FS791/92/94
Document #: 38-07343 Rev *A
Page 4 of 9
Loop Filter Selection Chart
Table 5 provides a list of recommended loop filter values for
the FS791/2/4. The FS79x products operate at both 3.3 and
5.0VDC. The loop filter shown in Figure 1 is representative of
the loop filter components in Table 5
SSCG Modulation Profile
The modulation frequency of the FS79x can be computed di-
viding the input frequency by this 720. The formula to compute
the Modulation Frequency is Fm = fin / 720.
Example: Fin
= 108 MHz
Range
= 1,1
Fm
= 108 MHz / 720 = 150 kHz
With the correct loop filter connected to pin 4, the following
profile will provide the best EMI reduction. This profile can be
seen on a Time Domain Analyzer. See Figure 3.
Table 4. Range Selection Table[3]
Part Number
S1 (Pin 3)
S0 (Pin 7)
Fin (Pin 1 & 2)
Modulation Rate
FSOUT (Pin 6)
FS791
1
1
80–140MHz
Fin/720
80–140MHz
FS792
1
0
40–70MHz
Fin/480
80–140MHz
FS794
0
1
20–35MHz
Fin/240
80–140MHz
R6
C7
LF (pin 4)
Figure 1. Loop Filter Selection
Table 5. FS79x Recommended Loop Filter Values[4]
+3.3 to 5.0VDC, ±5% (R6 = 3.3K)
Input (MHz)
BW = 1% (note 5)BW = 2% (note 5)BW = 3% (note 5)BW = 4% (note 5)
All C7 capacitor values are in picofarads (pF)
80
1000
220
190
170
90
510
190
130
90
100
250
150
96
65
110
200
120
74
47
120
150
80
52
38
130
120
55
33
26
140
100
43
22
16
Notes:
3.
Fin is the frequency of the crystal connected to pins 1 & 2 to form an oscillator circuit or the frequency of a clock source connected to pin 1, derived from other
means. When the clock source is from other than a crystal, pin 2 must be left unconnected.
4.
Component values are industry standards and are readily available from component suppliers.
5.
All bandwidths indicated above are total peak-to-peak spread, example: 1% = +0.5% to –0.5% and 4% = +2.0% to –2.0%
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