver squelch. The pilot tone frequency is chosen according to which of the 256 channels has been
selected by the frequency switch setting. The basic benefit of the pilot tone squelch system is that the receiver will
remain muted until it receives the pilot tone from the matching transmitter, even if a strong RF signal is present on the
carrier frequency of the system. The LM400 extends this concept even further by insuring that all transmitters in a
system have different pilot tone frequencies so that even spurious RF from the wrong transmitters can’t open the
receiver squelch.
WIDE-BAND DEVIATION
±75kHz deviation improves the capture ratio, signal to noise ratio and AM rejection of a wireless system dramatically,
compared to the more commonly used ±15kHz deviation.
LONG BATTERY LIFE
Switching power supplies throughout the design allow over 6 hours of operation using a single 9 Volt alkaline battery.
(A 9V lithium battery will provide over 20 hours of operation.) The battery contacts are spring loaded to prevent “rattle”
as the unit is handled.
FREQUENCY AGILITY
The transmitter section uses a synthesized, frequency selectable main oscillator. The frequency is extremely stable
over a wide temperature range and over time.
Two rotary switches, located on the side panel of the unit, provide 256 frequencies in 100kHz steps over a 25.5MHz
range. This alleviates carrier interference problems in mobile or traveling applications.
ANTENNA
The antenna on the LM400 consists of a permenantly attached flexible 1/4 wavelength bronze cable.
Rio Rancho, NM – USA
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