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Micrel, Inc.
Frequency Deviation under VCO Modulation
Three factors determine the deviation at which the data
stream is modulated: modulator clock frequency, capacitor
charge/discharge current, and modulator attenuator
setting. After each is presented in turn, the complete
formula for deviation is given.
The modulator clock frequency is determined by the
crystal oscillator frequency and the settings of the
Refclk_K and Mod_clkS fields:
MICRF507
To avoid saturation in the modulator, it is important not to
exceed maximum Mod_I. Maximum Mod_I for a given
f MOD_CLK is given by:
Mod_I MAX = INT(f MOD_CLK ? 28 × 10 -6 ) - 1
where INT() returns the integer part of the argument.
f MOD_CLK =
f XCO
Refclk_K × 2 (7 - Mod_clkS)
Set the modulator clock frequency to either 8x or 16x the
frequency deviation rate.
Figure 21. Two Different Modulator Current Settings
A third factor determining deviation is the modulator
attenuator , controlled by the Mod_A field. The attenuator is
used when the bit rate is small and/or the BT is small,
which gives a relatively slow modulator clock and therefore
long rise- and fall times, leading in turn to large frequency
deviations unless the signal is attenuated. Additionally, the
attenuator will improve the resolution in the modulator.
Figure 20. Two Different Modulator Clock Settings
Having f MOD_CLK set at 8 times the bit rate corresponds to a
baseband data signal filtered in a Gaussian filter with a
bandwidth-period product (BT) of 1. When BT is increased,
the waveform will be less filtered. Figure 20 shows two
waveforms with BT=1 (the minimum, with f MOD_CLK at 8
times the bitrate) and BT=2 (with f MOD_CLK at 16 times the
bit rate). Changing BT changes the charge-and discharge
times and therefore the frequency deviation, as seen in
Figure 20.
The capacitor charge/discharge current , set with the Mod_I
field, affects deviation, as shown in Figure 21, where
Mod_Ia>Mod_Ib. Higher current will give a higher
frequency deviation and vice versa. The effect of
modulator clock and Mod_I is as follows:
Figure 22. Two Different Modulator Attenuator Settings
The effect of the attenuator is given by:
f DEV
∝
Mod_I
f MOD_CLK
f DEV ∝
1
1 + Mod_A
October 2, 2013
31
Revision 2.2
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