Title: Low EMI Spread Spectrum Clock Oscillators

Title: Low EMI oscillators
Product: HM series
MERCURY
www.mercury-crystal.com
TN No.: TN-020
Date: March 3, 2004
Revision: 2
Page 1 of 1
Title: Low EMI Spread Spectrum Clock Oscillators
Background
Traditional ways of dealing with EMI (Electronic Magnetic Interference) problems include using EMI filters, ferrite beads,
chokes, adding power layer and ground plane to the board, more metal shielding and special coating and RF gaskets.
However, the principle sources of EMI problem come from the system clocks including frequency timing generators, crystal
oscillators, VCOs and PLLs. It is obvious that the most efficient and economic way to reduce EMI for the whole system is
to use low EMI spread spectrum clock (SSC) oscillators. The advantages of using low EMI oscillators are easy passage of
regulatory testing, short time-to-market and cost reduction.
Spread Spectrum Technology (SST)
Maximum allowable EMI radiation is normally referred to the peak EMI emissions not the averaged emissions. A
good approach is to spread out the concentrated mode energy on one particular frequency to a broader bandwidth and
controlled frequency range (for example: center frequency ±1%) with a controlled modulation rate. The total mode energy
remains the same but the peak energy got spread out to near-by frequencies. This frequency modulation technique is known
as Spread Spectrum Technology (SST). In stead of patching the EMI problems with filtering and shielding, SST provides
efficient and low cost solutions to the expensive EMI problems. Spread Spectrum Clock Oscillator (SSC) takes advantage
of the SST to provide low EMI frequency sources. In most of the cases, system designers face the EMI problem at the time
their products fail the EMI/EMC regulations at the test labs. Mercury HM series low EMI crystal oscillators provide drop-in
replacement solution for situation like these. The big plus is no board re-spinning.
As shown in the following spectrum comparison graphs, a conventional clock (un-modulated) has narrow band
width and peak radiation energy. By the SST, a 10 dB or more of the EMI reduction can be expected. The modulation
carrier frequency is usually in the range of 6 to 55 KHz (Mercury model and frequency dependent) which makes the
modulation process transparent to the oscillator frequency. Consequently, electronic devices have lower EMI emissions but
not affected by the resultant instantaneous frequencies.
Center Spread vs. Down Spread
The controlled modulation process can be on all of one side of the nominal frequency (down spread) or 50% up
and 50% down (center spread). Pick 100 MHz SSC as an example, its center frequency is modulated between 99.500 MHz
and 100.500 MHz with center spread of 0.5%; the frequency range is between 99.500 MHz and 100.0 MHz if down spread
at 0.5%. By moving the center frequency, a down spread 0.5% modulation can be considered a process equivalent to a
center spread of 0.25%. In another word, modulation between 99.500 MHz and 100.0 MHz (down 0.5%) is equivalent to
center spread 0.25% with center frequency at 99.750 MHz.
A.
100 MHz at center spread 0.5%
B.
100 MHz at down spread 0.5%
“B” is equivalent to 99.750 MHz at
center spread of 0.25%
Instantaneous
Frequencies (min.)
99.500
99.500
99.500
Center
Frequency
100.000
99.750
Instantaneous
Frequencies (max.)
100.500
100.000
100.000
The down spread is preferred if a system can not tolerate operating frequency higher than the nominal frequency
(over-clocking problem). In the above 100 MHz center spread 0.5% example, there is a period of time that the system
running between 100.000 MHz and 100.500 MHz, these instantaneous frequencies are higher than the system clock and may
erode system timing margin. Using down spread can avoid this problem with the sacrifice of a slightly slower clock rate.
Down-center spread (also called asymmetric spread), a compromised way of the two, is another choice and available in
some Mercury HM series
Title: Low EMI oscillators
MERCURY
www.mercury-crystal.com
Product: HM series
TN No.: TN-020
Date: March 3, 2004
Revision: 2
Page 2 of 2
Conventional Clock
Conventional Clock
dB
dB
EMI reduction
fo - (fo*1%)
Spread
spectrum
clock
EMI reduction
fc
fo - (fo*0.5%)
fc
fo=fc fo + (fo*0.5%)
F
F
fo + (fo*0.5%)
fo
T
fo
fo - (fo*0.5%)
T
fo - (fo*1%)
Modulation Carrier frequency
Modulation Carrier frequency
Figure 1: Spectrum comparisons. 1% down spread and ±0.5% center spread as examples
Modulation Carrier Frequency:
The modulation carrier frequency (sweep rate) is typically around KHz range which is relatively slower compared
with the MHz range of the clock frequency. As shown in figure 1, the output frequency is slowly swept within the pseudo
triangle shape wave envelope from the f(max). to fo(nominal) then to f(min) then to fo(nominal)., back and forth. The
resultant instantaneous frequencies are always between f(max) and f(min). The modulation percentage determines the
bandwidth of the span while the modulation carrier frequency determines the spacing of the spectral.
EMI Reduction at Harmonics:
As seen in figure 2 and figure 3, higher order harmonic frequencies do get stronger EMI reduction. They also show
that the greater modulation percentage reduces EMI emissions more. It needs to be pointed out that the fundamental
frequency as well as every harmonics all gets EMI reduction by the SST.
Date: March 3, 2004
Title: Low EMI oscillators
MERCURY
www.mercury-crystal.com
Product: HM series
Revision: 2
TN No.: TN-020
Page 3 of 3
EMI Reduction (dB)
Group: P
SST Fout = 125 MHz
28
25
22
19
16
13
10
7
1
3
5
7
9
11
13
15
17
HARMONICS
0.50%
1%
1.50%
2%
3%
3.5%
Figure 2: EMI reduction at harmonics for Mercury HM series group P
Group: W
SST Fout = 16.384 MHz
EMI Reduction (dB)
18
16
14
12
10
8
6
4
2
0
1
3
5
7
9
11
13
15
17
HARMONICS
1.25%
3.75%
Figure 3: EMI reduction at harmonics for Mercury HM series group W
19
19
Date: March 3, 2004
Title: Low EMI oscillators
MERCURY
www.mercury-crystal.com
Product: HM series
Revision: 2
TN No.: TN-020
Page 4 of 4
Measurement Data:
EMI reduction data shown below are actual measurement data taken from Mercury 3HM57 series group “R”. SSC
100 MHz at various spread percentages and harmonics at each percentage are shown.
TS305B Spectrum analyzer Waveform @SSC=OFF
TS305B Spectrum analyzer Waveform @SSC=+/-0.25%
0
0
-3.33dBm
-10
-20
-20
-30
-30
-40
-40
[dBm]
[dBm]
-10
-50
-50
-60
-60
-70
-70
-80
-80
-90
-90
-100
-100
96
97
98
99
100
101
102
103
-11.19dBm
96
104
97
98
99
TS305B Spectrum analyzer Waveform @SSC=+/-0.5%
102
103
104
103
104
0
-10
-12.98dBm
-20
-20
-30
-30
-40
-40
[dBm]
[dBm]
101
TS305B Spectrum analyzer Waveform @SSC=+/-1.5%
0
-10
-50
-60
-60
-70
-80
-80
-90
-90
-100
-100
97
98
99
100
101
102
103
-18.54dBm
-50
-70
96
104
96
97
98
99
Frequency[MHz]
-3.21
-10
100
101
102
Frequency[MHz]
TS305B Spectrum Analyzer Waveform @SSC=OFF
0
TS305B Spectrum Analyzer Waveform @SSC=+/-0.25%
0
-10.04
-20.15
-20
-24.25
-30
-10
-25.40
-40
-30
-50
-40
-60
-11.21
-20.19
-20
-35.04
[dBm]
[dBm]
100
Frequency[MHz]
Frequency[MHz]
-33.37
-37.72
-38.87
-48.97
-50
-70
-60
-80
-70
-90
-80
-90
-100
0
100
200
300
400
500
600
700
800
-100
900 1000 1100 1200
0
Frequency[MHz]
100
200
300
400
500
600
700
800
900 1000 1100 1200
Frequency[MHz]
TS305B Spectrum Analyzer Waveform @SSC=+/-0.5%
TS305B Spectrum Analyzer Waveform @SSC=+/-1.5%
0
0
-10
-10
-12.80
-20
-29.09
-30
-36.75
-40
-41.23
-42.47
-51.40
-50
-60
[dBm]
[dBm]
-19.46
-20
-24.45
-30
-80
-80
-90
-90
-100
-100
200
300
400
500
600
700
Frequency[MHz]
800
900 1000 1100 1200
-48.47
-58.77
-60
-70
100
-46.35
-50
-70
0
-41.22
-40
0
100
200
300
400
500
600
700
Frequency[MHz]
800
900 1000 1100 1200
Date: March 3, 2004
Title: Low EMI oscillators
MERCURY
www.mercury-crystal.com
Product: HM series
Revision: 2
TN No.: TN-020
Page 5 of 5
-24
-22
-18
-16
50 MHz
32.768 MHz
25 MHz
-14
-12
25 MHz
-10.1 dB
-15.5 dB
-17.6 dB
-22.9 dB
32.768 MHz
-9.9 dB
-15.6 dB
-17.8 dB
-23.0 dB
50 MHz
-10.1 dB
-16.3 dB
-18.2 dB
-23.3 dB
25 MHz, 32.768 MHz and 50.0 MHz
at center spread ±0.25%, ±0.5%
and ±1.5%.
-10
-8
off
0.25
0.5
1.0
0.75
1.5. SSC %
1.25
TS302B(Down Spread) Spectrum Analyzer Waveform@SSC=OFF
TS304B(Center Spread) Spectrum Analyzer Waveform @SSC=OFF
0
0
-3.78dBm
-20
-20
-30
-30
-40
-40
-50
-60
-50
-60
-70
-70
-80
-80
-90
-90
-100
-100
22
23
24
25
26
-3.76dBm
-10
[dBm]
[dBm]
-10
27
28
22
23
24
Frequency[MHz]
0
0
-10
-17.11dBm
-40
-50
-60
-40
-50
-60
-70
-70
-80
-80
-90
-90
-100
-100
24
25
Frequency[MHz]
28
26
27
TS3024
SSC :OFF - (+/-1.5%)
Reduction = 13.06dBm
-30
[dBm]
-30
23
27
-16.82dBm
-20
TS302B
SSC :OFF - (-3.0%)
Reduction = 13.33dBm
22
26
TS304B(Center Spread) Spectrum Analyzer Waveform @SSC=+/-1.5%
-10
-20
25
Frequency[MHz]
TS302B(Down Spread) Spectrum Analyzer Waveform @SSC=-3.0%
[dBm]
Gain (dB)
SSC is off
SSC=±0.25%
SSC=±0.5%
SSC=±1.5%
-20
28
22
23
24
25
Frequency[MHz]
25 MHz at down spread -3% and center spread ±1.5%. The EMI reduction is about the same.
26
27
28
Date: March 3, 2004
Title: Low EMI oscillators
MERCURY
www.mercury-crystal.com
Product: HM series
Revision: 2
TN No.: TN-020
Page 6 of 6
Start-up
Current
Tr
Tf
Time
Consumption Rise Time Fall Time
(u. sec.)
(mA)
(n. sec.) (n. sec.)
Duty
Cycle
(%)
Logic
“1”
Logic
“0”
200
SSC=OFF 290
200
780
Center
780
Spread
±0.25% 880
3.1
3.0
3.0
8.4
8.1
1.9
2.1
1.9
2.2
2.8
2.0
2.3
2.5
2.5
2.3
48.4
48.5
48.6
49.1
50.0
3.40
3.31
3.39
3.31
3.23
0.14
0.25
0.16
0.14
0.18
8.3
2.3
2.1
48.9
3.30
0.15
880
780
880
780
880
8.4
8.5
8.5
8.9
8.7
2.4
2.6
2.7
1.4
1.7
1.5
2.3
2.1
1.0
1.0
49.4
49.5
49.2
48.6
49.1
3.16
3.32
3.36
3.19
3.16
0.25
0.06
0.11
0.14
0.14
980
8.5
2.1
0.5
49.1
3.40
0.14
Center
Spread
±0.5%
Center
Spread
±1.5%
+3.3V, 25.000 MHz, group “R” oscillator parameter comparisons, Ta=25°C, CL=15 pF
SSC Block Diagram:
Spread spectrum technology can be simplified and expressed as follow.
Modulation carrier frequency in KHz range
÷
Ref.
Frequency
Input
÷
Phase
Detector
÷
ø
Low Pass
Filter
VCO
SSC output
Title: Low EMI oscillators
MERCURY
www.mercury-crystal.com
Product: HM series
Date: March 3, 2004
Revision: 2
TN No.: TN-020
Page 7 of 7
Jitter Due to Frequency Modulation:
Although the SST modulation is processed in the background and the modulation carrier is at least one thousand
times slower compared with the nominal frequency, one still concerns the jitter contributed to the whole system due to the
instantaneous frequency. A comparison between clocks with and without SST modulation shows that the modulation
process contributes less than 0.05% of the cycle-to-cycle jitter to the system. This negligible jitter contribution makes the
spread spectrum oscillators gain more popularity.
Mercury 3HM57 series has 250 ps typical and 300 ps max. for the group “R” and ±100 ps max. for the group “P”.
Concerns of using SSC on PLL
Most of the PLLs can work with SST clocks without any timing problem. However, downstream PLLs (defined as
PLLs that receive clock signal from other PLLs in the circuit) requires extra precaution in terms of tracking skew. There are
PLLs (Zero delay buffers ) available in the market specifically designed to work with spread spectrum clocks.
Another area to be concerned is the tracking rate of a PLL needs to be faster than the modulation rate of the SST.
All Mercury HM series has modulation carrier frequency below 60 KHz, downstream PLLs with 6 u sec tracking capability
will work fine.
Power Supply to the SSC
Power supply filtering plays important role to the EMI reduction and optimum jitter performance. Circuit below
shows the recommended power supply filtering configuration. This lowpass “Ð (PI)” filter can remove power supply noise
and prevent clock noise from feeding back to the supply. C1 is low frequency supply decoupling capacitor, a tantalum type
and 22 uF is recommended. C2 is high frequency supply decoupling capacitor, a 0.1 uF ceramic chip capacitor is
recommended. C3, a decoupling capacitor for the SSC, can be a 0.1 uF ceramic chip capacitor. If ferrite bead, C1 and C2 are
not available, tantalum type capacitor is preferred for the C3. All capacitors should be placed as close to the SSC as possible,
otherwise the increased trace inductance will negate its decoupling capability.
Ferrite Bead
System
Power
Supply
++
C1
22uF
Tantalum
Cap.
C2
0.1uF
Ceramic
Chip
Cap.
C3
5.00±0.15
VDD to SSC
Rs
SSC
Output
0.1uF
Ceramic
Chip
Cap.
Series Termination Resistor (Rs)
SSC output traces over one inch should use series termination. The output impedance of group “P” and group “R”
SSCs is 30 ohms. Therefore, for typical 50 ohm trace impedance boards, a 22 ohm chip resistor is recommended. 47 ohm is
recommended for 75 ohm trace impedance boards. The series resistor should be placed in series with the clock line and as
close to the SSC output pin as possible. The series resistor helps to maintain the signal integrity and enhance the EMI
emissions reduction.