Frequency Domain Split Band Scrambler
CMX264
1.6
Application Notes
1.6.1 General Use
The transmit scramble function may be implemented by connecting the device in the microphone line.
The receive function is achieved by connecting the device in the earpiece line. Some external pre- and
post-amplification and limiting may be necessary to ensure good signal to noise and dynamic range
performance.
Many existing cellphones incorporate the functions of SAT tone filtering and anti-splatter filtering so
these features are not included in the CMX264. The transmit and receive spectral shaping is designed
to make this interfacing easy.
It is anticipated that statutory transmission requirements will be met by filtering within the host
equipment e.g. for removal of a SAT tone at 4kHz. The filtering required for this is quite stringent (see
Figure 8) and so the Tx Upperband Output Filter (TXUBOPF) has been designed to avoid
unnecessary signal loss in the region of 3kHz. (see Figure 7).
Because this filter does not completely remove unwanted harmonics from the output of the upperband
balanced modulator, transmit output distortion and noise is specified at the output of the host transmit
filter (see Electrical Performance Specification in Section 1.7.1).
It is desirable to introduce any channel control signals (whether in-band or sub-audio) after the
scramble function and to remove the signals before descrambling.
1.6.2. Input Anti-Alias and Output Smoothing Filters
1.6.2.1 Input Anti-Alias Filtering
The internal circuitry of the device uses sampled techniques and so anti-alias filtering and post-filtering
(smoothing) may be required.
Input anti-alias filtering is supplied on chip at the MICIN and RXIN inputs. The filtering is first order with
a corner frequency of 25kHz.
Possible alias frequencies are:
(with input pre-emphasis selected)
63.337kHz
(with input pre-emphasis not selected)
85.261kHz
63.337kHz
52.780kHz
44.336kHz
184.734kHz
(Split point 1)
(Split point 2 and clear mode)
(Split point 3)
(Split point 4)
(All split points)
1.6.2.2 Output Smoothing
All output buffers are sampled circuits clocked at 63.337kHz but no post-filtering is provided on chip.
This is not included in this product as such circuitry is normally included elsewhere, so it may be
necessary for the host equipment to provide this filtering either to comply with transmission
requirements or to prevent aliasing within other sampled circuits in the system.
ã 1999 Consumer Microcircuits Limited
13
D/CMX264/1
CMLMICRO [ CML MICROCIRCUITS ]
