MICRF507 datasheet(20/46 Pages) MICREL

20 / 46 page

Micrel, Inc.

MICRF507

March 2010

M9999-032210-B

Frequency Synthesizer

The MICRF507 frequency synthesizer is an integer-N

phase-locked loop consisting of:

•

a reference source, made of an M-divider

clocked by the crystal oscillator

•

a voltage controlled oscillator (VCO)

•

a programmable frequency divider made of an

N-divider, an A-divider, and a dual modulus

prescaler

•

a phase/frequency detector

The loop filter is external for flexibility and can be a

simple passive circuit.

The phase/frequency detector compares the reference

frequency (from the M-divider) with the VCO output fed

through the programmable frequency divider. The

charge pump output of the phase/frequency detector,

after filtering, controls the VCO, closing the loop and forcing

the error between the reference frequency and the divided

VCO frequency to zero.

The block diagram, Figure 12, shows the basic elements and

arrangement of a PLL-based frequency synthesizer. The

MICRF507 has a dual modulus prescaler for increased

frequency resolution. In a dual modulus prescaler the main

divider is split into two parts, the main part N and an

additional divider A, where A < N. Both dividers are clocked

from the output of the dual-modulus prescaler, but only the

output of the N divider is fed into the phase detector. The

prescaler will first divide by 16. Both N and A count down

until A reaches zero, at which point the prescaler is switched

to a division ratio 16+1. At this point, the divider N has

completed A counts. Counting continues until N reaches

zero, which is an additional N-A counts. At this point, the

cycle repeats.

Field Name

Number

of bits

Location of bits

Description

Reg13[3:0],

Reg14[7:0]

M0 counter

Reg10[5:0]

A0 counter

Reg11[3:0],

Reg12[7:0]

N0 counter

Reg18[3:0],

Reg19[7:0]

M1 counter

Reg15[5:0]

A1 counter

Reg16[3:0],

Reg17[7:0]

N1 counter

CP_HI

Reg2[7]

High charge pump current (1= 500μA, 0 = 125µA)

VCO_Freq

Reg3[1:0]

Frequency setting of VCO (see Table 11)

VCO_IB

Reg3[4:2]

VCO bias current setting (see Table 11)

LD_en

Reg1[2]

Lock detect function on/off

Table 9. Register Bit Fields for Frequency Synthesizer

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