
Micrel, Inc.
Frequency Synthesizer
The MICRF507 frequency synthesizer is an integer-N
phase-locked loop consisting of:
MICRF507
after filtering, controls the VCO, closing the loop and forcing
the error between the reference frequency and the divided
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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
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
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,
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
M0
A0
N0
M1
A1
N1
CP_HI
VCO_Freq
VCO_IB
LD_en
Number
of bits
12
6
12
12
6
12
1
2
3
1
Location of bits
Reg13[3:0],
Reg14[7:0]
Reg10[5:0]
Reg11[3:0],
Reg12[7:0]
Reg18[3:0],
Reg19[7:0]
Reg15[5:0]
Reg16[3:0],
Reg17[7:0]
Reg2[7]
Reg3[1:0]
Reg3[4:2]
Reg1[2]
Description
M0 counter
A0 counter
N0 counter
M1 counter
A1 counter
N1 counter
High charge pump current (1= 500 μ A, 0 = 125μA)
Frequency setting of VCO (see Table 11)
VCO bias current setting (see Table 11)
Lock detect function on/off
Table 9. Register Bit Fields for Frequency Synthesizer
October 2, 2013
20
Revision 2.2