CMX972 Datasheet, PDF (34/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
7.7 VCO Phase Noise
The performance of the negative resistance VCO can be optimised by use of the VCONR bits in the VCO
control register ($2F, b1-b0, see section 6.8.1). For example, the typical change in phase noise and tuning
voltage with VCONR setting is shown in Table 18 for the 180 MHz VCO described in Figure 5 / Table 4.
VCONR setting ($2F, b1-b0)
Vtune
(V)
Phase Noise at 10kHz
offset, from f_vco/2
(dBc/Hz)
Maximum
Intermediate 1
Intermediate 2
Minimum
2.425
2.335
2.235
2.183
-105.7
-107.1
-109.0
-110.0
Table 18 Effect of VCONR bits, f_vco = 180 MHz, divide-by-2
The phase noise achieved by the CMX972 VCO depends on the VCO gain (K_v). The effect is shown in
Figure 21, which plots the phase noise measured at 90 MHz (180 MHz VCO as Figure 5 / Table 4, divide-
by-2 mode) as a function of the VCO gain.
-105
-106
-107
-108
-109
-110
6
8
10
12
14
VCO Gain Kv (MHz/V)
Figure 21 Effect of VCO Gain on Phase Noise
 2015 CML Microsystems Plc
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