MICRF506BML datasheet(13/28 Pages) MICREL

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Micrel

MICRF506

September 2004

M9999-092904

(408) 955-1690

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, 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, M, and A registers

are12, 12 and 6 respectively. The M, N, and A

values can be calculated from the formula:

fPDH =

XCO

VCO

31 N + A

where fPhD 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)

Adr

D7D6D5

0001001

‘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

C11

5.6pF

C10

5.6pF

TSX-10A

Pin 24

XTALOUT

Pin 23

XTALIN

Figure 5. Crystal Oscillator Circuit

The crystal should be connected between pins

XosIn and XoscOut (pin 23 and 23). In addition,

loading capacitors for the crystal are required. The

loading capacitor values depend on the total load

capacitance, CL, specified for the crystal. The load

capacitance seen between the crystal terminals

should be equal to CL for the crystal to oscillate at

the specified frequency.

+ Cparasitic

The parasitic capacitance is the pin input

capacitance and PCB stray capacitance. Typically,

the total parasitic capacitance is around 6pF. For

instance, for a 9pF load crystal the recommended

values of the external load capacitors are 5.6pF.

It is also possible to tune the crystal oscillator

internally by switching in internal capacitance using

5 tune bits XCOtune4 – XCOtun0. 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 3 shows the

tuning range for two different capacitor values, there

are 1.5pF and no capacitors.

The crystal used is a TN4-26011 from Toyocom.

Specification: Package TSX-10A, Nominal frequency

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