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MX909A

GMSK Modem Data Pump

MX-COM Inc  

GMSK Modem Data Pump

Page 7 of 37

MX909A PRELIMINARY 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 value of R1 should be calculated to give (0.2 x VDD)VP-

P at the RXAMPOUT pin for a received '...11110000...' sequence.

A capacitor may be fitted if AC coupling of the received signal is desired (see section 5.4), otherwise the DC

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

around VBIAS (VDD/2).

4.1.11 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 the signal is

suitable for extraction of 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. 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 powersave mode, the buffer is turned off and the TXOUT 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 and

capacitance of C5 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.

Suitable values being:

8000 bits/sec

4800 bits/sec

R4

100k:

100k:

C5

430pF

710pF

The signal at the TXOUT pin is centered around VBIAS and is approx. (0.2 x VDD)VP-P, going positive for a

logic '1' and negative for a logic '0', if the modem is not inverting the Tx data.

A capacitor may be fitted if AC coupling of the input to the frequency modulator is desired, see section 5.4.

The 'eye' diagram of the transmitted signal (after the external R4/C5 network) is shown in Figure 5.

¤2001 MX-COM, Inc.

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

Doc. # 20480134.005

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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