5
UHF Wireless Diversity Receiver
Rio Rancho, NM – USA
SURFACE ACOUSTIC WAVE (SAW) FILTER
A state-of-the-art SAW filter on the 244 MHz IF amplifier
offers sharp skirts, constant group delay, and wide
bandwidth in one filter. Though expensive, this special
type of filter allows primary filtering as early as possible,
at as high a frequency as possible and before high gain
is applied to the signal. Since these filters are made of
quartz, they are very temperature stable. (Conventional
LC filters at these frequencies don’t begin to perform as
well and in addition would drift unacceptably in the
elevated temperatures of an equipment rack or in the
field where temperatures are in constant flux.) After the
SAW filter, the 244MHz IF signal is converted to 10.7
Mhz IF and then to the low frequency of 300 kHz. Only
then is the majority of the gain applied, just before the
signal is converted to audio. Although 300 kHz is very
unconventional for an IF in a wide deviation (±75 kHz)
system, it offers outstanding AM rejection figure over a
very wide range of signal strengths and to produces an
excellent noise improvement at low signal strengths
(capture ratio).
DIGITAL PULSE COUNTING DETECTOR
The UCR301 receiver uses an advanced digital pulse
detector to demodulate the FM signal, rather than a
conventional quadrature detector. The common problem
with quadrature detectors is thermal drift, particularly
those that operate at higher frequencies like 10.7 MHz.
The UCR301 design presents an elegantly simple, yet
highly effective solution to this age old problem. A
stream of precision pulses is generated at 300kHz and
locked to the FM signal coming from the 300 kHz IF
section. The pulse width is constant, but the timing
between pulses varies with the frequency shift of the FM
signal. The integrated voltage of the pulses within any
given time interval varies in direct proportion to the
frequency modulation of the radio signal. Another way of
describing it is that as the FM modulation increases the
frequency, the circuit produces more pulses and as the
modulation decreases the frequency, the circuit
produces fewer pulses. More pulses produce a higher
voltage and fewer pulses a lower voltage. The resultant
varying voltage is the audio signal.
This type of detector eliminates the traditional problems
with quadrature detectors and provides very low audio
distortion, high temperature stability and stable audio
level. The counting detector also adds additional AM
rejection. Since the detector pulse amplitudes are
constant, level differences in the IF signal do not affect
them.
TRI MODE DYNAMIC FILTER
The detected audio signal is passed through a “dynamic
noise reduction circuit,” which automatically adjusts the
filter’s cutoff frequency by measuring the amplitude and
frequency of the audio signal and the quality of the RF
signal. The audio bandwidth is held only to that point
necessary to pass the highest frequency audio signal
present at the time. If the RF level is weak, then the filter
becomes more aggressive. This results in a dramatic
reduction of “hiss” at all times. During passages with a
high frequency content, this filter gets completely “out of
the way” and passes the signal with no decrease in high-
frequency response.
PILOT TONE MUTE
The UCR301 uses a pilot tone muting technique in order
to protect against the reception of stray signals. The
Lectrosonics transmitter adds an inaudible signal, known
as the pilot tone, to the transmitted signal. The receiver
detects (and removes) the pilot tone, and is thus able to
identify the desired signal and mute all others.
When the receiver is powered up, receive audio is muted
unless a proper pilot tone is detected. The pilot tone
must be present for approximately one second before
the signal is accepted.
A PILOT TONE BYPASS mode is available. In this
mode, the received audio remains unmuted regardless of
the presence or absence of a pilot tone. This mode is
useful for locating a clear frequency, since any potential
interference may be heard. Because this mode disables
the squelch, it may also be used in situations where
squelching behavior is undesirable.
SMART SQUELCH™
The UCR301 employs a sophisticated squelching system
to deliver the cleanest possible audio during marginal
conditions of reception. Any squelching system faces
inevitable trade-offs: squelch too much and valuable
audio information may be lost, squelch too little and
excessive noise may be heard; respond too rapidly and
the audio sounds “choppy”, respond too sluggishly and
syllables or entire words are cut off.
SmartSquelch
TM
achieves an optimal balance of these
trade-offs by combining several techniques that remove
distracting noise without the squelching action itself
becoming a distraction. Thse include: waiting for a
complete word or syllable before squelching, assessing
recent squelching history and RF signal strength and
assessing audio content to determine available masking.
By adjusting squelching behavior dynamically for the
optimal result under varying conditions, the receiver can
deliver acceptable audio quality from otherwise unusable
signals.
OUTPUT LEVEL ADJUST
Convenient front panel controls allow adjusting the audio
output in 1dB steps from -50 to +9 dBu.
POWER SUPPLY
The UCR301 may powered by a 9 VDC battery, or from
an external DC source (see Specifications and Features
section for allowed voltages.) A built in Poly-Fuse
