Lectrosonics UCR310 - Manual (Page 4)

DOUBLE BALANCED DIODE MIXERS
In all wireless receivers, a mixer is used to convert the carrier
frequency to the IF frequency where most of the filtering and gain
in the receiver takes place. After doing all the right things in the
front end, it would be a shame to waste the performance with a
second rate mixer. In other designs that is exactly what happens
since mediocre mixers cause more intermodulation problems than
mediocre front ends. The only solution was a high power, double
balanced diode mixer driven by a local oscillator with more output
power than most wireless transmitters (50 mW). The mixer in the
UCR310 produces output at only the sum and difference signals,
with minimal spurious signals. This mixer offers a very high
overload threshold and a high degree of isolation between ports.
The IF output of this mixer is at 71 MHz which is unusually high
for a wireless receiver. This high frequency was chosen to increase
the image rejection in the front end to as high or a higher level than
our fixed frequency designs. The mixer is followed by high cur­
rent, low noise amplifiers and SAW filters to preserve the superior
RF performance.
SURFACE ACOUSTIC WAVE FILTER
The UCR310 is unique in that it uses a state of the art SAW filter in
the IF section. The SAW filter is the only filter that can combine
sharp skirts, constant group delay, and wide bandwidth in one filter.
Though expensive, this special type of filter allows us to follow the
basic receiver rule of doing the primary filtering as early as pos­
sible, 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. After following the rule in a rigorous way, and due to the
sharp filtering action of the SAW filters, the 71MHz signal is
converted to 10.7Mhz and then to the low frequency of 300 kHz.
Lots of gain is then applied in a conventional IC and the signal is
then converted to audio. 300 kHz is very unconventional for a
second IF in a wide deviation (±50 kHz) system. We chose to use
300 kHz to obtain an outstanding AM rejection figure over a very
wide range of signal strengths and to produce an excellent noise
improvement at low signal strengths (capture ratio). To use an IF at
300 kHz requires an unusual circuit to convert the IF to audio.
DIGITAL PULSE COUNTING DETECTOR
The UCR310 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 ther­
mal drift, particularly those that operate at higher frequencies like
10.7 MHz. Though the quadrature detectors may work well at
room temperature, if they are not carefully compensated, they will
produce amplitude changes and audio distortion in the elevated
temperatures of an equipment rack. Some manufacturers try to get
around the problem by tuning their systems at higher temperatures
after they’ve been on for some time. This just means that for the
first hours in a cool room the receiver is well out of specification or
after a few hours in a hot rack.
The UCR310 design presents an elegantly simple, yet highly effec­
tive solution to this age old problem. The UCR310 detector
basically works like this: A stream of precision pulses is generated
at 300kHz 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. An­
other way of describing it is that as the FM modulation increases
the frequency, the circuit produces more pulses and as the modula­
tion decreases the frequency, the circuit produces fewer pulses.
More pulses produces 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, in addition to the limiting in the
IF section. The amplitude of the pulses is constant, so level
differences in the IF signal do not affect the pulse.
TRI MODE DYNAMIC FILTER
The audio signal is passed through a “dynamic noise reduction
circuit”. The cutoff frequency of this filter is varied automatically
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. Keep in mind that if hiss is
added to a signal, there is a psycho acoustic effect that makes the
sound seem brighter. The other side of this is that if hiss is removed
from a signal it will sound duller. Basically the ear’s detection
apparatus is pre-sensitized to high frequency sounds by small
amounts of high frequency hiss. Consider this effect when making
a judgment about the sound quality of various wireless systems and
this particular filter. We have satisfied ourselves through elaborate
tests that this filter is totally transparent.
PILOT TONE MUTE
The UCR310 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.
If the PILOT TONE BYPASS button is pressed, received audio
remains unmuted regardless of the presence or absence of a pilot
tone. This position is useful for locating a clear frequency, since
any potential interference may be heard. It may also be used in
situations where squelching behavior is undesirable. The “PILOT
TONE BYPASS” disables the squelch, as described below.
SMART SQUELCH
The UCR310 employs a sophisticated squelching system in an
attempt 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; re­
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