LMX2581_14 Datasheet, PDF (48/56 Pages) Texas Instruments – Wideband Frequency Synthesizer

LMX2581
SNAS601G – AUGUST 2012 – REVISED SEPTEMBER 2014
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10.2 Regulator Output Pins
The recommendation for the VregVCO and VbiasCOMP pins is a minimum of one 10 µF capacitor, but more
capacitance is better. These pins have a bias voltage of about 2.5 V, which means that capacitors of smaller
case size and voltage ratings can actually have far less capacitance the labeled value of the capacitor. If there is
insufficient capacitance on these pins, then the VCO phase noise may be degraded. This degradation may vary
with frequency and how insufficient the capacitance is, but for example, bench tests show a degradation of about
5 dB at 20 KHz offset for a 3 GHz carrier if these capacitors are reduced to 4.7 µF.
6
4.7uF
10uF
5
4
3
2
1
0
-1
1x10³
1x10⁴
1x10⁵
Offset (Hz)
1x10⁶
1x10⁷
D001
Figure 26. Impact of VregVCO and VbiasCOMP Capacitor on VCO Phase Noise
48
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