TEKTRONIX 2465A - Page 1
Forum section: electronics / beginners

author: ABSTA
author_rank: Contributor
timestamp: August 24, 2020, 06:20:33 pm
content: Hello ... i found this second hand Oscilloscope in facebook Market. I'm a beginner with these devices. I don't know why the curve has this shape. I tried to calibrate the probe ,but can't obtain the right curve.... is this oscilloscope ok?

author: tggzzz
author_rank: Super Contributor
timestamp: August 24, 2020, 06:29:37 pm
content: That looks OK, so far as it goes.Scopes are complex and subtle instruments. You need to RTFM (see the TekWikihttp://w140.com/tekwiki/wiki/2465A), and possibly a basic introduction to scopes. I'm sure there are many around; if nothing else have a look at TAoE III appendix O.You should also realise that the probe's shield is connected to mains earth, so do not go and probe the mains, lest there be loud noises and smoke. For basic probe safety, and other info, see the links athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: MarkL
author_rank: Unknown
timestamp: August 24, 2020, 08:09:52 pm
content: The waveform is that shape because the calibration output is not the best square wave generator on these scopes.You can clean up the waveform somewhat by connecting the ground lead on the probe to a banana plug plugged into the ground jack right next to the cal output. You could also hold the probe ground to one of the BNC outer shells for a temporary connection.If you're looking to compensate your probe, you'll want to move the horizontal sweep to the 1ms sweep speed which also reduces the cal output frequency. You can then adjust the probe for a flat response. You'll also get a better looking square wave at slower sweep speeds (lower cal frequencies).If these suggestions don't make sense, check out tggzzz's references.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 24, 2020, 08:15:41 pm
content: Connect the probe to channel 1 (or channel 2) and connect it to the calibrator output (where you have it already).Set the scope for 1 ms and 100 mV. Best to turn on the 20 MHz bandwidth limit too. Keep the VAR (variable) setting on the VOLTS/DIV knob turned (clicked) to off.Adjust the intensity on the display to get a sharp trace (might be good already, your photo looks a bit bright but that might just be the camera/photo).Assuming your probe has a compensation adjustment (looks like it does), adjust the probe (preferably with a small nylon or some non-metallic screwdriver blade) to make nice sharp square corners on the displayed waveform. You can use the VERTICAL POSITION knob for the respective channel to get the waveform to line up nicely on the graticule to help you see when the calibration setting produces very sharp corners.You can check/adjust the waveform on both AC and DC settings for 1 M ohm.Enjoy the 2465A! It's a classic.Edit: I think the main issue is that it looks like the scope might be set for 50 ns (hard to see for sure in the photo); try 1 ms and you are likely to get a much sharper waveform to help with probe calibration. Once you get it dialed in at 1 ms then speed up the SEC/DIV setting and watch what happens to the waveform as you step through each setting from 1 ms to 50 ns.- once you get it calibrated post another photo - if it's all working properly you should (will likely) have very sharp edges.As a next step you can use the cursors to measure from rising edge to rising edge, etc. at various SEC/DIV settings and get a feel for frequency, periods, etc. This is a great scope for learning lots of fundamentals. Enjoy!!PS, VERY COOLto have a new EEVer from Antarctica! Welcome!!!

author: bdunham7
author_rank: Super Contributor
timestamp: August 24, 2020, 08:35:20 pm
content: You have it on AC coupling and you are looking at some high frequency noise. Switch to DC coupling (not 50R) and change the timebase and voltage (gain) settings as others have suggested.

author: tggzzz
author_rank: Super Contributor
timestamp: August 24, 2020, 10:18:47 pm
content: Quote from: bdunham7 on August 24, 2020, 08:35:20 pmAC coupling would not affect the display.There is no high frequency noise visible, but the photo is strangely poor. Perhaps the lens needs cleaning.

author: bdunham7
author_rank: Super Contributor
timestamp: August 24, 2020, 11:25:28 pm
content: Quote from: tggzzz on August 24, 2020, 10:18:47 pmScopes can be very difficult to photograph in the wrong light with a phone or cheap camera. Letters and traces 'blossom' for some reason. It's pretty hard to actually see what is going on here.The cursor frequency display fooled me a bit, I can't tell where the other one is and I can't tell from the photo whether that is ms or ns. However, on my 2465B, I get a similar picture from the calibrator setup at 50ns/div, so that's what it is here. What is odd is that it appears to be at 100mV/div, but the calibrator output is 400mV, so it can't be a 10X probe. Yet, on mine, I only get that shape with a specifically miscalibrated 10X probe, not 1X. Hmmmm.And b/t/w, the AC coupling absolutely will change the calibrator waveform and in more ways than one (try it at 50ms) but at 50ns the only change is that it centers the signal around the ground position.EDIT: I see he likely has a probe with an attenuation coding pin, so the scope knows it is 10X.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 24, 2020, 11:27:01 pm
content: I'm pretty sure that if you hook a probe to the calibration test point and you set the scope for 100 mV and 50 ns you will get a waveform that looks pretty similar to what the OP posted. I think the OP just needs to slow it down to 1 ms and go from there.

author: bdunham7
author_rank: Super Contributor
timestamp: August 24, 2020, 11:49:12 pm
content: Wow, this forum is mangling photo uploads!Calibrator at 50ns with and without BW limit and at 50ms with and without AC coupling. Probe with coding pin and just a camera phone.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 24, 2020, 11:57:40 pm
content: Quote from: bdunham7 on August 24, 2020, 11:49:12 pm+1 for what's up with the photo uploads ? (figured out the portrait vs landscape issue: if the photo is taken in landscape orientation it comes up in portrait orientation on the forum post, and vice versa).Attached are images showing 1 ms and 50 ns with AC coupling - seem to have very little impact on the waveform shape vs DC coupled

author: tggzzz
author_rank: Super Contributor
timestamp: August 25, 2020, 12:03:31 am
content: Quote from: bdunham7 on August 24, 2020, 11:25:28 pmI recognised the "poor" calibrator waveform as being what you see with a fast sweep speed. The speed was confirmed by the 2.91MHz frequency.The non-square distortion in the waveform is not noise; it is just the calibrator waveform at a speed that is far higher than is useful for probe calibration.Of course, the AC coupling will change traceinverydifferent circumstances, especially if timing is changed by a factor of 1,000,000 (from 50ns to 50ms/div)! I didn't think it worth saying that.The AC coupling spec for a *10 probe is -3dB at <1Hz. The OP's pictures are, ahem, comfortably above that at 2MHz!When calibrating probe, the normal calout frequency is >1kHz, and you are only interested in frequencies higher than that. Again, >>1Hz, so the AC coupling won't affect those waveforms significantly.