micrf506 Micrel Semiconductor, micrf506 Datasheet - Page 24

• Manufacturer Part Number: micrf506
• Description: 410mhz And 450mhz Ism Band Transceiver
• Manufacturer: Micrel Semiconductor

Modulator
The modulator will create a waveform with
programmable amplitude and frequency. This
waveform is fed into a modulation varactor in the
VCO, which will create the desired frequency
modulation. The frequency spectrum can be
narrowed by increasing the rise-and fall times of the
waveform.
The modulator waveform is created by charging and
discharging a capacitor. A modulator clock controls
the timing, as shown in Figure19. For every rise-and
fall edge, 4 clock periods are being used. The
charging current during these 4 clock periods are not
equal, this is to reduce the high frequency
components in the waveform, which in turn will
narrow the frequency spectrum.
The frequency deviation can be set in three different
ways, as will be explained below. A formula for
setting the desired deviation is given at the end of
this chapter.
Modulator Clock
The modulator clock frequency is set by:
where f
19, f
a 6 bit number and Mod_clkS is a 3 bit number.
Mod_clkS can be set to a value between 0 and 7.
The modulator clock frequency should be set
according to the bit rate and shaping.
0000100
0000101
0000110
0000111
A6..A0
Micrel
September 2004
XCO
Figure 19. Modulator Waveform and Clock
MOD_CLK
Modulator Clock
Modulator Waveform
is the crystal oscillator frequency Refclk_K is
BitRate_clkS1
f
Mod_F2
Mod_clk
D7
is the modulator clock shown in Figure
-
-
=
Refclk_K  2
BitRate_clkS0
Mod_clkS2
Mod_F1
D6
-
f
XCO
(7
Mod_clkS )
Mod_clkS1
RefClk_K5
Mod_F0
D5
‘0’
Mod_clkS0
RefClk_K4
Mod_I4
D4
‘1’
24
BitSync_clkS2
A f
corresponds to a signal filtered in a gaussian filter
with a Bandwidth (Period-product (BT)) of 1. When
BT is increased, the waveform will be less filtered.
Minimum BT is 1 (Mod_clk is 8 times the bitrate).
Figure 20 shows two waveforms with BT=1 and
BT=2, i.e. the Mod_clk is 8 and 16 times higher than
the bit rate. When changing the BT factor, the
charge-and discharge times will also be changed,
and therefore the frequency deviation, as shown in
Figure 19.
Modulator Current
The current used during the rise- and fall times can
be programmed with the Mod_I4..Mod_I0 bit, the
last one being LSB. Figure 21 shows two waveforms
generated with two different currents, where
higher frequency deviation and vice versa. The
effect of modulator clock and MOD_1 is illustrated
by:
To avoid saturation in the modulator it is important
not to exceed maximum Mod_I. Maximum Mod_I for
a given fMOD_clk is given by:
Mod
RefClk_K3
Mod_A3
Mod_I3
Figure 20. Two Different Modulator Clock Setting
MOD_CLK
D3
_
Mod_clkb > Mod_clka
Ia
of 8 times the bit rate (as in Figure 20)
>
Mod_clka
Mod_clkb
MOD_I
BitSync_clkS1
Mod
RefClk_K2
Mod_A2
Mod_I2
f
DEVIATION
D2
MAX
_
= INT(f
Ib
. Higher current will give a
MOD_CLK


BitSync_clkS0
f
RefClk_K1
MOD_1
MOD_CLK
Mod_A1
Mod_I1
 28
10
D1
6
) - 1
[phone]
M9999-092904
MICRF506
BitRate_clkS2
RefClk_K0
Mod_A0
Mod_I0
D0