CMX991 datasheet(21/56 Pages) CMLMICRO

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RF Quadrature Transceiver / RF Quadrature Receiver

CMX991/CMX992

 2015 CML Microsystems Plc

D/991_992/20

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:

frf = flo - fTxIf

(wanted)

fimage = flo + fTxIF

(rejected)

Which mixing product is the wanted and which the unwanted image can be selected, see section 6.6.

TXOUTP

TXOUTN

AVDD

Divide by

2 or 4

Select

TXIP

TXIN

TXQP

TXQN

Divide by

2 or 4

IF Local Oscillator

-6dB, 0dB, +6dB

CMX991

Tx Output

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.

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