RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
configuration is shown in section 4.2.2, Figure 7 where the resistor R1 is used to define the resistive load
for the filter. The suggested value of 1200Ω can be varied depending on requirements, noting the trade-off
between voltage gain and Q of the matching arrangements. The input impedance varies slightly with VGA
setting but the effect of this is minimised by use of the terminating resistor R1.
The variable gain may be adjusted by a host processor based on the measured Signal Level Indicator
(SLI) value or on other criteria such as I/Q vector magnitude. The SLI output is an analogue output which
is single ended and referenced to ground. Following the IF amplifier there is a pair of mixers that perform
the final down-conversion either to an I/Q or low IF output. The I/Q demodulator has an output bandwidth
of 1MHz which allows a low IF output of up to 1MHz; typical values may be 450kHz, 455kHz or 465kHz.
5.2.3 I/Q Filters
The I/Q outputs include two filters that provide continuous time rejection to serve as anti-alias filters for
external ADCs. The default filter will give an I/Q bandwidth of 1MHz. A narrower filter of 100kHz bandwidth
is selectable to improve analogue rejection for narrow-band systems and guarantees image rejection for
typical (e.g. sigma-delta) ADC solutions.
Rx Mode Register ($13,b4)
‘Cal En’ = ‘1’.
Acquire I/Q Offset
CMX991
CMX992
Div
MIXINN
MIXINP
Switch
Circuitry disabled during
Acquire I/Q DC Offset
MIXOUT 1 MIXOUT 2
Figure 15 DC Offset Calibration Mode
5.2.4 DC Offset Correction
The CMX991/CMX992 does not provide direct compensation of dc offsets in the I/Q outputs from the
receiver, however it does provide a mode that allows the I/Q signals to be measured externally to support
easy compensation. To allow optimum measurement of dc offsets it is desirable to remove the input signal
to allow fast averaging of the output i.e. without the need to consider the possibility of modulation being
present. In this mode the areas of the CMX991/CMX992 that can generate dc offsets remain enabled.
The CMX991/CMX992 ‘Cal En’ mode (Rx Mode register $13, b4, see section 6.4.1) disables the early
stages of the receiver, as shown in Figure 15.
2012 CML Microsystems Plc
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D/991_992/18
CMLMICRO [ CML MICROCIRCUITS ]
