CMX589AP4 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX589AP4' PDF : 21 Pages View PDF
High Speed GMSK Modem
12
CMX589A PRELIMINARY INFORMATION
4.2.6 Rx Signal Quality
The effect of input Rx Signal quality on the Rx S/N output is shown in Figure 7.
% High Time
100
90
BT = 0.5
80
70
60
BT = 0.3
50
40
30
20
10
0
5
6
7
8
9
10
11
12
13
S/N (dB)
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 centered 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 500k
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 500k).
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.
The signal at Tx Out is centered around VBIAS, going positive for logic 1 (high) level inputs to the Tx Data
input and negative for logic 0 (low) inputs.
When the transmit circuits are put into a powersave mode (by a logic 1 to the Tx PS pin) the output voltage of
the Tx Filter will go to high impedance. When power is subsequently restored to the Tx filter, its output will
take several bit-times to settle. The Tx Enable input can be used to prevent these abnormal voltages from
appearing at the Tx Out pin.
1998 MX-COM, Inc.
www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480183.002
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
All trademarks and service marks are held by their respective companies.
Share Link: 
