MICRF405YML datasheet(28/46 Pages) MICREL

28 / 46 page

Micrel

MICRF405

April 2006

M9999-041906

(408) 955-1690

Openloop drift Vdd=3.6V

-700

-600

-500

-400

-300

-200

-100

100

-40

-20

100

120

Temperature (degC)

Figure 13. Carrier Drift in Open-Loop Modulation.

Divider Modulation

Modulation[1:0]=2, modulation

by switching between two sets of dividers, A0/N0/M0

and A1/N1/M1, is selected. In this case, the PLL

needs to settle at the new frequency for each bit-

shift. Therefore, the PLL bandwidth needs to be

sufficiently high enough to follow the modulation.

The PLL bandwidth/bit rate ratio controls the filtering

of the modulation. A large ratio gives little filtering

and a square shape to the data; while a small ratio

gives hard filtering of the data. To avoid large

overshoots, a large phase margin is desired, about

70 degrees will give about 10% overshoot. The

tradeoff is less rejection of the phase detector

frequency.

ASK Modulation

is selected when ASK_en=1 and

Modulation[2:0]=3. The ASK modulation depth is

controlled by the ASK[2:0] bits, and is equal to

{PA[2:0]-ASK[2:0]}*3dB. If PA[2:0]<=ASK[2:0], the

ASK modulation will be On Off Keying (OOK). For

example, PA[2:0]=7 and ASK[2:0]=7 is OOK, but if

ASK[2:0]=4, the modulation depth is –9dB (output

power for “1” is 10dBm and “0” is 1dBm). The

modulation depth is a tradeoff between occupied

bandwidth and sensitivity in the receiver. When

increasing the modulation depth, the pulling effect of

the VCO will also increase and can be seen as FM

components in the transmitted signal. To reject the

pulling of the VCO, shaping can be applied to the

ASK signal. The ASKshape[2:0] bits control the

shaping, where ASKshape[2:0] = 7 gives the most

shaping. The shaping is user selectable but a table

of recommended values is shown in Table 19.

ASK bitrate (kbps)

Recommended

ASKshape

< 4.8

< 9.6

< 19.2

< 38.4

< 50

Table 19. Recommended ASK Shaping.

A high PLL bandwidth will also help to avoid pulling

of the VCO. The internal third order loop filter should

be selected.

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