Shure UC2, Руководство по обслуживанию

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Shure UC2 Hand-Held UHF Transmitter
4 Characteristics 25A1044 (RI)
ETSI-Approved
This transmitter uses the same pcb as the domestic system, but it
requires a different topology and different parts.
A limiter is inserted between the pre-emphasis and the compressor
to limit the occupied bandwidth.
The buffer after the gain stage, U201A, is now an inverter with
pre-emphasis and 20 dB of attenuation. Previously bypassed, the
expander side of the NE575 (U202) is now activated and used as a
limiter. A sample of the audio is taken from the limiter output, U202,
pin 6, and sent to a control circuit that detects signals above a limiting
threshold. The comparator then sends a control signal to U202, pin 5,
which clamps the signal.
Rf Section
Processed audio enters R320, an internal potentiometer that is
adjusted for 45 kHz deviation (40 kHz for England), 100% modulation,
with a 6.8 dBu 1 kHz tone at the output of the compressor. For J fre-
quencies, R320 is adjusted for 5 kHz deviation with 1 kHz tone injected
into the mic input, to give –23.47 dBu at U201, pin 7 (TPA4).
The audio is then fed to the tuning voltage pin of the voltage con-
trolled oscillator (
VCO
), which modulates the carrier directly. A phase
locked-loop (
PLL
) frequency-synthesized system eliminates the need for
multiplier stages and results in a much higher degree of spectral purity.
The
VCO
is shielded to prevent external rf fields from affecting its
operation. Regulated 5 Vdc power from the low drop-out regulator,
U311, is provided to ensure frequency stability with changes in battery
voltage.
The
VCO
is capable of tuning from 782 to 806 MHz (
FCC
-approved),
with a 1 to 4 V tuning voltage range. At the output of the
VCO
, the rf
signal splits into two paths. Frequency variations by country are given
in a table in the
Service Procedures section.
The output of the
VCO
is coupled by C323 to the frequency control
pin of the synthesizer, U304. The synthesizer’s internal circuitry divides
the rf signal down, as necessary. The synthesizer contains a quartz-con-
trolled reference oscillator circuit operating from a 4.0 MHz crystal, Y302,
that is adjusted by trimmer C314.
The transmitter output frequency is user-selectable in groups of
compatible channels from 782 to 806 MHz for domestic groups. Fre-
quency selection is made via microprocessor U303, which interfaces with
the user by means of the Group/Channel switches, S301 and S302.
The output of the synthesizer is a series of pulses integrated by a
passive loop filter (R316, C319, R317, C353, R318, C320) to produce a
control voltage signal. The control voltage signal is then connected to
the
VCO
through buffer amplifier U306B, which is used to isolate the
PLL
filter from the audio modulation signals.