MICRF620Z datasheet(12/21 Pages) MICREL

12 / 21 page

Micrel, Inc.

MICRF620/MICRF620Z

July 2006

M9999-120205

Frequency Synthesizer

A6…A0

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, phase select

prescaler, programmable frequency dividers and a phase-

detector. The length of the N, M, and A registers are 12,

12 and 6 respectively. The N, M, and A values can be

calculated from the formula:

()

VCO

XCO

PhD

M ≠ 0

≤ A < N

fPHD: Phase detector comparison frequency

fXCO: Crystal oscillator frequency

fVCO: Voltage controlled oscillator frequency

fRF: Input/output RF frequency

There are two sets of each of the divide factors (i.e. A0

and A1). Storing the ‘0’ and the ‘1’ frequency in the 0- and

the 1 registers respectively, does the 2-FSK. The receive

frequency must be stored in the ‘0’ registers.

Crystal Oscillator (XCO)

Adr

0001001

‘0’

‘1’

XCOtune4

XCOtune3

XCOtune2

XCOtune1

XCOtune0

The crystal oscillator is a reference for the RF output

frequency and the LO frequency in the receiver. It is

possible to tune the internal crystal oscillator by switching

in internal capacitance using 5 tune bits XCOtune4 –

XCOtun0. The benefit of tuning the crystal oscillator is to

eliminate the initial tolerance and the tolerance over

temperature and aging. By using the crystal tuning feature

the noise bandwidth of the receiver can be reduced and a

higher sensitivity is achieved.

When XCOtune4 –

XCOtune0 = 0 no internal capacitors are connected to the

crystal pins. When XCOtune4 – XCOtune0 = 1 all of the

internal capacitors are connected to the crystal pins.

Figure 5 shows the tuning range.

-45.0

-35.0

-25.0

-15.0

-5.0

5.0

15.0

25.0

35.0

45.0

55.0

121620242832

[XCO_tune value]

Figure 5. XCO Tuning

The typical start up time for the crystal oscillator (default

XCO_tune=13) is ~750us. If more capacitance is added

(higher XCO_tune value), then the start-up time will be

longer.

To save current in the crystal oscillator start-up period, the

XCO is turned on before any other circuit block. When the

XCO has settled, rest of the circuit will be turned on. No

programming should be made during this period.

The current consumption during the prestart period is

approximately 280µA.

VCO

A6..A0

0000011

‘1’

‘0’

VCO_IB2 VCO_IB1 VCO_IB0 VCO_freq1 VCO_freq0

The VCO has no external components. It has three bit to

set the bias current and two bit to set the VCO frequency.

These five bits are set by the RF frequency, as follows:

RF freq.

VCO_IB2

VCO_IB1

VCO_IB0

VCO_freq1 VCO_freq0

410MHz

434MHz

450MHz

Table 7. VCO Bit Setting

The bias bit will optimize the phase noise, and the

frequency bit will control a capacitor bank in the VCO. The

tuning range the RF frequency versus varactor voltage is

dependent on the VCO frequency setting, and can be

shown in Figure 6.

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