micrf506 Micrel Semiconductor, micrf506 Datasheet - Page 25

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

where INT() returns the integer part of the argument.
Modulator Attenuator
A third way to set the deviation is by programming
the modulator attenuator, Mod_A2..Mod_A0, the last
being LSB. The purpose of the attenuator is to allow
small deviations when the bit rate is small and/or the
BT is small (these settings will give a relatively slow
modulator clock, and therefore long rise- and fall
times, which in turn results in large frequency
deviations). In addition, the attenuator will improve
the resolution in the modulator.
The effect of the attenuator is given by:
- Figure 22 shows two waveforms with different
attenuator setting:
is increased, the frequency deviation is lowered and
vice versa.
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_F2..Mod_F0, the least one being LSB. The
Mod_F should be set according to the formula:
Micrel
September 2004
Figure 21. Two Different Modulator Current Settings
Figure 22. Two Different Modulator Attenuator
Mod_Ab > Mod_Ab
Mod_la > Mod_lb
Mod_la
Mod_lb
Mod_Aa
Mod_Ab
f
DEVIATION
M od_A a <
Settings


1+ Mod_A
M od_A b
1
. If mod_A
25
Mod_F=0 disables the modulator filter and Mod_F=7
gives most filtering. Figure 22 shows a waveform
with and without the filter.
Calculation of the Frequency Deviation
The parameters influencing the frequency deviation
can be summarized in the following equations:
Where:
Figure 23. Modulator Waveform with and without
f
f
f
Refclk_K:
Mod_clkS:
f
Mod_I:
Mod_A:
C1:
C2:
DEV
XCO
RF
MOD_CLK
f
DEV
:
:
:
f
Mod_filter off
=
MOD_CLK
f
MOD_CLK
:
Mod_I
=
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
frequency,
Mod_clkS
Modulator
values between 0 and 31
Modulator attenuator setting,
values between 0 and 15
-2.17_10
82


Refclk_K  2
1+ Mod_A
Filtering
Mod_filter on
1
10
f
XCO
from

C
(7
Mod_clkS )
(
current
Refclk_K
1
+ C
[phone]
M9999-092904
the
2
MICRF506

f
RF
setting,
)
crystal
and