thank you Chuck for that explanation. I have A LOT of experiments to
conduct based on the responses here. All in due time. I realize that the 1
Khz calibration signal ain't much to speak of in terms of stress testing a
scope. But for me, at the very least, it demonstrates that something isn't
wildly wrong or dead in the scope using 1 Khz as the starting point. Plus,
since my primary application right now is audio amplifier repair.... a 1
Khz signal is typically the test signal I use anyway. I've got some radios
coming up soon though. We'll see!
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On Mon, Jun 10, 2019 at 11:09 AM Chuck Harris <cfharris@...> wrote:
A little study of Fourier transforms would do you wonders...
A square wave, shown in the time domain, is, well, square....
you know the picture... Up, straight across, down, straight
across, up...
If you instead take a tunable receiver, with a narrow bandwidth,
and feed it the square wave, as you tune the receiver from DC up
in frequency, you will notice that there are only signals at the
fundamental frequency of the square wave, and at the odd harmonics
of the fundamental frequency. Further, you will notice that the
amplitude of the signals the receiver sees diminishes rapidly
as the harmonic number goes up: actually by 1/harmonic number, so
by the time you get to the 9th harmonic, the amplitude is 1/9'th of
the fundamental's amplitude...
If you were an experimental researcher around Fourier's time, you
would probably try lots of things, like see what happens to your
picture of a square wave when you
eliminate all of the harmonics above a certain point...say, the
9th harmonic.
You would find, that if the harmonics below the 10th were kept, the
square wave would look pretty much like a square wave. It would have
some rounding of the leading and trailing corners, and some ripple
on the tops and bottom, but it would be a pretty good square wave.
Fast forward to a modern tektronix scope with its 1KHz square wave
scope probe calibrator signal.
If you display this signal on any scope that has a bandwidth of 10KHz
or more, it will look very much like a square wave... just as experimenters
around Fourier's time would have predicted.
When Harvey wrote "So your 1 Khz calibrator requires a decent frequency
response to (gasp!) 10 Khz"... he was indicating that the scope's square
wave calibration signal isn't much of a stress test for any modern scope,
as it could be displayed adequately on a scope with only 10KHz bandwidth.
-Chuck Harris
David Berlind wrote:
Harvey,
Thanks for the validation on the video. I'm glad someone with a good eye
took a look. When you said "So your 1 Khz calibrator requires a decent
frequency response to (gasp!) 10 Khz," are you saying that a 100Mhz probe
won't work as well as a 10Khz probe when reading off the calibrator?
In the video, did you notice the fluctuation in frequency? It wasn't
sitting on exactly 1Khz. There's a bit of back and forth movement. Is
that
normal and if not, what's the fix (not that I desperately need to fix
that.. but it'd be good to know).
Thanks.
