CMX589AP4 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX589AP4' PDF : 21 Pages View PDF
High Speed GMSK Modem
16
CMX589A PRELIMINARY INFORMATION
Ideally, the Rx demodulator should be DC coupled to the CMX589A RX Signal In pin (with a DC bias added to
center the signal at the RX Feedback pin at VDD/2 [VBIAS]). However, AC coupling can be used provided that:
The 3dB cut-off frequency is 20Hz or below (i.e. a 0.1F capacitor in series with 100k).
The data does not contain long sequences of consecutive ones or zeroes.
Sufficient time is allowed after a step change at the discriminator output (resulting from channel
changing or the appearance of a RF carrier) for the voltage into the CMX589A to settle before the
RXDCacq line is strobed.
5.1.3 Two-Point Modulation
When designing the CMX589A into a radio that uses a frequency synthesizer, a two-point modulation
technique is recommended. This is both to prevent the radio's PLL circuitry from counteracting the
modulation process, and to provide a clean flat modulation response down to DC.
Figure 12 shows a suggested basic configuration to provide a two-point modulation drive from the CMX589A
TX Output using MX-COM's MX019 Digitally Controlled Quad Amplifier Array. The MX019 elements provide
individual set-up, calibration and dynamic control of modulation levels. Level setting control of the
amplifiers/attenuators of the MX019 is via an 8-bit data word. Note that the MX019 frequency response
supports data rates as high as 8kbps.
With reference to Figure 12:
The buffer amplifier is required to prevent loading of the CMX589A external RC circuit.
Stage B, with R1/R2, provides suitable signal and DC levels for the VCO varactor; C1 is RF
decoupling. The drive level should be adjusted (digitally) to provide the desired deviation.
Stage C, with R3/R4, provides the Reference Oscillator drive (application dependent). This
parameter is set by adjusting for minimum AC signal on the PLL control voltage with a low-frequency
modulating signal (inside the PLL bandwidth) applied.
Stage D could be used with the components shown if a negative reference drive is required.
Stage A provides buffering and overall level control.
TX OUT
CMX589A
External RC
VSS See Fig.3
Buffer
+14dB to -14dB
A
With reference to the MX019 Data Sheet
Stage A = MX019 Channel 4
Stage B = MX019 Channel 1
Stage C = MX019 Channel 2
Stage D = MX019 Channel 3
Note:
1. All stages of the MX019 are 'inverting' stages.
2. Components R1-R6 should produce the proper output
signal levels for interface into the modulator.
CONTROL
+3dB to -3dB
B
R1 VVCO
R2
TX VCO
C1
+3dB to -3dB
C
To TX
REF Osc (+)
R3 VREF(+)
R4
+3dB to -3dB
D
To TX
REF Osc (-)
R5 VREF(-)
R6
Figure 12: An Example of Two-Point Modulation Drive with Individual Adjustment Using the MX019
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
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