CMX264D5 データシートの表示（PDF） - MX-COM Inc

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CMX264D5

Frequency Domain Split-Band Scrambler

MX-COM Inc  

Frequency Domain Split-Band Scrambler

7

CMX264 Advance Information

4 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 minimize power

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

Section 1.5.3.

4.1 Tx Channel

4.1.1 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.

4.1.2 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.

4.1.3 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.

4.1.4 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.

4.1.5 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.

4.1.6 Upperband Balanced Modulator

This modulates the output of the Tx Upperband Input Filter (TXUBIPF) to form two sidebands, a frequency

inverted lower sideband and a non-inverted upper sideband. Part of the inverted lower sideband is selected

by the Tx Upperband Output Filter (TXUPOPF) to form the transmitted upperband, i.e. that part of the audio

spectrum above the split point.

For each split point setting, the upper carrier frequency is chosen so that baseband frequencies close to

2770Hz are shifted to just above the split point. Conversely, baseband frequencies just above the split point

are shifted to the upper transmission limit of the channel. The result is that the upper carrier is the same

(approximate) distance above the lower carrier at all split point settings. The approximation arises because of

the need to have frequencies derivable from the crystal frequency and the fact that only certain divisors of

associated filter sample rates have been chosen as carriers, in order to avoid aliasing.

2000 MX-COM, Inc.

www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054

Doc. # 20480210.002

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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