CN-0147 Datasheet, PDF (3/4 Pages) Analog Devices – Powering a Fractional-N Voltage Controlled Oscillator with Low Noise LDO Regulators for Reduced Phase Noise

Circuit Note
100
V_OUT = 2.2V
I_L = 1mA
10
C_L = 10µF
C_L = 10µF C_NR = 0
C_NR = 10nF
C_L = 1µF
C_NR = 0
1
0.1
0.01
C_L = 1µF
C_NR = 10nF
0.001
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 4. ADP3334 Output Noise Spectrum
0.501
0.451
0.401
0.351
0.301
0.251
0.201
0.151
0.101
0.051
0.001
10
V_OUT = 1.8V
V_OUT = 2.8V
V_OUT = 3.3V
100
1k
10k
FREQUENCY (Hz)
100k
Figure 5. ADP150 output noise spectrum
CN-0147
Table 2. Calculation and Measurement of VCO Noise
Noise contribution from regulator
(nV/√Hz)
Noise contribution from regulator
(dBc/Hz)
Total calculated noise at VCO output
(dBc/Hz)
Measured VCO noise at 100 kHz offset
(dBc/Hz)
ADP3334 ADP150
150
25
−104
−119.5
−103
−109.5
−102.6 −108.5
Figure 6. ADF4350 Phase Noise at 4.4 GHz with ADP3334 Regulators
The noise contribution from the supply is then rss summed
with the noise contribution of the VCO (itself measured with a
very low noise supply) to give the total noise at the VCO output
with a given regulator.
These noise performances are rss summed together to give the
expected VCO phase noise:
L_(TOTAL) = L_(VCO)² + L_(LDO)²
Or expressed in dB
[ ( ) ( ) ] L_(TOTAL) = 10 log
10 ^{LVCO / 20}
+ 10 ²
LLDO / 20 ²
In this example, a 100 kHz noise spectral density offset is
chosen, a 6 MHz/V pushing figure is used, and −110 dBc/Hz is
taken as the VCO noise with an ideal supply.
Using a dedicated signal source analyzer (like Rohde & Schwarz
FSUP), the VCO phase noise is compared. At 100 kHz offset the
ADP3334 delivers −102.6 dBc/Hz (Figure 6), and in the same
configuration the ADP150 measures −108.5 dBc/Hz (Figure 7).
Figure 7. ADF4350 Phase Noise at 4.4 GHz with ADP150 Regulators
The integrated phase noise improves from 1.95° to 1.4° rms
also. The measured results correlate very closely with the
calculations and clearly show the benefit of using the ADP150
with the ADF4350.
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0147-DesignSupport.
Rev. C | Page 3 of 4