ANALOG DEVICES AN-1154 Service Manual

AN-1154

APPLICATION NOTE

dp

dQ

DEAD ZONE

BLEED CURRENT P US HE S THE
CHARGE PUMP O UT OF DEAD ZONE

10729-001

One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: [phone] • Fax: [phone] • www.analog.com

Optimizing Phase Noise and Spur Performance of the ADF4157 and ADF4158 PLLs

Using Constant Negative Bleed

by Robert Brennan and Dawid Powazynski

INTRODUCTION

The phase noise (PN) and integer boundary spur (IBS)
performance of the ADF4157 and the ADF4158 can be
improved by activating a constant negative bleed current. The
biggest improvement is achieved at frequencies at, or close to,
integer multiples of the phase frequency detector (PFD)
frequency. It is more visible for loop bandwidths greater than
60 kHz; however, it is recommended to use constant negative
bleed for all PLL loop bandwidths.

Constant negative bleed current works by adding a constant
offset to the charge pump (equivalent to a phase offset in
the PLL loop). This has the effect of linearizing the charge
pump by moving away from the nonlinear area near the origin
(sometimes referred to as the charge pump dead zone). Figure 1
illustrates this phenomenon.

Figure 1. The Effect of Bleed Current on Charge Pump

Without this constant current offset, sigma-delta quantization
(dQ) noise can fold back in-band and cause excessive noise or
spurs. This aliasing of sigma-delta noise in-band only happens
for high resolution sigma-delta modulators (equivalent to high
value of modulus) as used on the ADF4157 and ADF4158.
These parts require activation of a constant negative bleed
current to achieve optimal phase noise and spur performance.
This current is not needed for other Fractional-N PLLs with
lower value of modulus or Integer-N PLLs.

The constant negative bleed on the ADF4157 and ADF4158 is
activated by setting bits DB[24:23] in Register 4 to 0b11.

THE EFFECT OF BLEED CURRENT ON PHASE NOISE AND SPURS

The use of a constant negative bleed only improves phase noise
and integer boundary spurs for a certain range of charge pump
currents (I
can degrade. This phenomenon was measured for two PFD
frequencies, 12.5 MHz and 25 MHz. Each PFD frequency was
tested near two consecutive integer channels. The loop filter
configuration for each PFD frequency is show in the Appendix.

). For some values of ICP, phase noise and spurs

CP

HOW MEASUREMENTS WERE RECORDED

The measurements were recorded on an EV-ADF4157SD1Z
evaluation board. The loop filter was modified for each PFD
frequency.

1. The loop was locked at 5800.001 MHz using a PFD

frequency of 25 MHz.

2. The charge pump current was set to the minimum value

(0.31 mA).

3. Negative bleed was disabled.

4. The phase noise at a 5 kHz offset and the integer boundary

spur at 1 kHz were recorded.

5. Negative bleed was enabled.

6. The phase noise at a 5 kHz offset and the integer boundary

spur at 1 kHz were recorded.

7. Step 3 to Step 6 were repeated for every charge pump

current setting up to 5 mA.

8. Step 2 to Step 7 were repeated with the loop locked at

5825.001 MHz.

9. The loop filter was modified for a PFD frequency of

12.5 MHz, the loop was locked at 5800.001 MHz using a
PFD frequency of 12.5 MHz. Step 2 to Step 8 were
repeated.

Rev. 0 | Page 1 of 8

AN-1154 Application Note

TABLE OF CONTENTS

Introduction ...................................................................................... 1

The Effect of Bleed Current on Phase Noise and Spurs .............. 1

How Measurements Were Recorded .............................................. 1

Revision History ............................................................................... 2

REVISION HISTORY

5/12—Revision 0: Initial Version

Results .................................................................................................3

Analysis of Results .............................................................................4

Conclusion..........................................................................................4

Appendix ............................................................................................5

Rev. 0 | Page 2 of 8

Application Note AN-1154

0

PN (dBc/Hz) ; IBS (dBc)

0

PN (dBc/Hz) ; IBS (dBc)

0

PN (dBc/Hz) ; IBS (dBc)

0

PN (dBc/Hz) ; IBS (dBc)

RE S U LT S

PFD Frequency = 25 MHz

Output frequency = 5800.001 MHz

RF

= 5800.001MHz; PFD = 25MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

0.31 1.31 2.31 3.31 4.31

OUT

CHARGE PUMP CURRENT (mA)

Figure 2. Phase Noise and IBS at 5800.001 MHz with

a PFD Frequency = 25 MHz

Output frequency = 5825.001 MHz

PN AT 5kHz; BLE E D OFF
IBS; BLEED OFF
PN AT 5kHz; BLE E D ON
IBS; BLEED ON

10729-002

PFD Frequency = 12.5 MHz

Output frequency = 5800.001 MHz

RF

= 5800.001MHz; PFD = 12.5MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

0.31 1.31 2.31 3.31 4.31

OUT

PN AT 5kHz; BLE E D OFF
IBS; BLEED OFF
PN AT 5kHz; BLE E D ON
IBS; BLEED ON

CHARGE PUMP CURRENT (mA)

Figure 4. Phase Noise and IBS at 5800.001 MHz with

a PFD Frequency = 12.5 MHz

Output frequency = 5825.001 MHz

10729-004

RF

= 5825.001MHz; PFD = 25MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

0.31 1.31 2.31 3.31 4.31

Figure 3. Phase Noise and IBS at 5825.001 MHz with

OUT

PN AT 5kHz; BLE E D OFF
IBS; BLEED OFF
PN AT 5kHz; BLE E D ON
IBS; BLEED ON

CHARGE PUMP CURRENT (mA)

–10

–20

–30

–40

–50

–60

–70

–80

–90

0.31 1.31 2.31 3.31 4.31

10729-003

Figure 5. Phase Noise and IBS at 5825.001 MHz with

a PFD Frequency = 25 MHz

RF

= 5825.001MHz; PFD = 12.5MHz

OUT

PN AT 5kHz; BLE E D OFF
IBS; BLEED OFF
PN AT 5kHz; BLE E D ON
IBS; BLEED ON

CHARGE PUMP CURRENT (mA)

a PFD Frequency = 12.5 MHz

10729-005

Rev. 0 | Page 3 of 8