Lectrosonics UM195 User Manual download pdf (Page 5)

The signal to noise ratio of the 195 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 IF filtering stage in a 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 195 system. The dual-band compandor in the 195

Series system essentially provides a dynamic pre-emphasis/de-emphasis function with extremely low distortion.

The 195 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 such as with an automatic sound system.

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. 75kHz deviation is frequently used in the UHF spectrum, but it is

much harder to implement at VHF frequencies. The 195 system is the first to take advantage of this wider

deviation now allowed by the FCC in the VHF spectrum. Almost all other VHF wireless systems use 15kHz

deviation.

High efficiency circuits throughout the design allow over 7 hours of operation on the UHF version, and over 10

hours on the VHF versions, using a single 9 Volt alkaline battery. The battery compartment is a unique mechanical

design which automatically adjusts to fit any brand alkaline battery. The battery contacts are spring loaded to

prevent "rattle" as the unit is handled.

The transmitter section uses a crystal stabilized main oscillator followed by a quadrupler and three doubler stages.

The crystal controlled frequency is extremely stable over a wide temperature range and over time. Double tuning

used in the multiplier stages provides higher attenuation of spurious emissions which, in turn, minimizes the

possibility that the transmitter RF output would interfere with another transmitter/receiver system operating in the

same vicinity.

At UHF frequencies, where wavelengths and antennas are shorter than VHF, a resonant dangling wire is preferred.

The antenna on the UM195 consists of a flexible 1/4 wavelength wire, detachable via a twist lock connector. The

impedance of this connector is 50 Ohms at UHF frequencies.

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Lectrosonics UM195 User Manual Page 5