May 13, 2004

13

M9999-051304

MICRF505

Micrel

Frequency Synthesizer

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

The 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 lengths of the N and M and A registers are 12, 12 and 6 bits respectively. The M, N and A values can be calculated

from the formula:

where PhD is the phase detector comparison frequency.

PhD: Phase detector comparison frequency

fxco: Crystal oscillator frequency

fvco: Voltage controlled oscillator frequency

There are two sets of each of the divide factors (i.e. A0 and A1). If modulation by using the dividers is selected (that is

Modulation1=1, Modulation0=0), the two sets should be programmed to give two RF frequencies, separated by two times the

specified frequency deviation. For all other modulation methods, and also in receive mode, the 0-set will be used.

Crystal Oscillator (XCO)

A6..A0

D7

D6

D5

D4

D3

D2

D1

D0

0001001

‘0’

‘0’

‘1’

XCOtune4

XCOtune3

XCOtune2

XCOtune1

XCOtune0

The crystal oscillator is a very critical block. As the crystal oscillator is a reference for the RF output frequency and also for the

LO frequency in the receiver, very good phase and frequency stability is required. The schematic of the crystal oscillator’s

external components for 16MHz are shown in Figure 2.

C11

5.6pF

C10

5.6pF

Y1

TSX-10A

Pin 24

XTALOUT

Pin 23

XTALIN

Figure 5. Crystal Oscillator Circuit

The crystal should be connected between pins XoscIn and

XoscOut (pin 23 and 24). In addition, loading capacitors for

the crystal are required. The loading capacitor values depend

load capacitance seen between the crystal terminals should

frequency.

f

f

M

f

16 N

A

f

16 N

A

PhD

XCO

VCO / 2

RF

==

¥+

=

¥+