User's Manual

5
Frequency Agile UHF Miniature Belt-Pack Transmitter
Rio Rancho, NM – USA
There are actually two separate compandors in the 200 system, one for high frequencies and one for low frequencies.
A crossover network separates the frequency bands at 1kHz with a 6dB per octave slope, followed by separate high
and low frequency compandors. The attack and release times in the high frequency compandor are fast enough to keep
high frequency transient distortion at a low level, and the low frequency compandor uses slower time constants,
reducing low frequency distortion to well below that of a conventional compandor.
NO PRE-EMPHASIS/DE-EMPHASIS
The signal to noise ratio of the 400 and 200 systems is high enough to preclude the need for conventional pre-empha-
sis (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 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 conditioning
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 reinforcement
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 MM200, however, has only a single tone at 32.765 kHz. 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. 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.