CMX991 Datasheet, PDF (21/56 Pages) CML Microcircuits – Two-mode demodulator

RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
5.3 Transmitter (CMX991 only)
The transmitter requires analogue I and Q (baseband) signal inputs. This I/Q input is up-converted by
quadrature modulator(s) to a suitable IF (TxIF). This is the modulated signal with the desired modulation,
but at an IF of typically 45MHz or 90MHz, i.e. typically lower than the final desired (RF) transmit frequency.
The TxIF signal is available at the TXIFOUT pin or can be up (or down)converted to final frequency using
the CMX991 image reject up-mixer.
The TxIF signal can be optimised by selecting the correct setting of the IFH bit (register $11, b5 see
section 6.2.1) for IFs above or below 75MHz.
The IF LO input applied to the IF I/Q modulator(s) is generally developed internally (see section 5.4). The
LO is divided by either 2 or 4 to generate the quarature signals used in the modulator. The main LO, used
in image-reject up-converter, is generated off-chip.
5.3.1 Image-Reject Up-converter
The CMX991 transmitter architecture is shown in Figure 16. The image rejection process involves
generating TxIF signals with a quadrature phase relationship. The TxIF signals pass through filters to
remove harmonic content – this substantially reduces the spurious content of the final output. The
bandwidth of the filters is selectable as 45MHz, 60MHz, 90MHz or 120MHz (see also section 7.12). The
signals are then used in a modulator stage which converts to the final frequency. The process results in
image cancellation of the unwanted mixing sideband with default operation being high side mixing as
follows:
f_rf = f_lo - f_TxIf
f_image = f_lo + f_TxIF
(wanted)
(rejected)
Which mixing product is the wanted and which the unwanted image can be selected, see section 6.6.
TXIP
TXIN
TXQP
TXQN
-6dB, 0dB, +6dB
Divide by
2 or 4
Select
TXOUTP
AVDD
TXOUTN
C1
T1
Tx Output
IF Local Oscillator
Divide by
2 or 4
CMX991
Figure 16 CMX991 Transmitter Architecture
The image-reject function reduces the need for filtering following the modulator to remove spurious
products, however it is likely that some filtering will still be required to meet spurious emissions limits,
hence the additional filter as shown in Figure 16.
 2015 CML Microsystems Plc
21
D/991_992/20