Circuit Note

CN-0147

Rev. C | Page 3 of 4

100

10

1

0.1

0.01

0.001

10

100

1k

10k

100k

1M

FREQUENCY (Hz)

Figure 4. ADP3334 Output Noise Spectrum

0.501

0.001

0.051

0.101

0.151

0.201

0.251

0.301

0.351

0.401

0.451

10

100

1k

10k

100k

FREQUENCY (Hz)

Figure 5. ADP150 output noise spectrum

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:

2

2

(LDO)

(VCO)

(TOTAL)

L

L

L

+

=

Or expressed in dB

) (

)

]

2

20

/

2

20

/

10

10

log

10

LLDO

LVCO

(TOTAL)

L

+

=

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

Table 2. Calculation and Measurement of VCO Noise

ADP3334

ADP150

Noise contribution from regulator

(nV/√Hz)

150

25

Noise contribution from regulator

(dBc/Hz)

−104

−119.5

Total calculated noise at VCO output

(dBc/Hz)

−103

−109.5

Measured VCO noise at 100 kHz offset

(dBc/Hz)

−102.6

−108.5

Figure 6. ADF4350 Phase Noise at 4.4 GHz with ADP3334 Regulators

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.