
Micrel, Inc.
Figure 22 shows two waveforms with different attenuator
setting: Mod_Aa < Mod_Ab . If Mod_A is increased, the
frequency deviation is lowered and vice versa.
The resulting frequency deviation in terms of the
parameters discussed above is given in the following
equations:
MICRF507
f MOD_CLK =
f XCO
Refclk_K × 2 (7 ? Mod_clkS)
Figure 23. Modulator Waveform with and without Filtering
f DEV =
Mod_I
f MOD_CLK
×
1
1 + Mod_A
× ( C 1 + C 2 × f RF )
in which r b is the bit rate in bits/sec. Mod_F=0 disables the
modulator filter and Mod_F=7 provides the most filtering.
Figure 23 shows a waveform with and without the filter.
The modulator filter will not influence the frequency
-2.72 x 10
where:
f DEV :
f XCO :
f RF :
Refclk_K:
Mod_clkS:
f MOD_CLK :
Mod_I:
Mod_A:
C 1 :
C 2 :
Single sided frequency deviation [Hz]
Crystal oscillator frequency [Hz]
Center frequency [Hz]
6 bit divider, values between 1 and 63
Modulator clock setting, values
between 0 and 7
Modulator clock frequency, derived
from the crystal frequency, Refclk_K
and Mod_clkS
Modulator current setting, values
between 0 and 31
Modulator attenuator setting, values
between 0 and 15
10
85.2
deviation as long as the programmed cut-off frequency is
above the actual bit rate.
Note that the constants C 1 and C 2 are empirically derived.
Actual frequency deviation may differ by a few percent.
Modulator Filter
To reduce the high-frequency
components in the
generated waveform, a filter with programmable cut-off
frequency can be enabled. This is done using Mod_F[2:0],
bit 0 being LSB. The Mod_F field should be set according
to the formula:
Mod_F =
150x10 3 bps
r b
October 2, 2013
32
Revision 2.2