Calibrating a Tectronix 2225 without signal generator - Page 1
Forum section: electronics / projects

author: Shakesword
author_rank: Newbie
timestamp: August 10, 2017, 11:06:25 am
content: So I got lucky and was able to lay my dirty paws on an old Tectronix 2225.I do not have access to a fancy sinewave genrator.Is there some small accurate sinewave oscillator that I can make or build with an adurino or raspi or some other comon scrapp parts that would have the timebase accuracy to do a calibration ''good enough' on this scope?

author: grumpydoc
author_rank: Super Contributor
timestamp: August 10, 2017, 11:28:59 am
content: You don't need all that much accuracy from an analogue 'scope - they aren't precision instruments.For the time base a crystal oscillator and hardware counter will suffice, it doesn't matter how you achieve this. A 10MHz crystal and some 74 series CMOS TTL binary or decimal counters would be fine.For the voltage - well, you can check the DC calibration with a bench PSU and a DMM, or even a battery, a potentiometer and a DMM.The inbuilt "calibration" square wave is usually not very good for frequency but generally pretty close for voltage.

author: Shakesword
author_rank: Newbie
timestamp: August 10, 2017, 12:29:52 pm
content: Thanx for your advice grumpy. The thing is I am under the impression that need a sinewave to see the crossing of the trace to the screens grid with optimal accuracy.As seen in Daves video calibrating this scope.Using a square Wave, will it not be harder to see the exact flanks crossings? Considering the sharpe slope time of a '74' wave.I do not trust the inbuilt calibration thanks for confirming my suspicion.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 10, 2017, 12:53:14 pm
content: Square wave is fine for timebase accuracy as it is fairly easy to align with the graticule.The best is a sawtooth pulse with fast risetime and fall over about 0.2 divisions - the sharp triangular waveform is thenreallyeasy to align precisely with the graticule, however a narrow square pulse is almost as good and a 1:1 square wave is perfectly OK.Sine waves actually are not the best for timebase checking as it can be visually difficult to judge exactly where the peak of the waveform is, if i use a sinewave for the timebase checks I make sure I have one cycle per division so the peak is reasonably sharp.Sinewaves are fine for amplitude though and essential for bandwidth checks.

author: alm
author_rank: Super Contributor
timestamp: August 10, 2017, 02:41:19 pm
content: For fast timebases (not really an issue with a 50 MHz scope) it may be easier to get a fast sine wave than a fast square wave or triangle wave. A 250 MHz RF signal generator is cheaper / simpler than a 250 MHz square wave generator for obvious reasons (bandwidth). Even some purpose-made time mark generators used sine waves to calibrate the fastest timebases. For example, the Tek TG501 would generate sawtooth trains with periods down to 10 ns, and would produce sine waves with periods down to 1 ns for faster timebase settings.Obviously adjusting to sine waves is harder. Maybe the zero crossing point would be a more accurate place, since that has the highest slope? That would require careful vertical positioning.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 10, 2017, 02:57:23 pm
content: Quote from: alm on August 10, 2017, 02:41:19 pmThat seems to be that it would be the hardest to accurately line up - the peak of a sine wave is not too bad if you make it sharp enough.As you say it is often the only option at fast timebases.

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2017, 09:22:09 pm
content: Square waves or pulses are normally used for timebase calibration because they are easier to line up with the graticule than sine waves.The timebase settings are in groups with separate calibrations but usually you only need to calibrate one of them for the others to meet specifications because they all track. The same goes for the vertical calibration. If the calibration is off, then usually all deflection settings are off by the same amount unless something has failed.Before I got a dedicated oscilloscope amplitude calibrator and time mark generator, I used a 2 MHz function generator, frequency counter, AC voltmeter, and switched attenuator to do oscilloscope calibration.