Influence of oscilloscope probe on measurement bandwidth

Bandwidth is defined as the measurement system on the amplitude and frequency diagram is lower than the reference level 3dB The point of , Pictured 4.7 Shown , This shows that 3dB Response curve of point .

4.7. Bandwidth is defined as the decrease in amplitude in the response curve -3dB The frequency of

It is necessary to point out , The measuring system has a low amplitude in the rated bandwidth 3dB. This means that you can predict the frequency at the bandwidth limit , Amplitude measurement will have 30% The error of the .

Usually , Users will not use the oscilloscope with the full bandwidth limit . however , If amplitude accuracy is critical , Prepare to reduce the rated bandwidth of the oscilloscope accordingly .

for example , Consider the following figure 4.8 The expansion diagram of bandwidth roll off shown in . The horizontal scale in this figure shows that the result is better than 30% The rating reduction factor required for amplitude accuracy . If there is no rating reduction factor ( The coefficient is 1.0),100MHz Oscilloscope in 100MHz The amplitude error will be as high as 30%. If you want the amplitude measurement to fall on 3% Within the scope of , The bandwidth of this oscilloscope must be 0.3 The coefficient drops to 30MHz. When the frequency exceeds 30MHz when , The amplitude error will exceed 3%.

4.8. Rated bandwidth drop curve

The above example indicates the overall rule of thumb for oscilloscope selection . Yes 3% Amplitude measurement within , The specified bandwidth should be selected to be higher than the highest frequency waveform measured 3-5 Times the oscilloscope .

When the rising time or falling time is the main indicator , The following formula can be used to calculate the bandwidth of the oscilloscope (BW) The index is converted into rise time

indicators ：

Tr≈0.35/BW

Or for convenience ：

Tr(ns)≈350/BW(MHz)

Same as bandwidth , The rise time should be chosen to be faster than the estimated fastest rise time measured 3-5 Times the oscilloscope .( It should be pointed out that , The above bandwidth to rise time conversion assumes that the response of the oscilloscope is Gaussian roll off . Most oscilloscopes are designed for Gaussian roll off response .)

Oscilloscope probe bandwidth

Like other circuits , All oscilloscopes have bandwidth limits . Besides , Same as oscilloscope , The performance of the oscilloscope probe generally depends on the bandwidth . therefore , The bandwidth is 100MHz Your oscilloscope is 100MHz The amplitude response at point is lower than 3dB.

Allied , The oscilloscope probe bandwidth can also be expressed by the same formula used by the oscilloscope (Tr≈0.35/BW). Besides , For active probes , The following formula can be used to combine the rise time of oscilloscope and probe , Obtain an approximate probe / Rise time of oscilloscope system ：

Trsystem2≈Trprobe2+Trscope2

For passive probes , This relationship is more complicated , The above formula should not be used .

Generally speaking , The probe bandwidth should always equal or exceed the bandwidth of the oscilloscope to be used . If the probe bandwidth used is too low , It will limit the oscilloscope to achieve all measurement functions . chart 4.9 Further illustrates this point , It shows the same pulse jump measured by three probes with different bandwidths .

4.9. Influence on the rise time of three different oscilloscope probes ：(a)400MHz,10X Probe ;(b)100MHz,10X Probe ;(c)10MHz,1X Probe . All measurements use the same 400MHz Oscilloscope complete

Pictured 4.9a Shown , The first measurement is using matching 400MHz The combination of oscilloscope and probe . The probe used is 10 Megohm resistance and 14.1pF Capacitive 10X Probe . Be careful , The measured pulse rise time is 4.63ns. This falls well on 400MHz Oscilloscope / Probe combination 875ps Rise time range .

Now let's see how to use the same oscilloscope 、 Use 10X,100MHz What happens when the probe measures the same pulse , Pictured 4.9b Shown , Now the measured rise time is 5.97ns. This is more than previously measured 4.63ns Improved near 30%！

According to expectations , When using a probe with low bandwidth , The observed pulse rise time will become longer . Extreme examples are shown in the figure 4.9c Shown , Which is used on the same pulse 1X,10MHz Probe . here , The rise time has changed from the original 4.63ns Down to 27ns.

chart 4.9 The main conclusion is ： Not any probe can do this ！

To achieve the maximum performance of any oscilloscope , It is also a performance that you spend money on , Be sure to use the probe recommended by the manufacturer .

Bandwidth to oscilloscope probe tip

Generally speaking , It shall be possible to solve the probe bandwidth and the resulting probe according to the manufacturer's following specifications and recommendations / Oscilloscope system bandwidth . for example , Tektronix specifies the bandwidth of the probe working within the specified limit . These limits include global distortion 、 Rise time and scanning bandwidth .

in addition , When used with a compatible oscilloscope , Tektronix probe extends the bandwidth of the oscilloscope to the probe tip . for example , In compatible 100MHz When the oscilloscope is used , teck 100MHz The probe provides 100MHz Performance of (-3dB).

chart 4.10 The equivalent circuit in describes the industry recognized test setup used to verify the bandwidth to the probe tip . The specified signal source impedance of the test signal source is 50 ohm , Terminate at 50 Ohmic resistance , Lead to equivalent 25 Ohm source terminal impedance . Besides , The probe must use the probe head to BNC An adapter or equivalent device is connected to the signal source . The latter requirement of probe connection ensures the shortest grounding path .

4.10. The equivalent circuit used to test the bandwidth to the probe tip . Yes 100MHz System , The displayed rise time should be 3.5ns Or faster

When using the test setup described above ,100MHz Oscilloscope / The probe combination should result in the observed rise time <3.5ns. According to the bandwidth discussed above / Rise time relationship (Tr≈0.35/BW), this 3.5ns Rise time and 100MHz Bandwidth correspondence . For general purpose oscilloscope including standard matching probe , Most manufacturers promise 、 The declared oscilloscope bandwidth is provided on the probe tip .

however , Remember that the bandwidth on the probe tip depends on the figure 4.10 Test Method in . Because the actual environmental signal is rarely from 25 From ohmic signal source , Therefore, in the actual environment, the expected response and bandwidth should be lower than the optimal level to a certain extent , This is especially true when measuring higher impedance circuits .

Ground wire influence

When measuring the reference grounding , There must be two connections to the circuit or device under test . A connection is made through the probe , The probe senses the voltage of other measured parameters . Another necessary connection is to return to ground through the oscilloscope , Connect it back to the circuit under test . To complete the measurement of current path , It is necessary to realize the grounding circuit .

When the circuit under test and the oscilloscope are inserted into the same power socket circuit , The common potential of the power circuit provides a grounding loop . The signal circuit grounded through the power supply is generally indirect , The length is very long . result , You can't rely on this signal loop like a clean low inductance grounding loop .

Generally speaking , When measuring with any type of oscilloscope , The shortest grounding path should be used . The final grounding system is an online ECB( Etch circuit board ) To the probe head adapter , Pictured 4.11 Shown .ECB The adapter allows the probe tip to be directly inserted into the circuit test point , The outer barrel of the adapter should form a direct contact with the grounding ring of the probe head 、 Short ground contact . Measure the critical amplitude and timing , It is suggested that the established test points in the circuit board design include ECB/ Probe head adapter . This not only clearly indicates the location of the test point , And it ensures that the test points can be best connected , Realize the most reliable oscilloscope measurement .

4.11.ECB To the oscilloscope probe head adapter