CMX838 데이터 시트보기 (PDF) - CML Microsystems Plc

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CMX838

FRS/PMR446/GMRS Family Radio Processor

CML Microsystems Plc 

FRS/PMR446/GMRS Family Radio Processor

TOS

SUBAUDIO TONE IN

AUDIO IN

VBIAS

LOW R

LOW R

LOW R

LOW R

CMX838

Gain/Attenuation

TX/RX

TXMOD1

SUM VBIAS

Gain = +/-1

VBIAS

LOW R

LOW R

LOW R

LOW R

VBIAS

Gain/Attenuation

TXMOD2

VBIAS

Gain = +/-1

VBIAS

VBIAS

Figure 4: Digitally controlled amplifiers and switch matrix for adjusting and switching transmit audio

and subaudio signals.

4.1.2 Transmit Input Amplifier

The transmit input amplifier is a high gain low-noise operational amplifier. Figure 5 is a simplified schematic

showing the external components required for typical application with an electret condenser microphone. The

external component values should be selected such that the feedback resistor will be greater than 10kΩ and

the minimum gain should be greater than 6dB.

In some cases, it may be desirable to implement a pre-emphasis characteristic of appropriately configuring

the external component values around the TX input amplifier. In this case, the internal pre-emphasis should

be bypassed (via C-BUS).

Figure 5: TX Input Amplifier

4.1.3 Audio Switched Capacitor Filters

Four standard (composed of biquadratic sections) switched capacitor filters are used in the audio section. A

pre-emphasis filter (+6dB per octave from 300 to 3000 Hz intended for transmit only) is implemented using

2nd order switched capacitor network, which can be configured (via C-BUS) to be a 2nd order low-pass. A

6th order high-pass filter is used to remove subaudible tones and bandwidth limit the incoming receive or

transmit audio signal prior to being input to the limiter. A 4th order low-pass filter follows the deviation limiter.

This filter smoothes the transients generated by the deviation limiter. Finally, a de-emphasis filter (-6dB per

octave from 300 to 3000 Hz intended for receive only) is implemented using a 2nd order switched capacitor

network. See Section 5.1.2 for details on configuring audio filters.

 2003 CML Microsystems Plc

11

D/838/8

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