CMX972 Datasheet, PDF (14/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
5 General Description
The CMX972 is an RF integrated circuit providing a quadrature demodulator, integer-N synthesiser and an
IF VCO. Additional features include gain control and uncommitted differential amplifiers. A detailed block
diagram for the IC is shown in section 2. The device can support a wide range of modulation formats and
standards. The following sections describe the functionality of the CMX972.
5.1 Quadrature Demodulator
The quadrature demodulator is designed for IF/RF operation, having very low power consumption. Input
frequencies in the range 20MHz to 300MHz are allowed. The demodulator system has two gain-controlled
stages, one before and one after the I/Q down-converters, as shown in Figure 9. The two gain control
elements can be independently controlled (see section 6.3.1). This adjustability allows users to optimise
characteristics depending on their system requirements. Minimum noise figure can be maintained by
decreasing gain in VGA with VGB at maximum gain. Intermodulation performance can be optimised by
decreasing gain in VGA or VGB. A lower gain in VGA will tend to reduce dc offsets in the output I/Q signal.
For further information on the effects of control of VGA and VGB see section 7.4.
LO
Divide by
2 or 4
sin
IFIN
cos
Variable Gain
Stage B
(VGB)
Variable Gain
Stage A
(VGA)
RXIP
RXIN
RXQP
RXQN
Figure 9 Demodulator Gain Control
The output of the quadrature demodulator is provided as a differential signal (pins RXIP, RXIN, RXQP and
RXQN). The bandwidth of the I/Q signals depends on the OUTDRV bit (b7, $1C, Rx Control Register, see
section 6.3.1). The intermodulation performance of the CMX972 also depends on the OUTDRV bit, see
section 7.2 for further details.
The CMX972 provides for an optimisation of receiver intermodulation using the “IMD” bits in the VCO
control register, further details can be found in section 7.2.
5.1.1 I/Q Amplitude and Phase Correction
The LO path includes a correction circuit which may be enabled or disabled using the COR bit (b6 in the
Rx Control Register $1C), see section 6.3.1. This will improve the I/Q balance of the demodulator
particularly when using the local oscillator divide by two (LO/2) mode; enabling this mode (COR=’1’) will
give a small increase in current consumption of typically 0.5mA. The improvement is most noticeable with
higher frequency signals, e.g. circa 200 - 300MHz; at 45MHz the improvement is negligible.
 2015 CML Microsystems Plc
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