Lectrosonics UDR300c User Manual download pdf (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 UDR300C 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 UDR300C 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 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 pos-

sible 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 (±60 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 UDR300C 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 UDR300C design presents an elegantly simple, yet highly

effective solution to this age old problem. The UDR300C

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. 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

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.

Rio Rancho, NM – USA

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Lectrosonics UDR300c User Manual Page 5

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