Lectrosonics UDR200b - Manual (Page 5)

Wireless Diversity Receiver
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 (100 mW). The mixer in the UDR200B produces output at
only the sum and difference signals, with minimal spurious
signals. This mixer offers a very high overload threshold. The
IF output of this mixer is at 71 MHz which is unusually high for
a wireless receiver. This high frequency was chosen to in­
crease the image rejection in the front end to as high as our
fixed frequency designs. The mixer is followed by low noise
amplifiers and SAW filters to preserve the superior RF perfor­
mance.
SURFACE ACOUSTIC WAVE FILTER
The UDR200B 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
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 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 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 con­
verted to audio. 455 kHz is very unconventional for a second
IF in a wide deviation (±75 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 (capture ratio). To
use an IF at 455 kHz requires an unusual circuit to convert the
IF to audio.
DIGITAL PULSE COUNTING DETECTOR
The UDR200B receiver uses an advanced digital pulse detec­
tor 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. 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 equip­
ment 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 UDR200B design presents an elegantly simple, yet highly
effective solution to this age old problem. The UDR200B
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
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
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 con­
stant, so level differences in the IF signal do not affect the
pulse. The two resulting audio signals from the A and B re­
ceiver channels are then combined to achieve the maximum
benefits of diversity reception.
RATIO COMBINING DIVERSITY WITH OPTI­
BLEND
TM
Instead of the usual audio switching between the two receiv­
ers, we blend the audio outputs of the receivers in a ratio
controlled by the RF level of the received signals in the two
receivers. This totally eliminates any of the switching noise
sometimes heard in other designs and improves the signal to
noise ratio an additional 6 dB under weak signal conditions
and 3 dB in strong signal conditions.
TRI MODE DYNAMIC FILTER
After being combined in the Opti-Blend circuit, the audio signal
is passed through a “dynamic noise reduction circuit”. The
cutoff frequency of this filter is varied automatically by measur­
ing 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 appara­
tus 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.
There is one circumstance where a good argument can be
made for bypassing this filter, so a switch is provided to do just
that.
RF ONLY NOISE FILTER
A small switch on the rear of the receiver will allow the noise
filter to be set for RF ONLY. In this mode the filter is held out of
Rio Rancho, NM – USA 5