CMX991 Datasheet, PDF (19/56 Pages) CML Microcircuits – Two-mode demodulator

RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
5 General Description
5.1 Overview
The CMX991/CMX992 are RF Quadrature Transceiver and Receiver ICs respectively. Each incorporates
a superheterodyne receiver section along with IF local oscillator circuits. The CMX991 has an I/Q
modulator with image-rejecting up-converter. The CMX991/CMX992 I/Q architecture supports a wide
range of modulation types and various selectable functions maintain the performance across multiple
modulations and bandwidths. The demodulator outputs are analogue signals with a quadrature (I/Q) Zero-
IF signal format that simplifies connection to external ADCs. The receiver analogue signal interface also
supports a low IF output mode. The transmitter interface is analogue I/Q format. Control of the
CMX991/CMX992 is via the serial C-BUS (see section 6).
5.2 Receiver
The CMX991/CMX992 has a flexible multi-standard receiver designed to support multiple digital and
analogue radio systems of both constant envelope and linear modulation types. It is expected that the
applied input signal will have been amplified by an external Low Noise Amplifier (LNA). The user must
determine the need for, and design of, any external image reject filtering. The CMX991/CMX992 design is
optimised for an LNA gain of about 13dB². It is assumed there is some insertion loss prior to the LNA but
an overall noise figure of 4dB and gain of 8dB (approx.) should be provided by the circuits preceding the
CMX991/CMX992. A digital control signal is available from the chip, which can be used to enable/disable
the LNA. Use of this signal is recommended as it simplifies I/Q calibration of dc-offsets. A differential input
signal to the first mixer on the chip is recommended.
The receiver architecture is a superheterodyne type with a 1^st IF allowed in the range 10MHz to 150MHz,
some typical 1^st IFs being 10.7MHz, 21.4MHz, 45MHz, 70MHz and 150MHz. The CMX991/CMX992
provides a 1^st down converter mixer with excellent linearity and noise figure. The design is intended to
meet the challenging requirements of typical PMR/LMR radio systems.
5.2.1 Rx 1^st Mixer and IF Filtering
The Rx 1^st Mixer has a differential input with a nominal impedance of 300Ω and nominal input frequency
range of 100MHz to 1GHz. The 1^st mixer has selectable LO input dividers: these are /1, /2 and /4 to allow
common LO structures with the various Tx architectures, including use of the CMX998 with the transmitter.
The mixer RF LO input is differential but the normal configuration is single ended with the other input ac
coupled to ground (see section 4.4.1).
The mixer has two selectable outputs to allow the connection of two different 1^st IF filters, crystal or SAW
type, that may be separately enabled under host control. The type of filter used is dependent on the
application. The filter should provide rejection of blocking and intermodulation test tones for the
subsequent IF stages. This 1^st IF filter may also provide some useful adjacent channel filtering, but it is
likely that the majority of the adjacent channel rejection will come in subsequent stages.
5.2.2 IF Variable Gain Amplifier (VGA) and I/Q Down-converter Mixer
There are two selectable inputs to the IF amplifier, which is low noise and controlled through the C-BUS
serial interface (See section 6). The inputs are differential with a common inverting input (pin IFINN) which
should be decoupled locally to the ground plane used for the external IF elements. The IF inputs are high
impedance (see section 8.1.3.2) and this allows straightforward matching to IF filter components. A typical
2 _{The precise gain will depend on application and is often a trade-off between intermodulation performance}
and receiver noise figure. See also section 7.3.1.
 2015 CML Microsystems Plc
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