# Thread: 10073D Passive Probe Bandwidth

Replies: 8 - Pages: 1 - Oct 16, 2012 11:07 AM Last Post By: Fungible
 Posts: 1 Registered: 05/26/12
10073D Passive Probe Bandwidth
Posted: May 26, 2012 1:31 PM
 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 ?
 Posts: 7 Registered: 12/16/11
Re: 10073D Passive Probe Bandwidth
Posted: Jun 4, 2012 9:57 AM   in response to: montaigne7j
 Divider.bmp (332.0 KB)
 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.
 Posts: 7 Registered: 12/16/11
Re: 10073D Passive Probe Bandwidth
Posted: Jul 19, 2012 10:20 AM   in response to: montaigne7j
 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.
 Posts: 7 Registered: 12/16/11
Re: 10073D Passive Probe Bandwidth
Posted: Aug 14, 2012 9:46 AM   in response to: montaigne7j
 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.
 Posts: 7 Registered: 12/16/11
Re: 10073D Passive Probe Bandwidth
Posted: Sep 13, 2012 10:15 AM   in response to: montaigne7j
 plot2.bmp (1.1 MB) plot1.bmp (1.1 MB)
 Posts: 10 Registered: 07/29/12
Re: 10073D Passive Probe Bandwidth
Posted: Sep 15, 2012 12:42 PM   in response to: Fungible
 Freq_Comp.png (79.4 KB)
 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
 Posts: 7 Registered: 12/16/11
Re: 10073D Passive Probe Bandwidth
Posted: Sep 19, 2012 1:46 PM   in response to: Ed
 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.
 Posts: 10 Registered: 07/29/12
Re: 10073D Passive Probe Bandwidth
Posted: Sep 19, 2012 8:05 PM   in response to: Fungible
 Thank you
 Posts: 7 Registered: 12/16/11
Re: 10073D Passive Probe Bandwidth
Posted: Oct 16, 2012 11:07 AM   in response to: montaigne7j
 plot2.bmp (394.0 KB) plot1.bmp (394.0 KB)