+/+
YJGP VKOKPI KU ETKVKECN
YJGP VKOKPI KU ETKVKECN
SM532
Approved Product
Low EMI Sprectum Spread Clock
International Microcircuits,Inc.
8/31/98
525 Los Coches St., Milpitas, 95035 408-263-6300, FAX 408-263-6571
Rev. 1.4
http:/www.imicorp.com
Page 7 of 14
Loop Filters
The SM532 requires an external loop filter to provide the proper operation and modulation profile for a given input
frequency. The loop filter is connected to pin 5 (LF) of the SM532 and is a typical 2 pole low pass filter. Since the
SM532 operates over such a wide range of frequencies, the loop filter will change depending on the frequency of
operation. The following loop filter values are recommended for best performance and modulation profile at 3.0
volts and 5.0 volts VDD. Operating voltage is measured at the VDD pin of the SM532.
Notice that the selection of Loop Filter values only depends on the input frequency and VDD voltage, and
does not depend on the R and S settings.
Figure 3. Recommended Loop Filters
Recommended Loop Filter Values
Input
Range
VDD +/-5%
Input Frequency
Range (MHz)
R1 (K
Ω)
C6 (pF)
C7 (pF)
Loop
Filter #
Low
3.3
14.0 to 22.5
1.0
10,000
1,000
1
Middle
3.3
25.0 to 45.0
1.0
10,000
1,000
1
High
3.3
50.0 to 90.0
1.0
10,000
1,000
1
Input
Range
VDD +/-10%
Input Frequency
Range (MHz)
R1 (K
Ω)
C6 (pF)
C7 (pF)
Loop
Filter #
Low
5.0
14.0 to 19.9
1.0
10,000
1,000
1
Low
5.0
20.0 to 24.9
1.5
6,800
680
2
Low
5.0
25.0 to 29.9
2.0
3,900
390
3
Middle
5.0
30.0 to 39.9
1.0
10,000
1,000
1
Middle
5.0
40.0 to 49.9
1.5
6,800
680
2
Middle
5.0
50.0 to 59.9
2.0
3,900
390
3
High
5.0
60.0 to 79.9
1.0
10,000
1,000
1
High
5.0
80.0 to 99.9
1.5
6,800
680
2
High
5.0
100.0 to 120.0
2.0
3,900
390
3
Table 8
The component values listed in Table 8 are recommended values using commonly manufactured components.
Note that there are actually 3 different sets of loop filter values. Due to the VCO characteristics, the table is
divided in to 3 volt operation and 5 volt operation. Referring to the table above, it is apparent that one set of loop
filter values is all that is needed in the 3 volt operation. In the 5 volt operation, each input operating range is
divided into 3 sections which require a different loop filter for optimal performance. The best loop filter for any
application is the one that, provides the greatest EMI reduction, maintains system integrity, has a modulation
profile shown on page 7 and uses commonly available components.
LF
LF #1
LF
LF #2
LF
LF #3
C6
10,000 pF
C7
1,000 pF
R1
1 K ohm
C6
6,800 pF
C7
680 pF
R1
1.5 K ohm
C6
3,900 pF
C7
390 pF
R1
2 K ohm