Lectrosonics MM400 - Manual - Manual (Page 5)

Frequency Agile UHF Miniature Belt-Pack Transmitter
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 the pre-emphasized 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 alto­
gether. Neither of these problems occur in the 400 system.
LOW FREQUENCY ROLL-OFF
A 12 dB per octave low frequency roll-off is provided in the audio section, with the -3dB point at 70 Hz. The actual
roll-off frequency will vary somewhat according to the low frequency response of the mic capsule being used.
The low frequency roll-off is used to remove subsonic (or very low frequency) audio, often produced by air condition­
ing systems, automobile traffic and other sources from the audio signal. Excessive low frequency content in the
audio input can cause a variety of audio problems including driving the transmitter into limiting. In sound reinforce­
ment systems, as one instance, excessive low frequency content can cause excessive power amplifier drain or even
damage to loudspeaker systems.
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 pilot tone squelch system keeps the receiver 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 MM400 extends this concept even farther 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.
BATTERY LIFE
Switching power supplies throughout the design allow about 4.5 hours of operation using a single AA lithium battery.
(An alkaline or NiMh AA battery will provide about 2 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.
CIRCULATOR
The RF output circuit includes a one way circulator/isolator using a magnetically polarized ferrite. This device greatly
reduces RF intermodulation produced when multiple transmitters are used at separations of less than five feet. It
also provides additional RF output stage protection but is rarely seen in a wireless microphone transmitter due to its
high cost.
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