Micrel

MICRF506BML/YML

Frequency Synthesizer

The MICRF506 frequency synthesizer consists of a voltage-controlled oscillator (VCO), a crystal oscillator, dual

modulus prescaler, programmable frequency dividers and a phase-detector. The loop-filter is external for flexibility

and can be a simple passive circuit. The phase detector compares frequencies of two signals and produces an

error signal which is proportional to the difference between the input frequencies. The error signal is used to

control a voltage-controlled oscillator (VCO) which creates an output frequency. The output frequency is fed

through a frequency divider back to the input of the phase detector, producing a feedback loop. If the output

frequency drifts, the error signal will increase, driving the frequency in the opposite direction so as to reduce the

error. Thus the output is locked to the frequency at the other input. This input is called the reference and is

derived from a crystal oscillator, which is very stable in frequency. The block diagram below shows the basic

elements and arrangement of a PLL based frequency synthesizer. The MICRF506 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.

Phase

detector

Prescaler

N-Divider

M-Divider

Div/4

1800MHz

Loop filter

XCO

VCO

Charge pump

A-Divider

PA

A6…A0

D7

D6

D5

D4

D3

D2

D1

D0

0001010

-

-

A0_5

A0_4

A0_3

A0_2

A0_1

A0_0

0001011

-

-

-

-

N0_11

N0_10

N0_9

N0_8

0001100

N0_7

N0_6

N0_5

N0_4

N0_3

N0_2

N0_1

N0_0

0001101

-

-

-

-

M0_11

M0_10

M0_9

M0_8

0001110

M0_7

M0_6

M0_5

M0_4

M0_3

M0_2

M0_1

M0_0

0001111

-

-

A1_5

A1_4

A1_3

A1_2

A1_1

A1_0

0010000

-

-

-

-

N1_11

N1_10

N1_9

N1_8

0010001

N1_7

N1_6

N1_5

N1_4

N1_3

N1_2

N1_1

N1_0

0010010

-

-

-

-

M1_11

M1_10

M1_9

M1_8

0010011

M1_7

M1_6

M1_5

M1_4

M1_3

M1_2

M1_1

M1_0

July 2006

15

M9999-092904

+1 408-944-0800