Tek 2440 Oscilloscope Check - Page 1
Forum section: electronics / testgear

author: NSterenberg
author_rank: Newbie
timestamp: April 30, 2014, 11:17:57 pm
content: Hi-I recently got a used Tektronix 2440 digital oscilloscope. I am 16 and this is my first scope. Ive done a good deal of research into them, but I have a couple questions.First, I am slightly concerned with rise time. The tek 2440 is rated at 1.16ns rise time, but when I feed the scope a square wave, its rise time is somewhere near 100ns. Am I just confused, or is this in an indication that something is wrong?Second, I was having some trouble with the 2nd channel. With a square wave, it behaved normally at ~5kHz, but at 50kHz it turned into more of a RC charge discharge curve thing(see the picture), but then at 5MHz it matched channel 1, but with reduced amplitude. Anyway, I read through some manuals and found cap 311 was a tuning cap going into the pre-amp. When I touched the tuning screw with a small screwdriver, the signal (at 50kHz) snapped back to a square wave, and its been fine ever since. Im happy with that, just wondering if anyone knows why that happened.Third, when I get the signal up to >1MHz, it appears as a sine wave (2nd picture), not a square wave. If this scope has a 500MSa/s sample rate, shouldnt it be able to handle this?Thanks in advance. -Nick*EDIT* the 2nd picture was taken before i fixed channel two. Ignore the bottom waveform.

author: tautech
author_rank: Super Contributor
timestamp: April 30, 2014, 11:34:48 pm
content: You do not state what your waveform source is.Most do not supply an accurate square wave to the same frequency as they can do a sine wave.

author: NSterenberg
author_rank: Newbie
timestamp: April 30, 2014, 11:39:19 pm
content: I have used two sources for the waveform.One was an instek 8016G (I know its not the best), wasn't using this much because I know its not the most accurate.I also used the internal 50Hz to 5MHz square wave generator on the scope itself.-Nick

author: kc9qvl
author_rank: Regular Contributor
timestamp: April 30, 2014, 11:44:29 pm
content: With the test probes and probe compensation signal the 1 mhz the probe compensation signal starts to roll off. To really check the rise time you need a pulse generator or function gen that has a fast square wave.Tr

author: NSterenberg
author_rank: Newbie
timestamp: April 30, 2014, 11:55:52 pm
content: Ok well my primary concern is wether or not this oscilloscope is really out of spec or not. I got it from ebay and have 12 days left to return it no questions asked. Are there/what are any critical (and easy to perform with minimal resources) tests I can do on this scope?-Nick

author: kc9qvl
author_rank: Regular Contributor
timestamp: May 01, 2014, 12:01:42 am
content: Run through the extended tests and self calibration. Button to the left of the powerEdit signal compensation. Button to the left of the power then cal/diag

author: NSterenberg
author_rank: Newbie
timestamp: May 01, 2014, 10:11:04 pm
content: Well Im pretty sure my poor rise time is due to my signal source. Believe it or not, I hooked it up to an Arduino (dont judge me). It was just writing a pin high and low, creating a square wave, and the rise time was 15-20ns, WAY below what it was for the signal generated by the scope, so I think its ok.Also I cannot anything in the Extended calibration for signal compensation. From what I read in the manual, there really is nothing i can calibrate in those menus with out a calibration generator.-Nick

author: David Hess
author_rank: Super Contributor
timestamp: May 01, 2014, 11:38:56 pm
content: The calibrator output on the 2440 (or any oscilloscope) has a relatively slow but clean transition because it is intended for compensating attenuating passive probes. I do not think it is even very square at high sweep speeds. Function generators are not much faster and even their sync output is unlikely to be faster than 3.5 nanoseconds or so which is way to slow to evaluate the rise time on a 300 MHz or even 100 MHz oscilloscope.Also for transition time tests, the signal generator and coaxial cable impedance need to be matched at the oscilloscope which typically means turning on the oscilloscope's 50 ohm internal termination or using a feedthrough termination.The external calibration procedure is relatively simple but you have to change a jumper inside the oscilloscope to access the external calibration menus. I do *not* recommend doing that.The internal calibration and diagnostic menus are still available and can be used without screwing anything up. The oscilloscope has extensive built in self test routines that operate both during power on self test and manually.