泰克示波器探头基础
Application Engineer : 邓启华 QiHua.deng@
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示波器探头技术及应用
示波器 探头 DUT
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探头的类型
探头
Voltage
Passive
Logic
Current
Active
Temperature Optica
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有源探头
探头 C1 C2 CABLE R1 GROUND BUFFER AMP R2 仪器
R3
高输入阻抗缓冲放大器置于探头前端用于驱动传输线.
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Advantages
有源探头
– Low Input Capacitance – Wide Bandwidth – High Input R – Compatibility with 50 or 1 M Systems – No Compensation Necessary
探头输入阻抗
100M 10M 1M 100k 10k 1k 100 10 1 100 1k 10k 100k 1M 10M 100M 1G 10G
1X Passive 100 pF/1 M Z0 0.15 pF/500 Active 0.4 pF/100 k 10X Passive 11 pF/10 M
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小信号 (< 10 mv)
不是真正的地 在地线上噪声远大于信号
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1:1有源探头 (Length of Cable)
?6 feet PROBE TIP GROUND PROBE CABLE 8 - 10 pF/ft* ?1.5 ns/ft 1 M SCOPE 20 pF
Advantages:
– 1X (No Attenuation) – Inexpensive
.5 Equivalent Gates
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其他问题
补偿 系统的带宽 幅度的精度与频率的关系 接地 结构
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Model of Typical High Z 10X Passive Probe
C1 8 pF to 12 pF CABLE SCOPE C3 20 pF R3 1 M
R1 GROUND
+ -
电池供电
寄生电容
V-
大地
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方法四:差分探头
V+
C E
+
G
-
to Scope
V-
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隔离器与差分探头比较
源的阻抗
– B very low – A high
B A +350 Volts
将隔离探头联接到A端,可能危 及被测端.
Output
- 350 Volts
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隔离器与差分探头比较
frequency
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等效负载
CMOS Drive 25 pF Load 3.0 pF per 30 gate Equivalent
1X Passive 10X Passive Z0 Passive Active
Gates
3.5 Equivalent Gates
.25 Equivalent Gates
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10:1高阻无源探头
C1 8 pF to 12 pF CABLE 500 7-50 pF SCOPE C3 20 pF R3 1 M
R1 GROUND
9 M
R2 C2
电缆衰减引起的反射. R2C2 + SCOPE 对电缆来说不是理想的终端. C2 = 低频补偿----示波器的输入电容的变化. (Impedance probes, C2 is adjustable. In others, C1 is adjustable.)
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接地线效应
使用10X无源探头，输入电容10pF，6“接地线 振铃幅度＝50％ 误差 1 典型的振铃频率＝ 2π√ LC =50~70Mhz OR 上升时间=7~5ns
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接地线效应
Coaxial Cable
BNC Probe Adapter
1?Ground Adapter
3?Low Z Ground Lead
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差分探头
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所有测量是差分
Consider the Digital Multi-Meter:
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特殊的情况:示波器测量
传统的示波器探头: Differential measurement where one input is ground
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TDS 784A
p
p
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单端探头在什么情况下会有问题?
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低阻匹配探头
450 CABLE 50 0.5 pF SCOPE 由于电缆被它的特性阻抗终端, 输入端可看成50 , no L or C. 不需要补偿 0.5 pF 电容是一个加速电容用于补偿电缆的集肤效应 (skin-effect).
GROUND
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低阻匹配探头 Advantages/Disadvantages
10X Passive 100 MHZ 500 MHZ Probe Z0 Passive Probe Active Probe 3 GHZ 9 GHZ 750 MHZ 4 GHZ
3.5 ns - 8 pF 13 pF 700 ps
120 ps - 0.15 pF - 500 1 pF 40 ps 4.6 ns - 0.4 pF 2 pF 100 ps 100 k 10 M x p
λ Advantages
- Low Input C - Wide Bandwidth - Compatible with 50 Systems and 1 M with Termination Resistor - No Compensation Necessary
λ Disadvantages
- Low Input R - Must be Terminated into 50
参考点不是地
– Floating measurements – Balanced Signals
由于引入了交流地回路,以地作为测量参考会有问题
– Low Amplitude – High Bandwidth
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浮动测量
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TDS 784A
p
p
+350 Volts A
?
B Output
- 350 Volts
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10:1高阻无源探头
Advantages
– High Input R – Wide Dynamic Range – Inexpensive – Mechanically Rugged – Low Input C vs 1X Probe
Disadvantages
– High Input C – Not Compatible with 50 ?Systems – Must be Compensated
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不要悬浮示波器!
不要断开示波器的电源地线! – 非常的危险 – 得到一个错误的测量 – 可能危及到示波器 – 过分的容性负载可能危及到被测系统 – 有安全的变通办法!
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方法二:探头隔离
V+
变压器或光 隔离
ห้องสมุดไป่ตู้
C E
SCOPE G
+ -
寄生电容
V-
大地
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方法三:隔离示波器
V+
C E
G
Disadvantages:
– Terminated – Very Low Input R λ Unterminated
- Very High Reflections - Very High Input C - Low Bandwidth
* Typical 50 ?cable has about 30 pF/ft of capacitance
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探头对被测点的影响
Vcc
– Without probe Gain = - RC RE
Cc
Rc PROBE
f0 =
1 2 π RCCC
– With probe
VIN
RP RE
CP
Gain = -(RC||RP) RE f0 = 1 2 π (RC||RP)(CC+CP) NOTE: Vcc is an AC Ground
系统的带宽
BW(-3dB) BW(SYSTEM)= 1
√
(
1
BW(scope)
)2 + (
1
BW(scope)
)2
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接地线效应
Vsource
RIN 10 M GROUND LEAD L CIN (approx. 11 pF)
在实际的测量中,测量性能常常由探头的接地线来定,而不 是探头和仪器的带宽或输入阻抗.
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探头补偿
COMPENSATED 过补偿 欠补偿
1 ms/div
1 ms/div
1 ms/div
1 s/div
1 s/div
1 s/div
50 kHz
50 kHz
50 kHz
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系统的带宽
系统的上升时间 tr(10%~90%) tr(System)=√tr(scope)2+tr(probe)2
6?Ground Lead
The signal source is a 1ns rise time source.
No Ground Lead step from a 50
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探头传输延迟
4~8ns的延迟
–对不同类型的探头.
同类探头一般的延迟<200ps.