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10073D Passive Probe Bandwidth

Question asked by montaigne7j on May 26, 2012
Latest reply on Oct 16, 2012 by Fungible

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Sir,

The followings are spec. of 10073D Passive Probe
Bandwidth: 500 MHz
System risetime (calculated): <700 ps
Attenuation Ratio: 10:1
Input resistance: 2.2 MΩ (when terminated into 1 MOhm)

How can I calculate Probe Bandwidth ?
f3db not equal to 1/RC ?
R means input resistance, 2.2 Mohm,
C means input capacitance, 12 pF.
1/RC is about to 6 kHz that is far from 500 MHz,
What's relation between "bandwidth" and RC corner ?

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Fungible Jun 4, 2012 8:57 AM
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There is no direct relationship between the RC corner you calculated and the native bandwidth of the probe.

   At its core, the passive probe is a capacitively compensated resistor divider as shown in the attached schematic.This circuit has an input R of 2.2meg, input C of 12pf and a divider ratio of 10. When driven from a low impedance source, its frequency response is flat and the bandwidth has no limit.
   However, in the practical world, there is some limit to its bandwidth. That limit is caused by parasitic components not shown in the schematic. The parastics can be altered with careful selection of components, the circuit layout and creative ways of adding components to cancel or minimize parasitics. Thus, one implementation of the circuit could have a higher bandwidth than another depending on the lengths the designer is willing to go to maximize bandwidth.
   An added complication that occurs in passive probes, is that the connection between R2/C2 and R1/C1 is made using a coax cable. At the bandwidths of modern probes, the cable must be treated as a transmission line and techniques must be employed to handle the reflections that occur on the line.
   It is these internal factors that determine the bandwidth of the final probe design. And they are not directly reflected in the input R and C characteristics of the probe.
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- Divider.bmp332.0 KB
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Fungible Jul 19, 2012 9:20 AM
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Ever wonder what is being adjusted when adjusting the low frequency compensation of a passive probe?

All passive probes need to be adjusted when they are first connected to a scope channel. This process involves connecting the scope tip to a “Probe Comp” or “Cal” terminal on the scope and adjusting a trim
device in the probe to flatten the step response. In probe manuals, it typically states that this matches the probe capacitance to the scope input capacitance.

An analysis of the capacitively compensated resistor divider, shown in the first reply, will reveal that the DC or lower frequency attenuation is determined by the ratio of the two resistors and the high frequency attenuation is determined by the ratio of the two capacitors. For the overall frequency response to be flat, the ratio of the resistors and capacitors must be matched. In a 10:1 probe, when R2/R1=9 and C1/C2=9 the LF and HF attenuations are equal and the response is flat.

In the typical probe, R2 and C2 are fixed value components. R1 is either completely made up of the scope input resistance or is made up of the scope input resistance in parallel with a fixed resistor inside the probe. And C1 is made up of capacitance from the probe cable capacitance, the scope input capacitance and a variable capacitor. It is this variable capacitor that is being adjusted during the low frequency compensation process. Through the use of the variable capacitor, C1/C2 can be made equal to R2/R1 when the probe is connected to scopes (or scope channels) that have different input capacitances.

The calculation montaigne7j did using the probes input resistance and capacitance is a good approximation of the crossover frequency between the low frequency and high frequency attenuations.
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Fungible Aug 14, 2012 8:46 AM
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A word about scope input capacitance.

Since a passive probe must be adjusted to compensate for the scope’s input capacitance, it is standard practice for scopes to specify the input capacitance and for probes to specify the range of scope input capacitance they can compensate. These specification are typically found printed on the scope front panel and probe body or can be found in manuals or data sheets. With this information a user can determine if a scope and probe are compatible with each other.

The input of many scopes have two input impedances modes, 50ohm and 1megohm. Its important to understand that the input capacitance specification only applies to the 1megohm input impedance mode. This number should never be used to approximate the scope’s input circuit bandwidth. For example, the DSOX3054A has an input capacitance specification of 15pf. If the scope input mode is 50ohm and it is driven from a 50ohm source 1/(2*pi*25*15pf) = 425MHz. This appears to be in conflict with the 500MHz bandwidth specification of this model scope.

The explanation behind this is in the fact that much of the input structure leading to the input amplifier is made up of 50ohm transmission line. The most visible evidence of this is the input BNC. At the frequencies relevant to the probe LF compensation (several KHz), the transmission line looks like capacitance. And, thus, is included in the input capacitance specification. However, when the 50ohm input mode is selected and the input is driven with a 50ohm coax cable, this scope input transmission line simply becomes an extension of the external coax. Under these conditions, the input transmission line capacitance is no longer relevant.
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Fungible Sep 13, 2012 9:15 AM
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High Frequency compensation

The LF compensation of passive probes was discussed in the second reply. The typical passive probe will also have HF adjustments. In some probes, they are exposed as in the N2863A and N2862A probes. In others, they can only be reached by opening the compensation box at the BNC end of the probe as in the 10073C probe.

These adjustments affect the probe response up near the bandwidth of the probe. They are part of the network that deals with reflections in the probe’s coax cable and will have an influence on the probe rise time and overshoot (see the first reply). Typically they are set by the manufacture when the probe is built and are not touched by the user, which is why they are often hidden.

The plots attached show the how the two exposed adjustments in the 150MHz N2862A probe effect the time domain response. The bright trace is the correctly adjusted response. The two dim traces are the response after turning an adjustment clockwise and counter clockwise a bit. Note one adjustment influences the response flatness out to 15nsec after the edge with a small impact on the rise time and overshoot. The other adjustment has a strong influence on the rise time and overshoot but also has a secondary influence on the response out to 15nsec.

The adjustments are interdependent and it is an iterative process to get to the final response. The final response should be as flat as possible with a rise time and overshoot appropriate for the probe bandwidth. To make this adjustment, the probe must be driven with a high speed edge. One that is faster than the rise time of the probe. Connection to the source must be carefully made with a BNC to probe tip adapter, like the one shipped with the N2862A probe, and a 50ohm through termination. The response will be dependent on the quality of the connection.
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- plot1.bmp1.1 MB
- plot2.bmp1.1 MB
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- ed @ Fungible on Sep 15, 2012 1:04 PM
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  Thank you for your post. This information seems unusually hard to find.
  
  I would appreciate your comment on the following observation which is puzzling me:
  
  I performed the compensation as you described for the high frequencies on my N2863B probes and my DSOX3024A.
  
  When I use a 50 ohm RF signal generator to determine the scope's bandwidth into the 50 ohm vertical inputs of the scope, the 3 dB bandwidth is very close to 200Mhz and the amplitude frequency plot rolls off smoothly with a Gaussian shape. I expected that in a 200MHz scope.
  
  When I measure the bandwidth using the N2863B probing a 50 ohm load on the 50 ohm signal generator (so it is probing a 25 ohm impedance point), the 3 dB bandwidth increases to 280MHz and the shape of the response is very flat out to about 200MHz with about 1 dB of ripple between 130Mhz and 200 MHz. In other words a much more square frequency response when compared to the gradual roll off of the scope's 50 ohm inputs.
  
  Do you know if this is how the X3000 series/N2863B scope/probe system frequency response was designed? Or am I somehow incorrectly compensating the probes?
  
  If the Agilent folks read this, I would greatly appreciate their feedback as well.
  
  Edited by: Ed on Sep 15, 2012 1:22 PM
  
  Edited by: Ed on Sep 15, 2012 2:03 PM
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  - Freq_Comp.png79.4 KB
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  - Fungible @ ed on Sep 19, 2012 12:46 PM
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    There is no indication that the probe is incorrectly adjusted.
    
    Passive probes are the most common method of connecting a signal to a scope. And there is good reason for that. They are inexpensive, widely available, relatively durable, compatible with just about any scope (keeping in mind the LF compensation range) and very easy to use. They are great for quickly moving from point to point in a circuit and getting a very good picture of what the signal there is doing. However, they do have their limitations.
    
    After dealing with the reflections on the probe’s transmission line, there are very few degrees of freedom remaining to tailor the probes roll off to conform to a standard filter response (Butterworth, Gaussian, Chebyshev, etc.). The frequency response you have is very typical of passive probes. There will be ripple in the pass band and it rolls off quickly.
    
    When a person wants the maximum fidelity and accuracy in the time and frequency domain, they should consider using 50ohm probing or active probes.
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    - ed @ Fungible on Sep 19, 2012 7:05 PM
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      Thank you
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Fungible Oct 16, 2012 10:07 AM
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The ground connection is important

In the previous discussion about high frequency adjustments and frequency response, a BNC to probe tip adaptor was used to connect the probe to a source. This adaptor provides a very short coaxial interface between probe and BNC. This means a very short ground connection between probe and source ground.

When probing around on a circuit, the simplest thing to do is connect the probe ground lead provided with the probe (black wire with alligator clip on the end) to a convenient ground in the circuit. Then the probe can be moved from point to point in a 12cm radius (N2862A ground lead is 12cm) without changing the probe ground location. However, the ground lead length will have an effect on the probe response.

In the attached plot1 step response, the dim trace is using the BNC to probe tip adaptor between the source and a N2862A probe. The bright trace is using the supplied 12cm ground lead between source and probe.

In the attached plot2 step response, the dim trace is using the BNC to probe tip adaptor. The bright trace is using the supplied “ground spring” between source and probe.

It is clear the 12cm ground lead changes the probe response and that the ground spring has little impact on the response. The ground lead is a great tool for quickly inspecting signals. However, to get the best high frequency measurement from the passive probe, it is important to use the ground spring.
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- plot2.bmp394.0 KB
- plot1.bmp394.0 KB
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10073D Passive Probe Bandwidth

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