Audio Processor
7
MX806A
Recommended External Component Notes:
1. Input Op-Amp gain/attenuation components (voltage gain = 6.0dB) are shown in Inset 1 is a differential
configuration to demonstrate the versatility of this input. Components for a single (+ or -) input may be
used.
2. Resistor values R7 to R11 (summation components) are dependent upon application and configuration
requirements.
3. Xtal circuit capacitors C6 (CD) and C7 (CG) shown in Inset 2 are recommended in accordance with MX-
COM’s Crystal Oscillator Application Note. Circuit drive and drain resistors are incorporated on-chip.
Operation of any MX-COM IC without a Xtal clock input may cause device damage. To minimize damage
in the event of a Xtal/drive failure, you should install a current limiting device (resistor or fast-reaction
fuse) on the power-input (V
DD
).
4 General Description
The MX806A LMR audio processor is intended too primarily to operate as the “Audio Terminal” or radio
systems using the DBS800 Digitally Integrated Baseband Subsystem.
The MX806A half-duplex device has signal paths and level setting elements that are configured and adjusted
by digital information sent from the radio microcontroller using C-BUS protocol. (C-BUS is the serial interface
for all DBS 800 ICs.)
4.1
Signal Path
The Signal path is divided into three sections:
Input Process:
This stage has selectable TX/RX paths. Transmits voice signals pass through microphone
pre-amplifier, voltage controlled gain (VOGAD) and highpass filter stages. Received audio is de-emphasized.
This initial audio, after line-in gain adjustment, may be switched to external audio processes (such as
scrambling) or to the internal Main Process stages.
Main Process:
Conditioning for the input or external process signals is completed in this stage. It is
comprised of pre-emphasis, high and lowpass switched capacitor filters and a deviation limiter.
Mixing and Output Drives:
Main audio for transmission is mix with signaling and data from external sources
(other DBS 800 ICs) to provide the composite signal for the digitally adjustable transmitter modulation drives.
Received audio level is adjusted for output to loudspeaker circuitry.
If selected, signal level stability and output accuracy of the MX806A is maintained by a voltage-controlled gain
system using selectable signal-level detectors. Signal levels can be dynamically controlled to provide
“dynamic compensation” for factors such as temperature drift, VCO non-linearity, etc.
The MX806A audio stages can be completely disabled – or the whole IC can be placed into powersave mode,
leaving only clock and C-BUS circuitry active.
4.2
VOGAD
The overall Gain Control system of the MX806A consists of 2 selectable signal peak detectors whose output
is fed via external integrating components to adjust the gain of the Voltage Controlled Amplifier positioned in
the TX Input Process Path. The transmit input signals is presented to Peak Detector 1 or 2. The Peak
Detectors are enabled individually by a Mode Command. When the input signal exceeds the peak - to - peak
threshold of the detector, a 5-volt level is produced at the VOGAD Out pin. This level remains for as long as
the signal exceeds the threshold. The integrated level to the VOGAD In pin causes the Voltage Controlled
Amplifier gain to be reduced. As can be seen from Figure 3 and Figure 5, Peak Detector 1 allows control of
the audio level to the external audio process and Peak Detector 2 allows control of transmit deviation levels.
1998
MX-COM, Inc.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480064.006
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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CMLMICRO [ CML MICROCIRCUITS ]
