HMC704LP4E【PDF 14/44 页】IC FRACT-N PLL 16BIT 24QFN

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5 - 21

HMC704LP4E

v03.1211

8 GHz fractionaL-n PLL

For price, delivery, and to place orders: Hittite Microwave Corporation,20 Alpha Road, Chelmsford, MA 01824

Phone: 978-250-3343

Fax: 978-250-3373

Order On-line at www.hittite.com

Application Support: Phone: 978-250-3343 or apps@hittite.com

With this simplification the total integrated VCO phase noise,

Φ , in rads2 is given by

where

( )

is the single sideband phase noise in rads2/Hz inside the loop bandwidth, and

B is the 3dB corner frequency of the closed loop PLL

The integrated phase noise at the phase detector,

, is just scaled by N2 ie.

The rms phase jitter of the VCO (

) in rads, is just the square root of the phase noise integral.

since the simple integral of (EQ 5) is just a product of constants, we can easily do the integral in the log domain. For

example if the phase noise inside the loop is -110dBc/Hz at 10kHz offset and the loop bandwidth is 100kHz, and the di-

vision ratio is 100, then the integrated phase noise at the phase detector, in dB, is given by;

, or equivalently

Φ=

= 18urads = 1 milli-degrees

rms.

While the phase noise reduces by a factor of 20logN after division to the reference, due to the increased period of the

PD reference signal, the jitter is constant.

The rms jitter from the phase noise is then given by

jpn

In this example if the PD reference was 50MHz, Tpd = 20nsec, and hence Tjpn = 56 femto-sec.

PD Window Based Lock Detect

Lock Detect Enable “Reg 0Bh”[3]=1 is a global enable for all lock detect functions.

The window based Lock Detect circuit effectively measures the difference between the arrival of the reference and the

divided VCO signals at the PD. The arrival time difference must consistently be less than the Lock Detect window

length, to declare lock. Either signal may arrive first, only the difference in arrival times is counted.

2 f

()

2 f()

r2 Hz

ф(t)

фrms

Figure 30. Synthesizer Phase Noise and Jitter

(EQ 5)

( )

Φ= Φ

Φ=

( )

(

)

10log

= -110 + 5 +50 - 40 = -95 dBrads

pddB

βπ

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