Lectrosonics UCR195D User Manual download pdf (Page 7)

Diversity UHF Receiver

DIGITAL PULSE COUNTING DETECTOR

The UCR195D receiver uses an advanced digital pulse

counting detector to demodulate the FM signal, rather

than a conventional quadrature detector. The most

common problem with quadrature detectors is thermal

drift, particularly those that operate at higher frequencies

like 10.7 MHz. The UCR195D design presents an

elegantly simple, yet highly effective solution to this age

old problem.

The UCR195D detector basically works like this: A

stream of DC pulses is generated at 455kHz. The pulse

width is constant, but the timing between pulses varies

with the frequency shift of the FM signal. The pulse

stream is controlled by the FM signal coming from the

IF section which has been heavily limited. The average

voltage of the pulses within any given time interval

varies in direct proportion to the frequency modulation

of the radio signal, producing the audio signal.

This type of detector eliminates the traditional problems

with quadrature detectors and provides very low audio

distortion and no thermal drift. 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.

2:1 EXPANDER (Dual–Band Compandor)

This circuit is the other half of the dual-band compandor

in a 195 system. “Dual-band Companding” is a closed

loop system, that is, whatever is done in the transmitter

must be mirrored in the receiver. The transmitter com-

presses the audio signal in two separate audio bands

using two separate time constants in order to avoid the

inevitable trade-offs in a single-band compandor. The

companion circuit in the receiver then re-expands this

compressed signal restoring the original dynamic range

and frequency characteristics of the signal.

The audio signal leaves the Detector circuit and is fed

through an amplifier to the 23 kHz Low Pass Filter

where all the high frequency noise (including the 32 kHz

pilot tone) is filtered out. After the 23 kHz low pass

filter, the signal is split into two parts via a 1 kHz low

pass filter and a 1 kHz high pass filter. The separated

signals are then processed in separate channels of the

NE572 2:1 Expander. Each channel of the 2:1 Expander

is optimized for its respective frequency band. The two

outputs of the 2:1 Expander are then summed in an op-

amp summer and sent to the output amplifier as one

signal.

PILOT TONE MUTE (SQUELCH)

The 195 system utilizes a separate ultrasonic tone

modulation of the basic carrier to operate the receiver

squelch. In the transmitter, a 32kHz tone is injected into

the audio signal path just after the compandor. The

supersonic pilot tone is filtered out of the audio signal

immediately after the detector in the receiver so that it

does not influence the compandor or various gain stages.

The basic benefit of the pilot tone squelch system is that

the receiver will remain squelched (muted) until it

receives the pilot tone from the matching transmitter,

even if a strong RF signal is present on the carrier

frequency of the system. Once a pilot tone is received,

the receiver will remain open during all signal condi-

tions.

The mute circuit drives a relay which physically discon-

nects the output amplifier from the output. This provides

complete muting of the audio and the noise. The pilot

tone function may be bypassed with the Pilot Tone

Disable switch (located in the battery compartment.)

When the pilot tone has been disabled with this switch,

the MOD function of the LED bargraph meter on the

front panel is also disabled. The Pilot LED on the front

panel will glow when the pilot tone has enabled the

receiver audio output.

OUTPUT LEVEL ADJUST AND RANGE SWITCH

The front panel Output control will adjust the audio

output within the range set by the Low/High/Fixed range

switch (located inside the battery compartment.) In the

Low position (toward the front panel) the adjustment

range is from –50dBm to –20dBm, the High position

(center) allows an adjustment from –30dBm to 0dBm,

and the Fixed position (toward the rear panel) sets the

audio output to a fixed +8dBm with no front panel

control.

POWER SUPPLY

The UCR195D may be operated from internal 9 Volt

batteries, from the supplied CH12 adapter, or from an

external 12 to 18 VDC source. The power supply has

protection circuits which prevent damage to the receiver

if a positive ground voltage source is used. The power

supply also has a built in Poly-Fuse to protect the unit.

This fuse is self healing by simply turning off the

receiver for about 15 seconds.

Rio Rancho, NM – USA

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Lectrosonics UCR195D User Manual Page 7

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