Quadrature Modulator
CMX993/CMX993W
5
General Description
The CMX993 and CMX993W are RF quadrature modulators with additional features such as gain control
and uncommitted differential amplifiers. Detailed block diagrams for the ICs are shown in section 2. The
ICs can support a wide range of modulation formats and standards including TDMA operation.
The following sections describe the functionality of the ICs.
5.1
Quadrature Modulator
The quadrature modulator provides translation from baseband I and Q signals to a modulated RF signal.
The wideband inputs can be driven differentially or single-ended. In the case of single ended operation a
reference voltage equal to the nominal dc level of the modulation must be supplied. The input and filter
amplifiers allow single-ended signals to be translated to an appropriate dc level, one solution for an input
signal with 0V dc bias is shown in Figure 4/4a.
5.1.1 DC Offsets and Carrier Leakage
The modulator inputs (MODIN/MODIP and MODQN/MODQP) are differential and require a common dc
level or common mode voltage. Differences in the bias voltages on the pins will result in an increased level
of carrier present at the output. Care should be taken to minimise offsets, thereby minimising carrier
leakage.
Some systems implement carrier nulling before transmission. This results in a compensation of the small
internal offsets in the modulator and any offsets generated in external circuits. Digital to analogue
converters designed for I/Q systems, e.g. CMX981, often include registers which allow a programmable
offset to be applied to the I/Q signals, making this nulling process straightforward.
5.1.2 Wideband Noise and Gain Control
The wideband noise of this modulator is optimised to ensure a low noise floor at the output, compliant with
common product standards. This stage also provides gain control, which allows the output level to be
adjusted while maintaining the maximum possible signal-to-noise ratio.
To ensure optimum performance the I/Q input signals need to be free from noise. The input bandwidth of
the modulators is quite broad, so any noise on the I/Q signals will get translated to the RF output. Such
noise can generally be removed by simple RC filters, as shown in Figure 4/4a. Careful attention needs to
be paid to the bandwidth of these filters as, if unduly narrow, they can affect the modulation, degrading
image rejection and modulation accuracy.
5.2
Differential Amplifiers
5.2.1 Input Amplifiers
Two differential amplifiers are provided which may be used for signal conditioning, for example conversion
of differential input signals to single ended format, or to provide dc level translation. The amplifiers are
uncommitted, with the differential inputs and the output all available on pins. The stages are low power and
are enabled using the ‘General Control Register’ (see section 6.2). It is not possible to independently
control each amplifier, both are enabled with a common control bit.
2013 CML Microsystems Plc
11
D/993/10
CMLMICRO [ CML MICROCIRCUITS ]
