Micrel
MICRF405
April 2006
28
M9999-041906
(408) 955-1690
Openloop drift Vdd=3.6V
-700
-600
-500
-400
-300
-200
-100
0
100
-40
-20
0
20
40
60
80
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
7
< 9.6
6
< 19.2
4
< 38.4
2
< 50
0
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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