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
7
D/CMX264/1
CMLMICRO [ CML MICROCIRCUITS ]
