MDO3000 20MHz Bandwidth Filter - Page 1
Forum section: electronics / testgear

author: steffenmauch
author_rank: Contributor
timestamp: September 08, 2016, 04:25:35 pm
content: Hello,today I used intensively the 20MHz bandwidth limiting feature of the MDO3000. After a while I tried to find out what kind of filter it is with respect to the -3dB frequency, -20/-40/-80 dB per decade etc.Until now, I have not found the information that's why I tried to measure it out.Using the SDG2000X series waveform generation, I used one channel with active 20MHz bandwidth option (CH1) and another with 500MHz (Ch2).The waveform generator was connected via 50 Ohm BNC cables to the MDO3000.I did quite some measurements but stopped as when having a 50 MHz sine applied with 100mV resolution on Ch1.Switching to 50mV, 20mV, 10mV, 5mV, 2mV or even 1mV does not change the displayed data. When using the measurement functionality the value is reduced.E.g. switching from 100mV to 50mV halfs the value. But the signal being fed into the channel hasn't changed.So first question, does anyone have more information regarding the 20MHz bandwidth filter (e.g. a bode diagram or the order of the filter and filter type)?Second question can someone confirm such behavior when measuring with the 20MHz bandwidth filter with low resolution?Is this normal behavior?P.S. I even updated the firmware to V1.22 but it is the same with the new firmware.Thanks.

author: steffenmauch
author_rank: Contributor
timestamp: September 08, 2016, 07:17:20 pm
content: Well, after thinking for quite a while, it assume that the 20 MHz Bandwidth filter is applied digitally.My sinus had 4Vpp; for <100mV/div the ADC/analog front end is saturated.Usually, there is some warning displayed with respect to clipping; but maybe this is not implemented for the 20MHz Bandwidth filter case.If this is true, there should be a warning and the measurement should be disabled ...

author: steffenmauch
author_rank: Contributor
timestamp: September 23, 2016, 09:47:43 am
content: Well, I need some help of anyone with a MDO3000 as Tektronix support tells me that my hardware is not working correctlyBut as I checked everything with a MDO4000 having the same result, I would feel more comfortable when anyone can recheck it with its own MDO3000.Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2016, 12:20:24 pm
content: Quote from: steffenmauch on September 08, 2016, 07:17:20 pmI really doubt this; if it was implemented digitally then I would expect the manual or specifications to say so.

author: MarkL
author_rank: Unknown
timestamp: September 23, 2016, 03:01:31 pm
content: Quote from: steffenmauch on September 08, 2016, 07:17:20 pmI think you're correct that you're overdriving the input amplifier stage. The BW filter is happening after that stage and is making the clipped signal (probably a square wave at that point) look like a sine again. It is still an analog filter. I can get similar odd behavior on an Agilent scope. I also notice the offset doesn't work right because it is implemented as part of the input amplifier.I don't know the answer to your question, but to measure the slope of the filter I would go back to 4Vpp at say 1MHz and make sure nothing is clipping. Then use the FFT function to measure the peak and start moving up in frequency until you get to 50MHz or 100MHz or wherever you want to end. Record the peak dB values vs. frequency as your baseline. Don't assume the response of your scope or sig gen is flat.Then, turn on the BW filter and do the same. The FFT will provide additional sensitivity so you don't have move the attenuator setting and risk overdriving. If you can control the number of points in the FFT by increasing the points or slowing down the horizontal sweep, that will lower the noise floor if needed. The more points, the better.

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2016, 03:44:34 pm
content: An easier way to initially test it is to measure the transition time of a fast edge which is not all that demanding for a 20 MHz bandwidth filter. On a Tektronix oscilloscope, I have every expectation that it will come out to about 17.5 nanoseconds. The sync output on a function generator will work as a signal source for this if you do not have anything better.

author: w2aew
author_rank: Super Contributor
timestamp: September 23, 2016, 08:42:54 pm
content: I checked my MDO3000 and found that with a 20MHz sinewave input, the response was reduced to 74% of full amplitude with the 20MHz BW limit engaged (-2.6dB), and with 40MHz sinewave input was reduced to 28% of full amplitude (-11dB).