Lectrosonics LMa - Manual (Page 5)

Frequency-Agile UHF Belt-Pack Transmitter
Rio Rancho, NM 5
The pilot tone frequency is chosen according to which
of the 256 channels has been selected by the frequen-
cy switch setting. This ensures that all transmitters in
a system have different pilot tone frequencies so even
spurious RF from the wrong transmitters can’t open the
receiver squelch.
Circulator/Isolator
The transmitter RF output circuit includes a specialized
RF device called a “circulator/isolator” or simply “isola-
tor.” The device uses a magnetically polarized ferrite to
allow RF signals to pass through to the antenna, but it
blocks them from coming back into the output amplifier.
This greatly reduces RF intermodulation produced in
the transmitter output stages when multiple units are
used in close proximity (a few feet apart). The isolator
also helps protect the output stage from electrostatic
shock delivered to the antenna. Isolators are common
in broadcast and commercial applications, but because
of their high cost it is unusual to find them in wireless
microphone systems.
Input Limiter
The transmitter employs a digitally-controlled analog
audio limiter prior to 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 keep audio distortion low and preserve
short term dynamic changes in the audio.
No Pre-Emphasis/De-Emphasis
Because the signal to noise ratio of the hybrid system is
so high, there is no need for conventional pre-emphasis
(HF boost) in the transmitter and de-emphasis (HF roll
off) in the receiver. Thus, the possible distortion prob-
lems associated with pre-emphasis and de-emphasis
are eliminated.
Pilot Tone Squelch
The DSP in the transmitter generates one of 256 differ-
ent ultrasonic tones between 25 and 32 kHz to operate
the receiver squelch (audio muting). The benefit of a
pilot tone squelch system is that the receiver will remain
quiet 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 pilot tone also
eliminates noise (pops, thumps, etc.) from occurring
when the transmitter is powered on and off.
LMa Block Diagram
Freq
Switches
Bicolor
Modulation
LEDs
9V
Battery
Bicolor
Power
LED
+5v