Operator's Manual

4-1
September 2001
Part No. 001-5100-001
CIRCUIT DESCRIPTION
SECTION 4 CIRCUIT DESCRIPTION
4.1 GENERAL OVERVIEW
4.1.1 INTRODUCTION
The E.F Johnson 5100 series digital portable
radio is a microcontroller-based radio that uses a
Digital Signal Processor (DSP) to provide the
following modes of operation:
Narrowband Analog - FM modulation with a
maximum deviation of 2.5 kHz. This mode is usually
used in systems where the channel spacing is 12.5
kHz. Call Guard (CTCSS or DCS) subaudible squelch
signaling can be used in this mode.
Wideband Analog - FM modulation with a maximum
deviation of 5 kHz. This mode is usually used in
systems where the channel spacing is 25 kHz or 30
kHz. Call Guard (CTCSS or DCS) subaudible squelch
signaling can be used in this mode.
Project 25 Digital - The voice is digitized, error
corrected, optionally encrypted and transmitted using
C4FM modulation according to the Project 25 stan-
dard. This mode can be used in channel spacings of
12.5 kHz.
The DSP processes the received signals and
generates the appropriate output signals. The micro-
controller controls the hardware and provides an inter-
face between hardware and DSP.
PC Boards
This radio contains the following PC boards:
• RF Board
• Digital Board
• Keypad Board
• Four flex circuits that provide interconnection and
support for the volume, on/off, and LED controls.
• Encryption board (optional)
The Keypad Board provides the input/output
interface for the user. It accepts input from the keypad
and the various control knobs and sends the appro-
priate signals to the DSP on the Digital Board and to
the RF Board for proper configuration. It provides the
dual display information to inform the user of the
status of the radio. It also performs all RS-232 commu-
nications between the radio and remote computer
stations for the purposes of radio programming, tuning,
encryption key loading and software downloading.
4.1.2 ANALOG MODE
Receive Mode
The signal is routed from the antenna connector
to the RF Board where it is filtered, amplified, and
mixed with the first local oscillator frequency gener-
ated by the synthesizer. The resulting IF signal is also
filtered and amplified and sent to the ABACUS chip.
The signal is then mixed with the second local
oscillator frequency to create a second IF signal of 450
kHz. The second IF signal is then sampled at 14.4
Msps and downconverted to baseband. The baseband
signal is then decimated to a lower sample rate that is
selectable at 20 kHz. This signal is then routed via a
serial interface using a differential current output to
the ADSIC chip on the Digital Board.
On the Digital Board the ADSIC digitally filters
the input signal, performs frequency discrimination to
obtain the message signal and then routes the message
signal to the DSP. The DSP first performs a carrier-
detection squelch function on the radio. If a signal is
determined to be present, the audio portion of the
signal is resampled to an 8 kHz rate and then filtered
appropriately. The filtered signal is then routed back to
a D/A in the ADSIC to produce an analog signal for
output to the audio power amplifier (PA) and then the
speaker. Any detected signaling information is
decoded and the resulting information is sent to the
microcontroller.