### Application Notes

```Application Notes
PECL Output with Single Supply 50 ohm Termination Testing
Doc ID:
Author:
PAN0903041
Eddy Van Keulen
Nim Cho Lam
Joe Sung
Jason Fang
Date:
03-04-2009
Revision: 1
ABSTRACT:
Standard application and testing of PECL output with 50 ohm to VDD-2V termination requires two
power supplies and grounds. In case only a single power supply is used, a method has been developed
to match the ‘scope’ and ‘IC’ (Integrated Circuit) grounds.
STATEMENT OF CHALLENGE:
VDD
Scope : 50 Ohm DC coupled
COAX
50 Ohm
VDD-2V
0
Figure 1: Standard termination of PECL Output
Standard termination method (see figure 1) needs two power supplies with one being used as
the ‘scope’ ground. In a typical application, VDD is 3.3V; VDD-2V=1.3V (‘scope’ ground); GND=0V (‘IC’
ground). With a single power supply, the challenge is that ‘scope’ ground must equal ‘IC’ ground and that
is equivalent to terminating PECL output to VDD-3.3V which is incorrect.
ARCHITECTURE OF SOLUTION:
A transformation network that will let the PECL output see a 50 ohm to VDD-2V termination and
at the same time allows the scope to match impedance with a 50 ohm cable to IC ground is needed.
Figure 2 provides a method of testing PECL outputs when using a single power supply that matches the
impedance between the scope and the IC ground.
47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 1
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing
VDD
Scope : 50 Ohm DC coupled
VDD
R1
R3
P
Q
COAX
50 Ohm
R2
0
0
0
0
Figure 2 Recommended Method of PECL Output Testing with a Single Power Supply
In order to calculate the R1, R2, R3, values, we can follow the following simple steps:
Step 1: Using the Thevenin Equivalent, we can see that
3.3V
130
=
82
0
A
50
3.3V-2V
B
Figure 3 Thevenin Equivalent
Circuit A is Thevenin equivalent of circuit B in figure3.
Step 2: We would like to see a simple ratio of voltage amplitude between VP and VQ for convenience (see
figure 2). Therefore, we choose R3 ≈ 450 ohm so that VQ/VP ≈ 50/(450+50) ≈ 1/10. It is relating
measured amplitude to true amplitude by a simple factor of 10.
Step 3: We choose R1 =130 ohm which is common in some PECL applications.
Step 4: R2//(R3+50) = 82 ohm implies R2 is 98 ohm.
Notes:
With the recommended method, one should multiply the measured amplitude by 10 in order to
obtain the true PECL output amplitude.
47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 2
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing
RECOMMENDED IMPLEMENTATIONS:
A. Cable: SMA Cable
B. Resistors:
z 1% Tolerance Resistors:
z
R1=127Ω
R2=100Ω
R3=453Ω
Resistors’ size should be at least 0603.
TEST FIXTURE AND TEST RESULT:
Figure 4 Standard termination of PECL Output Testing with Two Power Supplies
Figure 5 Standard termination of PECL Output Testing with Two Power Supplies Text Result
47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 3
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing
Figure 6 Recommended Solution with a Single Power Supply
Figure 7 Recommended Method with a Single Power Supply Test Result
SUMMARY OF TEST RESULT:
sample 1
sample 2
sample 3
sample 4
sample 5
Recommend Method with
Single Power Supply (mV)
97.2
97.5
95.7
94.4
99.8
Standard Method with
Two Power Supplies (mV)
996
997
972
967
991
Ratio
10.24691
10.22564
10.15674
10.24364
9.92986
47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 4
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

0903041

Eddy van Keulen
Nim Cho Lam
Joe Sung
Jason Fang

（林念祖）
（宋乃仁）
（方伟杰）
2009 年 3 月 4 日
1

VDD
Scope : 50 Ohm DC coupled
COAX
50 Ohm
VDD-2V
0

1. 标准方法（图一）需要两组电源而且其中一个电源被使用在示波器接地端，在典型的应

2. 在使用 1 組电源的挑战是示波器接地端必須和 IC 接地端接在一起，而且 PECL 输出的传

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 5
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

VDD
Scope : 50 Ohm DC coupled
VDD
R1
R3
P
Q
COAX
50 Ohm
R2
0
0
0
0

Step 1：从著作文献里我們可以得知，下面 A 和 B 兩個图是等同的。
3.3V
130
=
82
0
50
3.3V-2V
A
B

Step 2：我們想要看見一個简单的比例输出振幅在 VP 和 VQ 之间（图二），因此我們选择 R3=450Ω，所以
。
Step 3：我們选择 R1=130Ω。
Step 4：由计算可以求出 R2 的值：
R2 // (R3+50Ω) = R2 // (450Ω+50Ω) = 82Ω => R2 = 98Ω

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 6
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

1. 传输线：使用 SMA 传输线。
2. 电阻：

R1=127Ω
R2=100Ω
R3=453Ω

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 7
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 8
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

sample 1

(mV)
97.2

(mV)
996
sample 2
97.5
997
10.22564
sample 3
97.5
972
10.15674
sample 4
94.4
967
10.24364
sample 5
99.8
991
9.92986

10.24691
47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 9
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

Eddy van Keulen
Nim Cho Lam （林念祖）
Joe Sung
（宋乃仁）
Jason Fang
（方偉傑）

2009 年 3 月 4 日

1

VDD
Scope : 50 Ohm DC coupled
COAX
50 Ohm
VDD-2V
0

3. 標準方法（圖一）需要兩組電源而且其中一個電源被使用在示波器接地端，在典型的應

4. 在使用 1 組電源的挑戰是示波器接地端必須和 IC 接地端接在一起，而且 PECL 輸出的

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 10
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

VDD
Scope : 50 Ohm DC coupled
VDD
R1
R3
P
Q
COAX
50 Ohm
R2
0
0
0
0

Step 1：從著作文獻裡我們可以得知，下面 A 和 B 兩個圖是等同的。
3.3V
130
=
82
0
50
3.3V-2V
A
B

Step 2：我們想要看見一個簡單的比例輸出振幅在 VP 和 VQ 之間（圖二），因此我們選擇 R3=450Ω ，所以
。
Step 3：我們選擇 R1=130Ω 。
47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 11
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

Step 4：由計算可以求出 R2 的值：
R2 // (R3+50Ω ) = R2 // (450Ω +50Ω ) = 82Ω => R2 = 98Ω

3. 傳輸線：使用 SMA 傳輸線。
4. 電阻：

R2=100Ω
R3=453Ω

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 12
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 13
Application Notes
PECL Output with Single Supply 50 ohm Termination Testing

(mV)
(mV)
sample 1
97.2
996
10.24691
sample 2
97.5
997
10.22564
sample 3
97.5
972
10.15674
sample 4
94.4
967
10.24364
sample 5
99.8
991
9.92986

47745 Fremont Blvd., Fremont, California 94538 Tel (510) 492-0990 Fax (510) 492-0991 www.phaselink.com
03/04/09 Page 14
```