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FX806A

AUDIO PROCESSOR

CML Microsystems Plc 

FX806A

AUDIO PROCESSOR

POWER

SUPPLY

CALIBRATION INPUT

#

(TX)

MIC. IN

MIC. & VOGAD

AMPS

LOW &

HIGHPASS

FILTERS

INPUT

PROCESS

#

(RX)

AUDIO IN

DE-EMPHASIS

FILTER

XTAL/CLOCK

XTAL

CLOCK

GENERATOR

CHIP

SELECT

SERIAL

CLOCK

COMMAND

DATA

C-BUS INTERFACE

MAIN

PROCESS

OUT

VOGAD SENSE

#

VOGAD

SENSE

#

#

GAIN

SET

BUFFER

#

VOGAD

SENSE

#

#

#

PRE-EMPHASIS

LIMITING

FILTERING

GAIN SETTING

MAIN PROCESS

# indicates logic control

To EXTERNAL AUDIO EXTERNAL AUDIO

PROCESSES

PROCESS IN

SUM

OUT

MOD.

IN

PRIMARY

and

SECONDARY

AUDIO

INPUTS

———

Voice.

Sub-/Audio

Tone.

FFSK.

etc.

#

ATTENUATOR

MODULATION 1

OUT

TRANSMITTER

#

MODULATION

DRIVES

ATTENUATOR

MODULATION 2

OUT

MODULATION

SUMMING

AMP

OUTPUT DRIVES

#

ATTENUATOR

LOUDSPEAKER

AUDIO

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.

Publication D/806A/3 July 1994

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 VBIAS.

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.

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