MICRF505BML datasheet(15/27 Pages) MICREL

15 / 27 page

May 13, 2004

M9999-051304

MICRF505

Micrel

Charge Pump

A6..A0

0000011

CP_HI

‘0’

OUTS3 OUTS2 OUTS1

OUTS0

The charge pump current can be set to either 125

mAor500mA

by CP_HI (‘1’ D 500

mA). This will affect the loop filter compo-

nent values, see “PLL Filter” section. In most cases, the

lowest current is best suited. For applications using a high

phase detector frequency and a high PLL bandwidth, the

500

mA can be a better choice.

PLL Filter

The design of the PLL filter will strongly affect the perfor-

mance of the frequency synthesizer. The PLL filter is kept

externally for flexibility. Input parameters when designing the

loop filter for the MICRF505 are mainly the modulation

method and the bit rate. These choices will also affect the

switching time and phase noise.

The frequency modulation can be done in two different ways

with the MICRF505, either by VCO modulation or by modu-

lation with the internal dividers (see chapter Frequency

modulation for further details). In the first case, the PLL

bandwidth needs to be adequately low not to cancel the

modulation. In the second case, using the dividers, the PLL

needs to lock on a new carrier frequency for every new data

bit. Now the PLL bandwidth needs to be adequately high. It

may be necessary to use a third order filter to suppress the

phase detector frequency, as this is not suppressed as much

as when doing modulation on the VCO.

A schematic for a second and third order loop filter is shown

in Figure 5. For a second order filter, C3 is not connected and

R2 is 0

Pin 27

CP_OUT

Pin 29

VARIN

Figure 8. Second and Third Order Loop Filter

Table 9 shows three different loop filters, the two first for VCO

modulation and the last one for modulation using the internal

dividers. The component values are calculated with RF

frequency = 915MHz, VCO gain = 67MHz/V and charge

pump current = 125

mA. Other settings are shown in the table.

The varactor pin capacitance (pin 29) of 6pF does not

influence on the component values for the two filters with

lowest bandwidth. For the 12kHz bandwidth filter, a third

order loop filter is calculated. The third pole is set by R2

¥C3.

Here C3 is chosen to be 6pF, the same as the varactor input

pin capacitance (pin 29). C3 can therefore be skipped.

Baud Rate

PLL BW

Phase

(kbaud/sec)

(kHz)

Margin (

∞)

Detector Freq. (kHz)

VCO

>20

0.8

500

56nF

560nF

1.2k

0NC

VCO

>125

500

3.9nF

560nF

4.7k

0NC

Internal

>10

300

56nF

560pF

27k

220k

Table 9. Loop Filter Components Values

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