FX909AD2 データシートの表示（PDF） - CML Microsystems Plc

部品番号

コンポーネント説明

一致するリスト

FX909AD2

Wireless Modem Data Pump

CML Microsystems Plc 

Wireless Modem Data Pump

FX909A

1.5.1.6

CRC Generator/Checker

A circuit which generates (in transmit mode) or checks (in receive mode) the Cyclic Redundancy

Checksum bits, which are included in transmitted Mobitex data blocks so that the receive modem can

detect transmission errors.

1.5.1.7

FEC Generator/Checker

In transmit mode this circuit calculates and adds the Forward Error Correction (4 bits) to each byte

presented to it. In receive mode the FEC information is used to correct most transmission errors that

have occurred in a Mobitex Data Block or in the Frame Head control bytes.

1.5.1.8

Interleave/De-interleave Buffer

This circuit interleaves data bits within a data block before transmission and de-interleaves the

received data block so that the FEC system is best able to handle short noise bursts or signal fades.

1.5.1.9

Frame Sync Detect

This circuit, which is only active in receive mode, is used to look for the user specified 16-bit Frame

Synchronisation pattern which is transmitted to mark the start of every frame.

1.5.1.10

Rx I/P Amp

This amplifier allows the received signal input to the modem to be set to the optimum level by

suitable selection of the external components R1 and R2. The value of R1 should be calculated to

give 0.2 x VDD pk-pk at the RXFB pin for a received '...11110000...' sequence.

A capacitor may be fitted if ac coupling of the received signal is desired (see section 1.6.4), otherwise

the dc level of the received signal should be adjusted so that the signal at the modem's RXFB pin is

centred around VBIAS (½ VDD).

1.5.1.11 Tx/Rx Low Pass Filter

This filter, which is used in both transmit and receive modes, is a low pass transitional Gaussian filter

having a loss of 3dB at 0.3 times the selected bit rate (BT = 0.3). See Figure 4.

In transmit mode, the bits are passed through this filter to eliminate the high frequency components

which would otherwise cause interference into adjacent radio channels.

In receive mode this filter is used with an increased BT factor (0.56) to reject HF noise, so that the

signal is suitable for extraction of the received data.

1.5.1.12

Tx Output Buffer

This is a unity gain amplifier used in transmit mode to buffer the output of the Tx low pass filter. In

receive mode, the input of this buffer is connected to VBIAS. When changing from Rx to Tx mode the

input to this buffer will be connected to VBIAS for 2 bit periods to prevent unwanted signals, from the

low pass filter, at the output. When the modem is set to power save mode, the buffer is turned off

and the TXOP pin connected to VBIAS via a high value resistance. When exiting from power save

mode the Tx output is only reconnected to the buffer after 2 bit periods, to prevent unwanted signals,

from the low pass filter, at the output.

Note: The RC low pass filter formed by the external components R4 and C5 between the Tx Output

Buffer and the input to the radio's frequency modulator forms an important part of the transmit signal

filtering. These components may form part of any dc level-shifting and gain adjustment circuitry. The

ground connection to the capacitor C5 should be positioned to give maximum attenuation of high

frequency noise into the modulator. R4 and C5 should be chosen so that the product of the

resistance of R4 (in Ohms) and capacitance of C5 (in Farads) is 0.34/bit rate (bit rate in bits per

second). R4 should be not less than 47k and the value used for the external capacitor should take

into account parasitic capacitance.

© 1996 Consumer Microcircuits Limited

8

D/909A/4

Share Link: