Theory Overview; Operation; Control Head (Spectra); I. General (See Functional Block Diagram) - Motorola Spectra Service Manual

I. GENERAL (See Functional Block Diagram)
The Spectra ® 900 MHz radio is a wideband, fre-
quency synthesized, fixed-tuned mobile radio covering
the 896-902; 935-941 MHz band. The Spectra line
includes models that are capable of either trunked or
conventional operation, and certain models that are
capable of both. Receiver frequencies are in the
935-941 MHz range; transmitter frequencies are
896-902 MHz for normal operation and 935-941 MHz
for talkaround (mobile-to-mobile) operation. 900 MHz
service is a 12.5 kHz channel spacing, 2.5 kHz deviation
system.
The Spectra radio consists of six major assemblies,
four of which are in the main transceiver housing.
These are:
• Command board - includes the microprocessor,
the plug-in memory board, and miscellaneous
audio and control stages; the HearClear
plugs into the command board.
• Receiver front end assembly.
• RF board contains receiver IF, demodulator,
and the synthesizer logic and filtering.
• VCO/doubler board.
The remaining two assemblies are:
• Dash- or remote-mount control head assembly,
which is connected, directly or remotely, to the
front of the transceiver by the interconnect
board, or remote interconnect board and control
cable. This assembly contains two printed cir-
cuit boards: the control head board and an
interconnect board.
• Transmit power amplifier (PA) which plugs into
the rear of the housing.
II. OPERATION
When the radio is receiving, the signal comes from
the antenna connector, passes through the antenna
switch on the transmit PA, to the receiver front end. The
signal is mixed with the low-side injection signal from the
injection doubler, and then fed to the 109.65 MHz IF on
the RF board. After another mix to the 450 kHz second
IF, the signal is detected and fed to the command
board, processed by the HearClear board and the audio
filter IC, and finally fed to the audio PA and speaker. All
signalling, filtering, and detection is performed on the
command board.
The audio PA (U450) is a dc-coupled bridge-
type amplifier with its outputs directly connected
to the external speaker. Therefore, the speaker
outputs must never be grounded. Use an audio
isolation transformer (for example, Motorola Part
No. SLN6435A) to isolate test equipment from
the audio PA (U450)/speaker.
4

THEORY OVERVIEW

CAUTION
When the radio is transmitting, microphone audio is pro-
cessed by the HearClear board, routed through the
audio filter IC for pre-emphasis and splatter filtering, and
then fed to the synthesizer modulation ports. The injec-
tion is fed to the transmit power amplifier. The
amplification level is controlled by the power control cir-
cuitry on the command board via the control voltage fed
to the PA module.
The synthesizer controls the VCO in both receive
and transmit. The VCO operates at one-half the desired
injection frequency, and is doubled and fed through the
injection switch to the appropriate stage. Synthesizer
frequency programming is done by the microprocessor,
and is controlled by information stored in the radio
EEPROM. The radio microprocessor also controls all
audio routing and switching, signalling generation and
detection, reference oscillator tuning, transmit deviation,
transmit PA power level and current limiting, and, via the
TM board
serial bus, control head interfacing.
III. CONTROL HEAD (SPECTRA)
The Spectra control head has solid-state micropro-
cessor circuitry that operates the standard and optional
features built into the system. See paragraph 1.6 for
Spectra 9000 control head.
A. Description of Controls (See Figure 1)
POWER SWITCH - a push-type switch for turning the
radio on and off (pushbutton control heads). A rotary
switch is used on the rotary control head models, which
also controls volume.
MODE - A rocker switch (pushbutton control heads) or
rotary switch (rotary control heads) for selecting modes
and programming menus.
VOLUME - rotary switch, which also controls power
(rotary control heads) or rocker switch (pushbutton con-
trol heads) for selecting volume level, editing names,
and making certain radio adjustments.
DIM - button for controlling intensity of the display.
HOME - button for restoring preprogrammed mode,
establishing programmed mode, selecting programming
entries, etc.
MICROPHONE - connector; accepts any Spectra or
Spectra 9000 microphone. With remote control head
option, the microphone can also plug into the rear of the
control head.
DISPLAY - Vacuum fluorescent (VF) display; eight-
character alphanumeric display shows all radio states or
statuses; also, functions as the ON/OFF indicator.
PRI, N PRI, DIR - indicators built into the VF display;
these are acronyms for priority and non-priority (used
for scanning), and Direct, used for talkaround).