FX806A
A
UDIO
P
ROCESSOR
CHIP
SELECT
POWER
SUPPLY
C-BUS INTERFACE
CALIBRATION INPUT
PRIMARY
and
SECONDARY
AUDIO
INPUTS
———
Voice.
Sub-/Audio
Tone.
FFSK.
etc.
SERIAL
CLOCK
COMMAND
DATA
MAIN
PROCESS
OUT
SUM
OUT
MOD.
IN
#
MODULATION 1
OUT
ATTENUATOR
#
(TX)
MIC. IN
MIC. & VOGAD
AMPS
LOW &
HIGHPASS
FILTERS
VOGAD SENSE
#
VOGAD
SENSE
#
VOGAD
SENSE
#
#
TRANSMITTER
MODULATION
DRIVES
#
#
GAIN
SET
INPUT
PROCESS
#
PRE-EMPHASIS
LIMITING
FILTERING
GAIN SETTING
(RX)
AUDIO IN
DE-EMPHASIS
FILTER
XTAL/CLOCK
CLOCK
XTAL
GENERATOR
BUFFER
ATTENUATOR
To EXTERNAL AUDIO
PROCESSES
EXTERNAL AUDIO
PROCESS IN
Fig.1 FX806A Audio Processor
Brief Description
Intended primarily to operate as the “Audio Terminal” of
Radio Systems using the DBS 800 Digitally-integrated
Baseband System, the FX806A is a PMR Audio Processor
which meets EIA and CEPT audio specifications.
Using a unique filter line-up, the FX806A offers lower
distortion versus modulation level figures than conventional
filter/limiter configurations.
The FX806A is a half-duplex device whose signal paths
and level-setting elements are dynamically configured and
adjusted by digital information sent from the Radio
µController
using “C-BUS” hardware and software protocol.
Figure 5 shows a complete functional block diagram of
the FX806A signal paths which can be viewed as 3 sections:
q
Input Process
Selectable transmit or receive input paths.
The transmit path with low-noise input and VOGAD
amplifiers and bandpass filtered stages provides good signal-
to-noise performance at low input levels and minimum
distortion for high-drive modulation signals.
De-emphasis is software selectable at the Rx Audio Input
for FM or PM radio configurations.
This initial audio, after in-line gain adjustment, is
available for switching to either external audio processes
(such as scrambling) or internally to the Main Process
stages.
q
Main Process
Conditioning for Input or External Process signals with
gain/pre-emphasis, high and lowpass switched capacitor
filters and a transmitter deviation limiter. The Main Process
Output may be switched to V
BIAS
.
q
Summation and Output Drives
Main “voice audio” from the Main Process is combined
with signalling and data from other DBS 800 facilities, to
provide the composite (in and outband) signal for the digitally
adjustable Transmitter Modulation Drives.
Received audio is level (volume) adjusted for output to
loudspeaker circuitry.
Signal-level stability and therefore output accuracy, of
the FX806A is maintained by a voltage-controlled gain
system (VOGAD) with specific gain sensors that are selected
automatically by the Internal/External Mode Command. The
VOGAD system permits high deviation with low distortion.
This is achieved by reducing the path gain (and so reducing
the distortion introduced by the Peak Deviation limiter) when
the input signal is large.
Signal levels can be controlled to provide ‘dynamic-
compensation’ for such factors as temperature drift, VCO
non-linearity, etc.
FX806A audio output stages can be completely disabled
or the whole microcircuit placed into a “Powersave” mode,
leaving only clock and “C-BUS” circuitry active.
The FX806A is a low-power, 5-volt CMOS integrated
circuit and is available in 24-pin DIL cerdip and 24-pin/lead
plastic SMD packages.
Publication D/806A/3 July 1994
#
#
#
#
ATTENUATOR
MODULATION 2
OUT
MAIN PROCESS
MODULATION
SUMMING
AMP
OUTPUT DRIVES
# indicates logic control
#
LOUDSPEAKER
AUDIO
CMLMICRO [ CML MICROCIRCUITS ]
