Tektronix 465B square wave not square - Page 1
Forum section: electronics / testgear

author: necessaryevil
author_rank: Regular Contributor
timestamp: August 30, 2014, 12:14:02 pm
content: Hello,I'm quite a newbie when it comes about oscilloscopes. I've bought a second hand Tektronix 465B, but I can't get a good square wave from the integrated calibrator.Since i'm new to oscilloscopes, I'm not sure if it's the oscilloscope or me.My observations:*AC or DC coupling does not make much difference (only the vertical position of the trace changes)*With the 20 MHz bandlimit on the screen is less fuzzySome pictures:With the 20 MHz bandwith limit on:

author: Whales
author_rank: Super Contributor
timestamp: August 30, 2014, 12:18:35 pm
content: Only seeing the 'top' and 'bottom' of the waves is normal for analog scopes. The brightness of the screen phosphor is dependant on how long the scanning electron beam stays over any one area. The 'vertical' parts of the square wave (which last almost zero time) are drawn by the beam moving verticallyvery fast,so they are practically invisible.Distortion of the top and bottom: try using shorter leads. It may be the inductance of your current ones.

author: andrewfernie
author_rank: Newbie
timestamp: August 30, 2014, 12:26:40 pm
content: Have you adjusted the compensation on your probes? There is normally a small screw head on each probe that needs to be tweaked to get a nice square wave. Google "oscilloscope probe compensation" for more.

author: necessaryevil
author_rank: Regular Contributor
timestamp: August 30, 2014, 01:41:38 pm
content: Andrewfernie,There is no such thing on myscopeprobe. Can it be that I have to do the compensation on the oscilloscope? Or do I have the wrong probes?By the way, the square is 1 kHz.

author: cs.dk
author_rank: Unknown
timestamp: August 30, 2014, 01:48:22 pm
content: It is possible, that you have fixed 1x probes, they lack the compensation-screw.Any name/numbers on the probes?

author: Shock
author_rank: Super Contributor
timestamp: August 30, 2014, 02:00:00 pm
content: If the probes have a slider switch on them (or are 10:1 probes) there could be a compensation screw adjustment on the bnc connector or on the probe itself somewhere.

author: EV
author_rank: Frequent Contributor
timestamp: August 30, 2014, 02:02:04 pm
content: Quote from: necessaryevil on August 30, 2014, 01:41:38 pmIt is not on the scope. It is on the probe!

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 30, 2014, 02:06:27 pm
content: Quote from: EV on August 30, 2014, 02:02:04 pm

author: necessaryevil
author_rank: Regular Contributor
timestamp: August 30, 2014, 02:06:46 pm
content: It says Tektronix, but that's just on the removable top part. I 've found no switches or screws on the probes for so far.//I typed scope where I meant probe in my previous post. I fixed it.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 30, 2014, 02:22:22 pm
content: There's something weird going on there. If I read it correctly the square wave should have an amplitude of 300mV, yet your screen shows slightly more than 1 div with the attenuator set to 50mV. Looks like 5:1 probes but they are not very common. Evenso, if they are not 1:1 there should be a compensation possibility.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 30, 2014, 02:44:28 pm
content: Quote from: PA0PBZ on August 30, 2014, 02:22:22 pmThose probes are similar to the ones that Tektronix supply matched to one particular model of scope where probe is (as good as) captive. E.g. Tek 222 series.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2014, 02:55:24 pm
content: Don't recognize those probes, but they can't be very good if they don't even have simple compensation. Dump 'em.Tim

author: necessaryevil
author_rank: Regular Contributor
timestamp: August 30, 2014, 03:47:24 pm
content: Will those probes work with my oscilloscope?http://www.ebay.nl/itm/2x-Digital-Oscilloscope-Probe-100MHz-P6100-with-clip-new-/130808400734?pt=LH_DefaultDomain_0&hash=item1e74c9cb5e&_uhb=1

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2014, 09:17:39 pm
content: 2 Comments:Tek hook grabbers will fit many other probes, so they may not be Tek probes.The eBay probes look like the P6*** series and are OK quality and fine for general purpose work.EditQuoteOne normally would only adjust probe compensation at 10:1 probe settings to correct waveform overshoot/undershoot.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2014, 11:26:38 pm
content: Quote from: necessaryevil on August 30, 2014, 03:47:24 pmI have some of these; they compensate correctly into most scopes. Don't expect great flatness past 100MHz. They do extend into the 100s of MHz, so as long as you aren't trying to measure precision settling or anything.Tim

author: miguelvp
author_rank: Super Contributor
timestamp: August 31, 2014, 06:01:16 am
content: Morton controls has some UNI-T probes under discount, but the 100 MHz are long gone (well there is one but that's a HV 1:100 probe single)https://www.mortoncontrols.com/index.php/clearance.html?dir=asc&limit=all&order=nameI got the 300MHz ones for just $32 for a pair and I'm happy with them. I believe the 100MHz ones went for around $12 a pair but I don't recall.Edit: It was $11 for the pair, not that it matters since those ones are gone:https://www.eevblog.com/forum/reviews/cheap-china-p6xxx-probes-good-bad/msg485886/#msg485886

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2014, 06:36:29 am
content: I agree with PA0PBZ. There is something weird going on with the probes. The levels shown are wrong.Quote from: necessaryevil on August 30, 2014, 03:47:24 pmIt is difficult to tell from the photo but if those are AideTek P6100 probes, then they will work fine. They are not quite as good as the Tektronix 100 MHz probes that I have but it takes me a lot of effort to measure the difference.

author: Whales
author_rank: Super Contributor
timestamp: August 31, 2014, 09:06:12 am
content: My analog scope's vertical amplification board starting playing up once before, but I simply had to clean the contacts on its jumpers. It is worth switching which of your 2 probes goes into which input and comparing the results

author: PaulAm
author_rank: Frequent Contributor
timestamp: August 31, 2014, 04:34:07 pm
content: I had a 468 that behaved as if all of the attenuation factors were off by a factor of 3 (1 V/div was .3V/div). That was only on 1 channel though. Tracked that down to a dirty AC/Gnd/DC coupling switch.This could be the probes, the calibrator output or dirty contacts in the attenuators. Not enough info to say which though.Although I would not recommend it at this point, before a complete diagnosis has been made, cleaning the attenuator contacts is possible. Use only IPA, lookup and follow the Tek cleaning procedure exactly. Be extremely careful. It's easy to mess up the tiny gold contacts and render them useless. Do not under any circumstances use any other contact cleaner.

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2014, 04:40:46 pm
content: Quote from: PaulAm on August 31, 2014, 04:34:07 pmIf the problem is not the probes, then it is likely to be something in common with both channels which would exclude the attenuators and input circuits.QuoteListen and be warned.

author: necessaryevil
author_rank: Regular Contributor
timestamp: August 31, 2014, 08:20:50 pm
content: Thank you for your replies.A few things:*The volt/division error is definitely the probes. I have used a 'normal wire' with the use of a bnc to banana plug converter. The shape of the wave remained the same, but the amplitude becomes 6 divisions, which seems right for a 300 mV square wave with the knob at 50mV/div.*There is still something else wrong. Sometimes I see this:*I'm gonna buy the P6100 probes right now!

author: necessaryevil
author_rank: Regular Contributor
timestamp: September 06, 2014, 09:09:39 pm
content: I'm still waiting for the P6100 probes, but while waiting I scored a Tektronix 2205 for free. I used the probes which came with my 465B on the 2205, and yes, you were right, It was the probes.I have to mention that I'm really proud on my new acquistion. Of course I didn't forget to give the guy which gave it away some beer.

author: israel
author_rank: Contributor
timestamp: September 07, 2014, 06:20:46 pm
content: try the cleaning the switches "AC-GND-DC" usually cause problems of this type. You will have to remove the cover of the attenuators to reach them.Regards,

author: necessaryevil
author_rank: Regular Contributor
timestamp: September 13, 2014, 09:02:11 am
content: I 've got the probes!With the 20 MHz bandwidth limit on I can get the square wave as it should be. However, with the bandwidth limit off the trace is very fuzzy. First thing I'm going to do is to clean the rotary switches and the ac-gnd-dc (as suggested). Thank you for your replies for so far!UPDATE:I've got another question. My squarewave is square now, but it doesn't seem to matter much if I turn the compensation screw on my new probes. Shouldn't be like this while turning: undershoot-GOOD-overshoot? I have this problem on both of my oscilloscopes.I feel I have a long way to go.....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2014, 01:30:46 pm
content: They're on 10x right?The trace naturally becomes fuzzier at high bandwidth. If the noise level of the amplifiers is constant with respect to frequency, then increasing the bandwidth by 4x doubles the noise amplitude. 100/20MHz is a factor of 5, so you can expect the trace width to effectively double.Tim

author: HiassofT
author_rank: Newbie
timestamp: September 13, 2014, 02:17:02 pm
content: Quote from: necessaryevil on September 13, 2014, 09:02:11 amSame here on my tek, the calibrator output seems to be quite noisy. I even had to set the trigger options to HF reject on my rigol to get a stable picture. The cal output of the rigol is a lot better better.I'd say this is nothing to worry about if everything else is working fine.so long,Hias

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2014, 05:34:47 pm
content: That noise level is a little excessive but with x10 probes and a fast oscilloscope, it is easy to pick up lots of ambient radio frequency interference. Some older oscilloscopes had a BNC for the calibrator output so it could be attached to the x10 probe coaxially which makes a huge difference in noise.The probe compensation adjustment should make a big difference in x10 mode and practically no difference in x1 mode.

author: necessaryevil
author_rank: Regular Contributor
timestamp: September 13, 2014, 07:08:27 pm
content: Thank you for your replies! I' ll compare whats on my display with your pics!//editThe problem was in the probes!