Tektronix 2225 bought on ebay - beginner's question - Page 1
Forum section: electronics / beginners

author: mtr
author_rank: Newbie
timestamp: June 29, 2014, 08:13:41 pm
content: I bought a used Tektronix 2225 from ebay and it was sold as fully functional with two probes. I have tried to compensate the two probes with the best result shown in the attached picture. I used a different set of probes from a rigol ds2072a with the same results for both channels. However, the square wave does not look like the one shown in the user manual for a proper compensated probe. Is there a problem with the scope or is the result ok? Another question is: why does the second channel in chop mode shows no straight line? I have tried to terminate the input of the second channel with the same result.Thank you very much for your help in advance,Michael

author: edy
author_rank: Super Contributor
timestamp: June 29, 2014, 08:28:45 pm
content: You may need to adjust the probe as there is a small screw/pot adjustment. Also check if they are in 1:1 or 10:1 mode. It looks like the signal is "ringing" by over shooting the square and settling back (although fairly quickly). What are you using as the source? The calibration output of the scope?

author: Wim_L
author_rank: Regular Contributor
timestamp: June 29, 2014, 08:44:53 pm
content: See if you can find specifications for both the scope inputs, and probe. The scope input will have a certain capacitance specified. The probe will specify a capacitance range. If the scope input isn't within the compensation range available to the probe, it might not be possible to trim the probe for a perfect waveform (though it's always worth trying, the range is often a little bigger than what the probe specs claim).

author: mtr
author_rank: Newbie
timestamp: June 29, 2014, 08:54:00 pm
content: Thank you edu and Wim_L for the quick reply.edyI have used the calibration output of the scope and have adjusted the probe's trim pots with the best results shown in the picture. The probes are 10:1 fixed - no switching possible.Wim_LThe scope input (as labeled on the 2225) is 1M Ohn with 25pf. I have searched for the probe's specification but with no success. However, I was able to simulate under- and over-compensation, therefore I assume that the compensation range of the probe might be adequate.Could this be a problem of the scope?Michael

author: xwarp
author_rank: Frequent Contributor
timestamp: June 29, 2014, 09:07:32 pm
content: Which channel are you using to comp the probes?If you get nice clean square by adjusting all 4 probes on one channel and not the other, then you might have an issue with one of the channels.

author: w2aew
author_rank: Super Contributor
timestamp: June 29, 2014, 09:51:22 pm
content: Also make sure you use a short ground lead.

author: macboy
author_rank: Super Contributor
timestamp: June 30, 2014, 01:04:20 pm
content: Quote from: mtr on June 29, 2014, 08:13:41 pmThere is nothing wrong with the scope or the probes, they are just not matched perfectly to each other.Do you have original Tek probes, or aftermarket?What wave shape do you see when the compensation signal is connected directly to the input with a short coax cable (not a probe)?The compensation adjustment screw is a low-frequency compensation adjustment. You should adjust this screw to get the flattest possible result in the last (right most) 2/3 of the square wave. In your case, the mid- or high-frequency compensation is not adjusted properly. Typically, this is not user adjustable, because most people would do more harm that good when adjusting this. Most probes have these adjustments, as does the scope input itself. In order to adjust the HF compensation properly, you need a good quality fast pulse generator that gives nice flat top pulses with minimal high frequency artifacts. You have a 100 MHz scope so you would want a ~ 2 ns or faster pulse generator. I don't know the rise time of the 2225 cal signal, but if it is fast enough, then you might be able to adjust the mid frequency compensation using it. I don't recommend trying these adjustments without the proper equipment and knowledge, as you can really mess up the frequency response of the scope and/or probes.

author: robrenz
author_rank: Super Contributor
timestamp: June 30, 2014, 01:33:06 pm
content: Use your position knobs to separate the two traces so they don't overlap at the center.

author: mtr
author_rank: Newbie
timestamp: June 30, 2014, 02:37:22 pm
content: Thank you all very much for your replies, suggestions and help!!xwarpI have just tried what you have suggested but I always get the same results as.w2aewusing a very short ground lead did not improve the situation eithermacboyThe probes are no Tek probes. I have connected both channels directly to an audio generator which improved the waveform - see picture 1. I also have checked the audio generator's output with a borrowed dso and the square wave looks like expected - see second picture.robenzI separated the two channels as recommended but still the square wave looks distorted and the second channel signal shows no straight line. However when I use 'ALT' instead of 'CHOP' I get a straight line as expected.Kind regards,Michael

author: robrenz
author_rank: Super Contributor
timestamp: June 30, 2014, 02:49:47 pm
content: I misinterpreted your first trace picture. I see now that the second channel is being influenced by the channel 1 square wave when you are on chop mode only.Are you setting channel 2 to ground on the input selection (AC GND DC) when you are trying this?

author: mtr
author_rank: Newbie
timestamp: June 30, 2014, 02:52:45 pm
content: yes I tried GND and in addition terminated the second channel, but still the second channel is influenced by the square wave from channel 1

author: robrenz
author_rank: Super Contributor
timestamp: June 30, 2014, 03:07:39 pm
content: For now I would just us ALT and not chop. Others more knowledgeable than me may have input on that issue.On the compensation issue, it may be that therange of compensation adjustment availableon the probes you are using does not cover the capacitance of the 2225 inputs at 1M 25pF

author: mtr
author_rank: Newbie
timestamp: June 30, 2014, 03:12:55 pm
content: ... I will try to get some original Tek probes P6103

author: robrenz
author_rank: Super Contributor
timestamp: June 30, 2014, 03:15:01 pm
content: They don't have to be Tek probes, just make sure the compensation range covers the 25pF of the 2225.

author: mtr
author_rank: Newbie
timestamp: June 30, 2014, 03:22:44 pm
content: ... ok but what does this mean?Can I use a probe with an input capacitance of e.g. < 12 pF and a compensation range of say 10 ... 55pF?

author: robrenz
author_rank: Super Contributor
timestamp: June 30, 2014, 03:31:59 pm
content: I am no expert but I think so

author: w2aew
author_rank: Super Contributor
timestamp: June 30, 2014, 05:34:20 pm
content: Quote from: mtr on June 30, 2014, 03:22:44 pmYes. The first number (<12pF) is the capacitive portion of the load that this probe placed on the circuit you connect it to. The compensation range (10-55pf) is the range of scope-channel input capacitance that this probe can be properly compensated for.Have you checked out my video on probe compensation (what it is, why it's done, etc.)?

author: mij59
author_rank: Frequent Contributor
timestamp: June 30, 2014, 05:35:32 pm
content: Hi,This is not about probe compensation, it is some kind of crosstalk between the channels.I can replicate the symptom on my Tek 2465B feeding a 1 vpp signal in to channel 1, channel 2 set to ground and setting channel 1 to 50 mV per division.The waveform of channel 1 is to big to be displayed, the amplitude of the signal on channel 2 is about 0.1 division.The signal on channel 2 is independent of the sensitivity setting of channel 2, so it is not caused by the front end amplifier.

author: w2aew
author_rank: Super Contributor
timestamp: June 30, 2014, 05:44:43 pm
content: This is not an unusual amount of crosstalk in these analog scopes. The 2565 service manual states that for an 8-div high signal on CH1, that the acceptable crosstalk (at 100MHz test frequency) is 0.08 divisions on other channels.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 01, 2014, 01:51:48 am
content: Quote from: w2aew on June 30, 2014, 05:44:43 pmYou beat me to it!I have observed this effect on "chop" in Tek,HP,Iwatsu ,& other Oscilloscopes.If I recall correctly, setting it for minimum crosstalk was one of the steps we did in the "two yearly" checks on our 500 & 7000 series 'scopes.(Telecom Aust used to send around a road case with the required test gear as per the Handbook).I would suggest that the OP use "alternate",& in any case,wind the channel without any signal on it out of the way,whilst endeavouring to get the best compromise of compensation without the confusing second trace.

author: mtr
author_rank: Newbie
timestamp: July 01, 2014, 09:27:10 am
content: ... thank you all for your valuable input!!!w2aewI always was confused about the capacity values of probes. But with your explanation I final got it!!! Yes I have seen your video but for a newbie's like me, it is quite difficult to grasp every aspect even when you watch the video twice. Furthermore, it is good to know that the crosstalk I have observed on my Tek seems to be normal.w2aew could you please explain why certain probes have one or two trimmers. What is the difference in having two trimmers?

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2014, 10:07:13 am
content: Quote from: mtr on July 01, 2014, 09:27:10 amAs w2aew has gone offline may I answer.1 is for the standard Cal signal on your scope @ 1 KHz.The other is for high frequency. What that frequency is, is specified in that probe's documentation.This can be 1 MHz.Probe compensation must be adjusted at 10:1 on a 10:1/1:1 probe.I have probes to 300MHz BW that have only 1 trimmer.I have a borrowed 500MHz probe that has only 1 trimmer however another model of this probe has 2.

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2014, 03:32:28 pm
content: Quote from: macboy on June 30, 2014, 01:04:20 pmThe probe compensation output is not even close to fast enough for high frequency transient response adjustment. If it was, then it would need to have a coaxial connection as well.

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2014, 03:48:05 pm
content: Quote from: w2aew on June 30, 2014, 05:44:43 pmThere is a big difference though between 100 MHz and the 1 kHz probe compensation signal. I checked my 2232 which is my closest match to a 2225 and no crosstalk was visible with the probe compensation signal still on the CRT. When overloaded, crosstalk was only visible in CHOP mode.The 2232 and 2225 use a very similar channel switch design but I would want to see how a different 2225 performs to be sure. Unless the probe is causing the transient response problem, my suspicion is that something is wrong with the oscilloscope around the vertical channel switch.

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2014, 04:07:27 pm
content: Quote from: mtr on June 30, 2014, 02:37:22 pmAssuming the Rigol is showing the waveform from the audio generator accurately, it looks to me like there is a problem with the transient response on the 2225. I find it difficult to believe that there could be both that severe a crosstalk problem *and* a transient response problem so I suspect they are related. My guess is that the output capacitors on the low voltage power supply are weak and causing poor load regulation.QuoteI tested my 2232 which uses a very similar design and crosstalk with the probe compensation signal was only visible in CHOP mode when the compensation signal was deliberately moved off of the CRT in the vertical direction causing overload.