CMX589AP4 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX589AP4' PDF : 24 Pages View PDF
GMSK Modem
4.2.6 Rx Signal Quality
The effect of input Rx Signal quality on the Rx S/N output is shown in Figure 7.
CMX589A
Figure 7: Typical Rx S/N Output High Time (%) vs. Input S/N
4.3 Transmit
4.3.1 Tx Signal Path Description
The binary data applied to the Tx Data input is retimed within the chip on each rising edge of the Tx Clock and
then converted to a 1-volt peak-to-peak binary signal centred at VBIAS (for VDD= 5.0V)
If the Tx Enable input is high, then this internal binary signal will be connected to the input of the Lowpass Tx
Filter, and the output of the filter connected to the Tx Out pin.
Tx Enable
1
0
Tx Filter Input
Data @
VDD
5
VP-P
e.g. 1VP-P for VDD=5V
VBIAS
Tx Out Pin
Filtered ‘Tx Filter Input’
VBIAS via 500kΩ
A ‘low’ input to the Tx Enable will connect the input of the Tx Filter to VBIAS, and disconnect the Tx Out pin from
the filter, connecting it instead to VBIAS through a high resistance (nominally 500kΩ).
The Tx Filter has a lowpass frequency response, which is approximately gaussian in shape as shown in Figure 9,
to minimize amplitude and phase distortion of the binary signal while providing sufficient attenuation of the high
frequency-components which would otherwise cause interference into adjacent radio channels. The actual filter
bandwidth to be used in any particular application will be determined by the overall system requirements. The
attenuation-vs.-frequency response of the transmit filtering provided by the CMX589A has been designed to meet
the specifications for most GMSK modem systems that are -3dB bandwidth switchable between 0.3 and 0.5 times
the data bit-rate (BT).
Note: An external RC network is required between the Tx Out pin and the input to the Frequency Modulator
(see Figure 2 and Figure 3). This network, which can form part of any DC level shifting and gain adjustment
circuitry, forms an important part of the transmit signal filtering. The ground connection to capacitor C1 should
be positioned to give maximum attenuation of high-frequency noise into the modulator.
© 1998 Consumer Microcircuits Limited
12
D/589A/3
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