CMX264D5 Просмотр технического описания (PDF) - CML Microcircuits

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CMX264D5

Frequency Domain Split Band Scrambler

CML Microcircuits 

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.

© 1999 Consumer Microcircuits Limited

13

D/CMX264/1

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