CMX972 Datasheet, PDF (31/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
The control of gain using VGA and VGB has an impact on the performance of the CMX972 demodulator
section. The variation in noise figure (NF) is straightforward, with the RF/IF gain control (VGB) having a
direct impact on NF but, due to the gain before the I/Q section, VGA has little impact on NF (see Figure
17). The variation of intermodulation (IMD) is more complex, as shown in Figure 18, where performance is
characterised by ‘Input Third Order Intercept Point’ (IIP3). At maximum gain IIP3 is at a minimum and as
would be expected, the IIP3 increases as the RF/IF gain is reduced (VGB). The improvement plateaus
beyond the –18dB gain setting as the input stages limit performance at this level. Reduction in gain with
VGA (I/Q gain control) also has a positive effect on IIP3. This is perhaps less intuitive but indicates that the
intermodulation performance of the CMX972 demodulator chain is dominated by the output stages rather
than RF/IF or mixer stages. Thus –6dB or even –12dB VGA gain control settings can be used to achieve
improved IMD performance for negligible change in noise figure (Figure 17), as long as the reduction in
gain can be tolerated.
40
35
30
25
VGA
20
VGB
15
10
5
0
-30
-25
-20
-15
-10
-5
0
VGA / VGB Attenuation
Figure 17 Variation in CMX972 Demodulator Noise Figure with VGA/VGB Control
10
5
0
VGA
-5
VGB
-10
-15
-20
-30
-25
-20
-15
-10
-5
0
Gain Control / dB
Figure 18 Variation in Input Third Order Intercept Point with VGA/VGB Control
 2015 CML Microsystems Plc
31
D/972/2