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CMX838

FRS/PMR446/GMRS Family Radio Processor

MX-COM Inc  

FRS/PMR446/GMRS Family Radio Processor

11

4 General Description

CMX838 Advance Information

4.1 Audio

The audio signal processing is designed to meet or exceed the requirements for basic audio filtering, gain

control and deviation limiting in a FRS radio. Figure 3 is a block diagram of the audio circuitry.

AUXPUPEN

AUX I/O

MICOUT

MICIN

VREF

RXIN

PREEMPHASIS OR 2ND

ORDER LPF

LP

PRE

1

AUDIO

INPUT 1

SELECT

PRE LPF CTRL

AUDIO

LEVEL

6TH ORDER HPF

1

HPF BYPASS

AUDIO OUT

SELECT

AOUT

BOUT

VBIAS

TOS

VREF

AIN

VLL

VLH

BIN

AUDIO

INPUT 2 LPF BYPASS

LIMITER BYPASS

SELECT

DEVIATION LIMITER AND POST-LIMITER LPF

TONE GENERATOR

1

TX/RX

TX SUBAUDIO (From On-Chip Subaudio Tone Generator)

DEEMPHASIS NETWORK

DE

1

DEBP

RX AUDIO OUT

LEVEL

RXOUT

TOS

TXMOD

SWITCH

MATRIX and

PHASE

CONTROL

See Figure 4

TXMOD1

TXMOD2

Figure 3: Audio Processing Block Diagram

4.1.1 Digitally Controlled Amplifiers (DCA)

There are five DCAs on-chip. They are used to set signal levels for audio in/out, subaudio in/out, receive

audio out (volume control), modulation out1, and modulation out2. The audio in/out DCA is adjustable in

0.5dB steps over a +7.5dB to –7.5dB range, see Section 5.1.2.3. The volume control level DCA is adjustable

in 1.5dB steps over a +12dB to –33dB range, see Section 5.1.2.4. The subaudio signal level in/out DCA is

adjustable in 0.5dB steps over a +7.5dB to –7.5dB range, see Section 5.1.2.19

The modulation level controls are composed of two DCAs, and a switch matrix, see Figure 4. Each

modulation level DCA, modulation out1 and modulation out2, can be switched to select either the output of

the audio processor, or the output of the tone generator, or the addition of the audio and tone. In addition,

there is an internally generated DC volume (labeled ‘TOS’ in Figure 4), which can be sent to the MOD1 and

MOD2 DCA’s. This signal is not generally applicable to FRS radios. However, in some cases it may be

desirable for testing or signal generation. The modulation out1 DCA is adjustable in 0.5dB steps over a

+7.5dB to –7.5dB range and the Modulation Out2 DCA is adjustable in 0.25dB steps over a +3.75dB to

-3.75dB range, see Section 5.1.2.6. To obtain inverse signals of mod 1and mod 2, the MSB from the first byte

(bit 7) and the MSB from second byte (bit 15) have to set to logic 1, see Section 5.1.2.6.

¤2001 MX-COM, Inc.

www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054

Doc. # 20480221.002

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.

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