Lectrosonics UM300b - Manual (Page 5)

Frequency Agile UHF Belt-Pack Transmitter
NO PRE-EMPHASIS/DE-EMPHASIS
The signal to noise ratio of the 300 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 noticeable 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 300 system. The dual-band compandor in the 300 Series system essentially provides a
dynamic pre-emphasis/de-emphasis function with extremely low distortion.
PILOT TONE SQUELCH
The 300 system utilizes an ultrasonic tone modulation of the carrier to operate the receiver squelch. This “pilot tone”
consists of a 32kHz signal mixed with the audio signal following the microphone preamp, just after the compandor, to
control the audio output muting of the receiver. The pilot tone is filtered out of the audio signal immediately after the
detector in the receiver so that it does not influence the compandor or various gain stages. 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. This is extremely important
in applications that include an automatic microphone mixer.
WIDE-BAND DEVIATION
±50kHz 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
High efficiency circuits throughout the design allow over 7hours of operation using a single 9 Volt alkaline battery. (A
9V lithium battery will provide over 17 hours of operation.) The battery compartment is a unique mechanical design
which automatically adjusts to fit any brand of 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 a maximum of 256 frequencies in 25kHz steps
minimum over a maximum range of 25.5MHz. This alleviates carrier interference problems in mobile or travelling
applications.
ANTENNA
At UHF frequencies, where wavelengths and antennas are shorter than at VHF frequencies, a resonant length wire is
preferred over using the microphone cable as the antenna. The antenna on the UM300B consists of a flexible 1/4
wavelength bronze cable, detachable via an SMA connector. The impedance of this connector is 50 Ohms.
Rio Rancho, NM – USA 5