MX589 Consumer Microcircuits Limited, MX589 Datasheet - Page 11

• Manufacturer Part Number: MX589
• Description: Mx589 High Speed Gmsk Modem
• Manufacturer: Consumer Microcircuits Limited

High Speed GMSK Modem 4k to 64kbps
4.3
4.3.1
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 V
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.
A ‘low’ input to the TX Enable will connect the input of the TX Filter to V
from the filter, connecting it instead to V
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 MX589 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 V
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 be undefined.
Enable input can be used to prevent these abnormal voltages from appearing at the Tx Out pin.
©2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
When power is subsequently restored to the Tx filter, its output will take several bit-times to settle. The Tx
TX CLK
TX Data
RX Data
RX CLK
Transmit
TX Signal Path Description
SAMPLE RX DATA AT THIS TIME
EXTERNAL CIRCUITS SHOULD
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1.0 s Min.
1.0 s Max.
TX DATA SAMPLED BY
THE MX589 AT THESE
μ
μ
Tx Enable
1
0
1 BIT PERIOD
Figure 8: Rx and Tx Clock Data Timings
INSTANCES
BIAS
BIAS
Tx Filter Input
V
V
through a high resistance (nominally 500kΩ).
DD
BIAS
, going positive for logic 1 (high) level inputs to the Tx Data input
/5V
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P-P
Data
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1.0 s Min.
1.0 s Max.
μ
μ
TX CLOCK AND RX CLOCK OUTPUTS
(MARK/SPACE) DUTY CYCLE NOMINALLY 50%.
Tx Out Pin
Filtered Data
V
BIAS
via 500kΩ
BIAS
BIAS
, and disconnect the TX Out pin
(for V
DATA INVALID
DON'T CARE
DD
= 5.0V)
Doc. # 204800103.011
DATA MUST
DATA VALID
BE VALID
MX589