CMX969 데이터 시트보기 (PDF) - MX-COM Inc

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CMX969

MOTIENT/ARDIS RD-LAP MDC4800 Modem

MX-COM Inc  

MOTIENTSM/ARDISSM RD-LAPTM MDC4800 Modem

8

CMX969 Advance Information

4.1.8 Interleave/De-interleave Buffer

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

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

4.1.9 Auto Frame Sync Detect, Rx Level & Timing Extraction, Rx Data Symbol Extraction

This block, which is only active in receive mode, is used to look for the Frame Synchronization pattern which

is transmitted to mark the start of every frame and to extract the received symbols from the received signal

using extracted signal level and timing information.

4.1.10 Rx Input 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 dc level of the received signal should be adjusted so

that the signal at the modem's RXFB pin is centered around VBIAS (VDD/2). See Section 6.1.3 for details of the

optimum levels.

4.1.11 Low Pass Filter

This filter, which is used in both transmit and receive modes, is a linear-phase low pass filter having a

frequency response automatically switched to suit RD-LAP or MDC operation.

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

that would otherwise cause interference into adjacent radio channels.

Binary

Data

data

encoding

binary -

symbol

2 or 4-level

symbols

Transmit

filter

Filtered

baseband

signal

Frequency

modulator

Modem

Figure 4: Generation of Filtered Tx Baseband Signal

In receive mode, the filter is used to reject HF noise and to equalize the received signal to a form suitable for

extracting the received data.

4.1.12 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 unless the RXEYE bit of the Control Register is '1', when it

is connected to the received signal. When changing from Rx to Tx mode the input to this buffer will be

connected to VBIAS for 8 symbol times in RD-LAP mode, 2 symbol times in MDC mode, while the low pass

filter settles.

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

input to the radio's frequency modulator forms an important part of the transmit out of band spurious signal

filtering. These components may form part of any dc level-shifting and gain adjustment circuitry. C5 should be

positioned to give maximum attenuation of high frequency noise into the modulator.

4.1.13 Clock Oscillator and Dividers

These circuits derive the transmit symbol rate (and the nominal receive symbol rate) by frequency division of

a reference frequency which may be generated by the on-chip Xtal oscillator or applied from an external

source.

Note: If the on-chip xtal oscillator is to be used, then the external components X1, C3, C4 and R3 are

required. If an external clock source is to be used, then it should be connected to the XTAL input pin, the

XTAL pin should be left unconnected, and X1, C3, C4 and R3 not fitted.

¤2001 MX-COM, Inc.

www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054

Doc. # 20480211.002

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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