High Speed GMSK Modem 4k to 64kbps
Page 15 of 20
MX589
5.1.3 Two-Point Modulation
When designing the MX589 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 MX589 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.
With reference to Figure 12:
The buffer amplifier is required to prevent loading of the MX589 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.
CONTROL
+3dB to -3dB
TX VCO
C1
B
C
D
R1
V
VCO
+14dB to -14dB
TX OUT
Buffer
A
MX589
R2
External RC
See Fig.3
V
SS
+3dB to -3dB
R3
To TX
REF Osc (+)
V
REF
(+)
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:
R4
+3dB to -3dB
R5
To TX
REF Osc (-)
V
REF
(-)
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.
R6
Figure 12: An Example of Two-Point Modulation Drive with Individual Adjustment Using the MX019
2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 204800103.011
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
All trademarks and service marks are held by their respective companies.
CMLMICRO [ CML MICROCIRCUITS ]
