RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
The half IF response of the CMX991/CMX992 is dependant on the output matching of the 1st mixer. The
arrangements described in 7.4 should be applied and matching can be optimised for a particular design.
The half IF performance has some variability between devices and care needs to be taken to ensure
sufficient margin exits for device variations, a standard deviation of 1.9 may be used for such calculations.
7.10 Receiver I/Q Filters
Two filtering modes are selectable using the ‘I/Q Filter’ bit (b5 of the Rx Mode Register, $13). The default
setting of the ‘I/Q Filter’ bit is ‘0’ and in this case the I/Q output bandwidth is limited to about 100kHz. With
the ‘I/Q Filter’ bit set to ‘1’ the I/Q output has a bandwidth of approximately 1MHz. Typical frequency
responses for the two modes are shown in Figure 27.
0
-10
-20
-30
100kHz Mode
-40
1MHz Mode
-50
-60
-70
-80
10
100
1000
10000
Frequency / kHz
Figure 27 Typical Receiver I/Q Frequency Response
7.11 Modulation Accuracy
To check the phase error of the transmitter, a CMX910 Evaluation Kit (EV9100) was used to drive the Tx
I/Q modulator differentially with GMSK. This setup tested 9.6kbps GMSK with a BT of 0.4, to fit in a 25kHz
channel. The plot in Figure 28 shows the Tx Output ACP performance as ~77dBc and Table 20 shows the
RMS phase error as being less than 1 degree (~0.8 degrees). The differential output level from the
EV9100 is 2Vp-p, therefore the 0dB I/Q gain setting was used.
2012 CML Microsystems Plc
42
D/991_992/18
CMLMICRO [ CML MICROCIRCUITS ]
