Quadrature Modulator
CMX993/CMX993W
4.2
Quadrature Modulator
Typical values for the quadrature modulator output circuits are shown in Figure 3/Table 3 and typical
configuration and values for the input circuits in Figure 4/4a and Table 4. Concerning Figure 3, the
transformer T1 should be located close to the CMX993/CMX993W and tracks from the MOP and MON
pins to T1 should be as close to equal in length as possible, preferably with a symmetrical layout. The
decoupling capacitor (C1) should be close to the centre tap of T1.
Note: Details of a simpler alternative output configuration are provided in an Application Note available
from www.cmlmicro.com.
MOP
MODQP
MODQN
T1
Output
R3
R4
AVDD
C1
MODIN
MODIP
MON
VREF
BVREF
C4
Bandgap
CMX993
CMX993W
Figure 3 Modulator Output External Components (30MHz to 1GHz)
C1
C4
10nF
1µF
T1
R3, R4
Balun 4:1 (See Note 1)
(See Note 2)
Note 1: For example TC4-14+ from mini circuits; for applications
between 200MHz and 1GHz or Coilcraft WBC4-1WL for
applications between 30MHz and 300MHz.
Note 2: The resistors R3 and R4 are optional. Fitting these with
110 resistors will give a good broadband match
however will reduce output level available. For many
applications they will be unnecessary.
Table 3 Quadrature Modulator Output Components
The input configuration in Figure 4/4a is a possible solution for a single ended input with 0V dc bias. The
input amplifiers are used to translate the input to BVREF. In order to balance the small bias current drawn
by the modulator, the filter amplifiers are used to buffer BVREF. RC filters are used on the input to the
modulator to remove wideband noise generated by the differential amplifiers. The values of the resistors
and capacitors should be selected based on the bandwidths of the modulation. It is important to keep the
values of R6, R7, R8 and R9 the same so that small dc offsets generated by the modulator input bias
currents are matched, thereby providing the best carrier rejection. The networks R6/C1, R7/C2, R8/C3 and
R9/C4 should be placed close to the modulator input pins of the CMX993/CMX993W to ensure optimum
noise performance. It is important to note that due to small input bias currents, offset voltages and
component tolerances it is impractical to expect this type of input configuration to give ideal carrier
suppression. To achieve optimal carrier suppression it is recommended to have the ability to finely adjust
the dc offsets (see section 5.1.1).
2013 CML Microsystems Plc
8
D/993/10
CMLMICRO [ CML MICROCIRCUITS ]
