CMX972 Datasheet, PDF (15/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
250MHz
45MHz
Condition
RXIP/RXQP RXIN/RXQN RXIP/RXQP RXIN/RXQN
$1C, b6 = ‘0’
$1C, b6 = ‘1’
92.0°
89.8°
92.0°
89.8°
90.1°
89.9°
90.2°
89.9°
Table 8 Typical Phase Balance, LO/2 Mode
At 250MHz I/Q amplitude balance is typically 0.12dB with COR = ‘0’ and 0.04dB with COR = ‘1’. Enabling
the correction circuit also reduces the I/Q path gain, particularly at higher frequencies. This can be
compensated by setting the FREQ bits (b3-0 in the Rx Mode register $1D) to ‘1111’, instead of the default
value of ‘0000’. I/Q path gain is restored at the expense of a slight degradation in I/Q phase balance of
≈0.5°. For many applications, the ‘1111’ setting will be adequate.
At all frequencies, phase correction accuracy is improved by using a lower setting of the FREQ bits (b3-0
in the Rx Mode register $1D). However, care should be taken to avoid significant gain degradation, which
occurs if a setting near ‘0000’ is chosen for a high frequency. Table 9 is a guide for the appropriate setting
of the FREQ bits, so as to obtain the best phase balance (typically better than 0.06°) with only a small gain
reduction (typically less than 0.6dB). Where frequency ranges overlap, either setting of the FREQ bits can
be used.
Bit _{b3 b2 b1 b0} Frequency
0 1 0 0 20MHz to 40MHz
1 0 0 0 40MHz to 80MHz
1 1 0 0 80MHz to 200MHz
1 1 0 1 200MHz to 240MHz
1 1 1 0 240MHz to 300MHz
Table 9 Recommended FREQ bit Settings in the Rx Mode Register
60
59
58
57
56
55
54
53
52
51
50
49
48
47
46
45
0
Cor=ON, $1D=00
Cor=ON, $1D=0F
Cor=OFF, $1D=00
50
100 150 200 250 300
Frequency / MHz
Figure 10 Frequency Response, showing effect of COR bit ($1C, b6) and FREQ bits($1D, b3-b0)
 2015 CML Microsystems Plc
15
D/972/2