Lectrosonics UCR205d - 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 transmit­
ters (50 mW). The mixer in the UCR205D 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 current,
low noise amplifiers and SAW filters to preserve the superior RF
performance.
SURFACE ACOUSTIC WAVE FILTER
The UCR205D is unique in that it uses state of the art SAW
filters in each IF section. The SAW filters are the only filter that
can combine sharp skirts, constant group delay, and wide band­
width 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 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. Conven­
tional 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 the low frequency of
455 kHz. Lots of gain is then applied in a conventional IC and
the signal is then converted to audio. 455 kHz is very uncon­
ventional for a second IF in a wide deviation (±50 kHz) system.
We chose to use 455 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 (cap­
ture ratio). To use an IF at 455 kHz requires an unusual circuit to
convert the IF to audio.
DIGITAL PULSE COUNTING DETECTOR
The UCR205D receiver uses an advanced digital pulse detector
to demodulate the FM signal, rather than a conventional quadra­
ture detector. The common problem with quadrature detectors
is thermal drift, particularly those that operate at higher fre­
quencies like 10.7 MHz. Though the quadrature detectors may
work well at room temperature, if they are not carefully com­
pensated, 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 UCR205D design presents an elegantly simple, yet highly
effective solution to this age old problem. The UCR205D
detector basically works like this: A stream of precision pulses
is generated at 455KHz locked to the FM signal coming from
the 455 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 modu­
lation of the radio signal. Another way of describing it is that as
the FM modulation increases the frequency, the circuit pro­
duces more pulses and as the modulation decreases the
frequency, the circuit produces fewer pulses. More pulses pro­
duces 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 automati­
cally 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 re­
sponse. 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 UCR205D 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.
With the power switch in the normal (“ON”) position, 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.
In the “PILOT OFF” position, received audio remains unmuted
regardless of the presence or absence of a pilot tone. This
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