Tektronix 2440 Self Cal vs. Ext Cal - Page 1
Forum section: electronics / testgear

author: rheb1026
author_rank: Regular Contributor
timestamp: April 03, 2017, 06:53:15 pm
content: Does anyone happen to know the difference between the Tek 2440 self cal and ext cal? I was looking to replace the internal battery in this scope and would potentially have to recalibrate it. I've looked through the service manual for information, but couldn't find anything about what one vs the other does (or maybe I missed it in the 500 pages).Is the ext cal only for the external trigger inputs on the back of the instrument? How well would the scope work if I were only able to do the self cal?I should be able to do the dc voltages for the ext calibration, but also don't know what kind of pulse is needed for the "fast pulse" portion. Would a repetitive square wave suffice?http://www.tek.com/oscilloscope/2440-manual/2440-manual/2440

author: LazyJack
author_rank: Frequent Contributor
timestamp: April 03, 2017, 07:41:01 pm
content: The service manual pretty much details all calibration steps, I'm not sure why you haven't found it.Self cal is completely automatic, and calibrates much of the functions of the instrument, relative to its own reference.However, there are a couple of steps that cannot be self cal'd. These are the internal reference itself, the attenuators for the both channels and both trigger inputs (they are on the front, not the back), the delay between CH1 and CH2 and the mysterious CTE, which I believe has to do with the CCD shift registers performance.For self cal, you do not have to do anything, just run it.For ext cal, you need to provide various DC voltages (such 0.2, 2 and 20V) on the vertical and the trigger inputs and you also need 100kHz 60mV p-p square wave, with fast rise time and good signal integrity. The manual recommend the PG506 calibrator generator, but you can use anything else that can provide these.The voltages have to be accurate, as the internal 10V reference is comapred to them, and if off by a couple of %, then that cal step will fail. Same for the square wave during CTE cal, if not the right voltage, frequency or much distortion, it will fail. But no harm will be done, just rerun the failed step with proper settings.It's not hard to do, the scope will guide you trough the ext cal process, follow the instruction on the screen. Do read the service manual in depth, and understand what each step is for. However be aware, that there may be differences. For example later software versions ask for GND instead of 0.2mV during the trigger ext cal. This does not make any real difference, but it may not be what is in the manual.If you clear the calibration because of a flat nvram battery, both self and ext cal must be performed to get the scope back to spec. You also may want to check the internal adjustments section, but unless being confident and having the proper signal source, do not touch the CCD clock setting. The CTR adjustment however is worth checking.Oh, and the most important. For ext cal or internal adjustments, you have to remove the calibration disable jumper. Also, do not run the cope without the cover, unless you use an extra fan to cool the CCD arrays, Otherwise overheat damage will occur.

author: rheb1026
author_rank: Regular Contributor
timestamp: April 03, 2017, 08:23:07 pm
content: Thanks very much for the information. I was able to find the info about calibration procedures, but I couldn't find anything about what exactly the extended cal procedures calibrated and if they were necessary. I also wasn't able to find (or maybe missed it) that the pulse should be a 100kHz 60mV square wave.

author: LazyJack
author_rank: Frequent Contributor
timestamp: April 04, 2017, 06:50:15 am
content: Actually, I may be mistaken here, it is 60mV peak, but 120mV peak-to-peak. But this will become evident because the scope sets itself up for the measurement and will give instructions to provide a 6 division high signal, centered vertically. I think it is at 20mV/div at that time, hence the 120mV p-p. The manual doesn't mention this, because it simply refers to the PG506 calibrator, which gives the proper signals.For the delay time calibration, it does not really matter, any square wave will do. But for the CTE calibration you need to provide the above voltages. BTW, CTE cal has to be executed as the last calibration step. The scope will warn about this anyway.