Lectrosonics Inc LM400 ユーザーズマニュアル
5
Frequency-Agile UHF Belt-Pack Transmitter
Rio Rancho, NM – USA
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 perfor-
mance that rivals that of wholly digital systems, without the power and bandwidth problems inherent in digital transmis-
sion. 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.
Hybrid simply encodes the signal to use a noisy channel as efficiently and robustly as possible, yielding audio perfor-
mance that rivals that of wholly digital systems, without the power and bandwidth problems inherent in digital transmis-
sion. 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 sys-
tems, the Digital Hybrid has done away with the analog compandor and its artifacts.
such as excellent range, efficient use of RF spectrum, and long battery life. However, unlike conventional FM sys-
tems, 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.
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
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.
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.
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.
(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.
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.
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.