CMX264 データシートの表示（PDF） - CML Microcircuits

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CMX264

Frequency Domain Split Band Scrambler

CML Microcircuits 

Frequency Domain Split Band Scrambler

CMX264

1.5 General Description

This device has been designed to be compatible with mobile station baseband channels. All signal processing

blocks such as lowpass filters, pre-emphasis, de-emphasis, and balanced modulators use switched capacitor

(SWC) techniques.

In each mode of operation, all blocks not in the signal path are turned off in order to minimise power

consumption. All internal settings and signal paths are selected by means of the serial interface according to

Section 1.5.3.

1.5.1 Tx Channel

1.5.1.1 Tx Channel Pre-Emphasis

A pre-emphasis circuit at the input of the Tx scramble block. It has a slope of 6dB per octave between

280Hz and 3140Hz. In scramble mode, it may be selected to “whiten” the audio signal prior to split

band inversion. It may be used in conjunction with the optional de-emphasis at the Rx output of the

descrambler.

1.5.1.2 Tx Lowerband Input Filter (TXLBIPF)

A lowpass filter whose input is the externally amplified and limited baseband audio signal from the

microphone. It selects that part of the audio spectrum which is below the split point. Its output signal is

processed by the lowerband balanced modulator and output filter to form the transmitted lowerband.

The bandwidth of the lowerband is controlled according to the split point chosen by varying the

bandwidth of this filter. The bandwidth is proportional to the sampling clock frequency and so an

appropriate clock frequency is internally selected for each split point.

1.5.1.3 Tx Lowerband Balanced Modulator

This modulates the output of the Tx Lowerband Input Filter (TXLBIPF) to form a frequency shifted

upper sideband and a frequency inverted lower sideband. It is the frequency inverted lower sideband

which is eventually transmitted as the lower frequency part of the scrambled signal, i.e. the lowerband.

The lower carrier frequency varies with split point and is always about 230Hz above the corner

frequency of the preceding lowpass filter (TXLBIPF). This means that baseband frequencies around

230Hz below the lower carrier frequency are translated to approximately 230Hz in the scrambled

audio. These frequencies form the lowest corner frequency of the transmit spectrum.

1.5.1.4 Tx Lowerband Output Filter (TXLBOPF)

A lowpass filter whose input is the output signal from the lowerband balanced modulator. Its function is

to select the frequency inverted lower sideband and remove the upper sideband. The resulting output

signal from this filter forms the lowerband part of the scrambled audio. It is summed with the output of

the Tx upperband channel to form the complete scrambled signal for transmission. This filter's corner

response also eventually defines the lower corner frequency of the recovered audio.

1.5.1.5 Tx Upperband Input Filter (TXUBIPF)

A lowpass filter whose cutoff frequency represents the upper limit of the baseband audio which is

scrambled, transmitted and descrambled. The output from this filter is processed by the upperband

balanced modulator, the output filter and any external channel filtering to form the transmitted

upperband.

© 1999 Consumer Microcircuits Limited

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D/CMX264/1

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