EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Skyfox on April 22, 2015, 11:27:43 pm
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Hi all, new member, first time poster.
I have an old analog Protek P-3502 oscilloscope that I'd like to calibrate. In the manual where it covers calibration it says the required equipment includes a square wave generator [1 kHz~1mHz, reset time <5ms], an oscillator [1kHz~over 20mHz], and a time mark generator [pulse ranges from 0.1?s to 0.5ms ±1%].
I take it the square wave generator just means a function generator. But my question is, what specifically is the oscillator and the time mark generator it's calling for, and how do they differ from a function generator?
Thanks!
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A square wave generator does what you think - generates square waves. Usually, the period, pulse width, amplitude and polarity are adjustable. An "oscillator" usually just refers to a sinewave generator. A function generator can be considered a superset of both, usually having sine, square, triangle, ramps, etc., with lots of adjustments. However, most function generators have relaxed specs and aren't particularly accurate. A time mark generator is a very specific piece that puts out sharp rise-time narrow pulses at specified intervals. Often, the intervals can only be selected from some pre-set choices, so it is more restrictive than a general purpose function generator. However, many time mark generators (e.g. Tektronix) had ovenized oscillators inside that made them very acccurate for adjusting the time base of an oscilloscope. Simply punch in the interval you desire and line up the pulses with the scope's graticule as you adjust the horizontal time base calibration pot. For most low cost scopes, the time base accuracy spec isn't all that great anyway, and you can probably accomplish everything you want with a function generator (and perhaps a counter to verify the function generator's settings).
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A leveled sine generator is useful to check bandwidth on scopes. You just crank it up until the display shows the -3db point and read off your bandwidth from the generator.
You can make a serviceable time mark generator with a canned 10 MHz crystal oscillator, some decade counters and a one shot to generate the narrow pulses. Analog scopes generally were spec'd around 3% or so on the horizontal, so you don't even need to ovenize it. It'll be good enough. You can use a good frequency counter to get it dead on, but it really won't make any practical difference.
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A leveled sine generator is useful to check bandwidth on scopes. You just crank it up until the display shows the -3db point and read off your bandwidth from the generator.
I saw this in an Agilent doc:
"Signal attenuation at the -3 db frequency translates into approximately -30% amplitude error"
So that would mean using your approach the sig gen should be set for a specific reference frequency (well within the expected scope bandwidth) to output a given level (in Volts) and when the scope reads that Voltage - 30% the scope would then be showing the - 3db level?
For example, using this approach I set my sig gen to .707 Volts (2 Vpkpk) at 9 kHz (the low end of my sig gen) and then increased the sig gen freq until the scope showed 1.4 Vpkpk (2Vpkpk – 30%). Using this approach my Rigol DS2072 reaches approximately 110 MHz before it hits the 1.4 Vpkpk (– 3 db) level. Does that sound correct/feasible? Thx
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Yes, that's correct. -3 dB on a linear scale is 10-3/20, which is .708, or -30 % below unity reference.
You could also use a fast pulse generator and measure the 10-90 risetime ; the equivalent bandwidth of a 1st order RC network is then 0.35 / risetime, which usually matches quite well with the -3 dB bandwidth of scopes.
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Thanks for your input, everybody! As I am a total newbie with stuff like this, some of what you guys were talking about is over my head, and knowing how to build something that doesn't just come in a kit I have to solder together would be out of the question. I've been looking to buy a decent used function generator, so I'll add a frequency counter to the shopping list as well.
I did find that using the tone generator on my audio editing program and connecting to the headphone output can get a beautiful sine wave on the oscilloscope screen, but without knowing precisely what the output voltage is (and not knowing which of my multimeters is most accurate) I can't use it for any calibration. Plus, the square/pulse wave generation on the audio program doesn't make nice square corners on the wave form; at each corner there's quite a bit of oscillation as the program tries to keep the wave form smooth instead of having sharp transitions like that.