Harris DX 25U, Техническое руководство

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Section J
Analog Input Board (A35)
J.1
Introduction
This section includes a description of the Analog Input Board
and troubleshooting information.
The Analog Input Board includes audio and power control cir-
cuits. The output is an audio signal with a DC component, which
goes to the Digital to Analog Converter. The DC component
determines the transmitters carrier power output, and the audio
component amplitude modulates the transmitter.
The Analog Input Board is located on the left inside wall of the
Center Control Compartment.
Refer to SECTION V, Maintenance, for adjustment and pc board
maintenance procedures.
J.2
Circuit Description
Refer to Schematic 839-7855-100 in the Drawing Package.
The Analog Input Board includes the following circuits:
a. Bessel Filter, to optimize audio overshoot performance.
b. Transformerless, balanced audio input stage.
c. Maximum Power Adjustment.
d. Power supply hum and noise cancelling.
e. Digitally controlled potentiometer, for power control.
f. “Dither” circuit for optimizing transmitter noise perform-
ance.
g. Audio sample for the DC Regulator, B- signal.
h. A “PA Turn on/Turn off” circuit.
i. On-board voltage regulators.
J.2.1
Audio Input
The audio input signal from the External Interface enters at either
J1, J2 or J3. Back-to-back zener diodes on the External Interface
provide protection against transients and excessive voltage at the
input.
J.2.2
Bessel Filter
The Bessel Filter uses passive components L1 through L4, C1
through C4, and R7 through R12. The Bessel Filter rolls off
frequencies above the audio band without introducing overshoot.
Audio frequency response of the transmitter is less than 0.9 Db
down at 10 kHz, and square wave overshoot is minimized when
the filter is properly matched to the source impedance of the
processing equipment. Processing equipment with IC output
stages typically have a low source impedance. Consult the tech-
nical manual for your processing equipment.
The Analog Input Board includes three audio input connectors
(J1, J2 and J3), each with different series resistors for various
source impedances. Use J1 if the source impedance is 600 Ohms
or more, J2 if it is between 150 and 600 Ohms, and J3 if it is 50
Ohms or less.
If you do not know the source impedance use either J1 or J2. If
the source impedance is greater than the input impedance, some
overshoot will result. If the source impedance is less than the
input impedance, high frequency response will change slightly.
J.2.3
Instrumentation Amplifier (U6, U9)
An Instrumentation Amplifier configuration is used to convert
the balanced input to an unbalanced output and to provide
rejection to any noise on the input signal.
The audio signal from the Bessel Filter is AC coupled through
C5, C6, C7 and C8 to the input stage. Back-to-back zener diodes
CR1 and CR2, and series resistors R11 and R12 provide input
protection.
The input to the first section of the instrumentation amplifier is
U6-5 and U6-3. Both non-inverting amplifiers, U6A and U6B,
have the same gain and Audio Gain Adjust control R15 allows
inputs from -10 dBm to +10 dBm to be used. The outputs U6-3
and U6-1 are sent to U9-2 and U9-3.
Voltage divider R68 compensates for the gain difference between
the inverting and non-inverting inputs of U9.
Note that the 10k resistors used in this circuit are each one section
of DIP (Dual Inline Package) resistor arrays R68 and R69. All
resistors are labeled “R68" or ”R69," and the DIP terminals for
each resistor section are given.
J.2.4
Buffer Amplifier (U7)
The output of U9-1 goes through a voltage divider to the input
of Buffer Amplifier U7-3. The input to the buffer amplifier can
be observed at TP1. When the transmitter is modulated at 100%
with a sine wave, the audio signal amplitude at TP1 will be about
1.5 Vp-p with no DC component.
The signal is amplified by U7 (gain of 2) and sent to the
maximum power circuit U7-6.
J.2.5
Maximum Power Adjust (U7, R27)
The Max PWR Adjustment R27 adds a DC component to the
audio signal to set the maximum carrier power of the transmitter.
A negative DC voltage from R27 is fed to the non-inverting input
U7-5 where it is summed with the audio signal. The output of
U7-7, TP-4, will be a -(Audio + DC) signal. The audio compo-
nent from TP1 is amplified by U7.
With the “maximum power adjust” set, the DC voltage at TP4
will be about -1.0 VDC at 30 kW with no modulation. With 100%
modulation, the -(Audio + DC) signal at TP4 will be a 2 Vp-p
audio signal with a -1.0 VDC component. The voltage at TP4
will NOT change when the RAISE or LOWER controls on the
front panel are operated or when the transmitter is switched
between LOW, MEDIUM, or HIGH powers. If Maximum Power
Adjust is set for less than 30 kW, the DC component at TP4 will
be less than -1.0 VDC.
888-2297-002 J-1
WARNING: Disconnect primary power prior to servicing.