Input Limiter; No Pre-Emphasis/De-Emphasis; Pilot Tone Squelch; Wide-Band Deviation - Lectrosonics LM Operating Instructions Manual

Frequency Agile UHF Belt-Pack Transmitter

INPUT LIMITER

The LM transmitter employs a digitally-controlled analog audio limiter just before the analog-to-digital converter. The
limiter has a range greater than 30 dB for excellent overload protection. A dual release envelope makes the limiter
acoustically transparent while maintaining low distortion. It can be thought of as two limiters in series, connected as
a fast attack and release limiter followed by a slow attack and release limiter. The limiter recovers quickly from brief
transients, so that its action is hidden from the listener, but recovers slowly from sustained high levels, to both keep
audio distortion low and preserve short term dynamic changes.

NO PRE-EMPHASIS/DE-EMPHASIS

The signal to noise ratio of the hybrid 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 10 dB 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 hybrid 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 400 Series 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

Employing ±75 kHz dramatically improves the capture ratio, signal to noise ratio and AM rejection of our wireless
system compared to those that use the more common ±15 kHz deviation.

LONG BATTERY LIFE

The use of switching power supplies throughout the design allow over 6 hours of operation using a single 9 volt
alkaline battery and over 13 hours of operation with a 9 volt lithium battery. 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 100 kHz steps over a 25.5 MHz
range. This alleviates carrier interference problems in mobile or traveling applications.

ANTENNA

The antenna on the LM consists of a permanently attached flexible 1/4 wavelength galvanized steel cable.
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