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 V )V
DD P-
at the RXAMPOUT pin for a received '...11110000...' sequence.
P
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 V
(V /2).
DD
BIAS
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 V
. When changing from Rx to Tx mode the input to this
BIAS
buffer will be connected to V
for 2 bit periods to prevent unwanted signals, from the low pass filter, at the
BIAS
output. When the modem is set to powersave mode, the buffer is turned off and the TXOUT pin connected to
via a high value resistance. When exiting from power save mode the Tx output is only reconnected to
V
BIAS
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:
R4
C5
8000 bits/sec
4800 bits/sec
100k:
100k:
430pF
710pF
The signal at the TXOUT pin is centered around V
and is approx. (0.2 x V )V , going positive for a
DD P-P
BIAS
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.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480134.005
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