Lectrosonics MM200 - Manual - Manual (Page 5)

Frequency Agile UHF Miniature Belt-Pack Transmitter
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 400 system. The dual-band compandor in the modeled 200 Series system essentially
provides a dynamic pre-emphasis/de-emphasis function with low distortion.
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 200 system utilizes an ultrasonic tone that modulates the carrier to operate the receiver squelch. The “pilot tone”
frequency is 32.765 kHz which is low enough to be passed by the system but not so low that it can be heard. The
pilot tone squelch system will keep 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.
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.
Rio Rancho, NM – USA 5