Test Equipment: Oscillocopes

[TinselKoala]

That little switch under the channel vertical V/div setting knob is the most misunderstood scope control ever, and is a particular "pet peeve" of mine.

This is the channel's _input coupling_ switch.
AC -Ground-DC.

It does NOT refer to the "type of signal" that you are measuring! It refers to how the signal is _coupled_ into the oscilloscope.

The "ground" setting is, or should be, obvious. It disconnects the probe _tip_ from the scope and connects the input circuitry directly to the scope's chassis ground. This means that the scope's input sees "zero volts". No signal. So you select this when you want to set the channel's baseline, or zero voltage display, on some reference marker on the screen, like the center graticule line or some other graticule line of your choosing using the vertical position control. It essentially turns that channel's input off, removes the signal from the probe, connects the scope's input to ground (and leaves the probe tip floating so it doesn't short-circuit the device under test.)

The "DC" setting is called "DC Coupling". I believe it should more properly just be called "Direct Coupling", since that is what it does: It connects the probe tip _directly_ to the scope's input circuitry. Just that. (Here I'm disregarding the input and probe impedances or 10x voltage divider, which are irrelevant to this discussion.)

The "AC" setting is the one that causes confusion. This is "AC Coupling".... the switch simply puts a capacitor in series with the scope probe tip. Just that. This is a "high pass filter" arrangement that works to filter out, or eliminate, a DC component to the signal (the very slowly changing or unchanging part) and only allows the rapidly changing part of the signal to pass through to the scope's input circuitry.

The "AC coupled" setting has two major effects on what you see on the screen.

First, it removes the DC component of an oscillating signal. So, if you are interested in seeing the small oscillating ripple riding on top of a larger DC signal.... this is what you would use to view it. For example, say you have a ripple of 20 mV sitting on top of your +55 V DC power supply. How do you display such a thing? If Direct Coupled, and you use a V/div value that keeps the 55 V on the screen, the ripple is so small compared to that, you'll never see it accurately. So you need to _remove_ that +55 volts DC from the signal and magnify the 20mV ripple so that you can see it displayed at sufficient resolution to measure it. You select "AC Coupled", and then you can turn the V/div setting to some low value like 10mV/div and the ripple will be displayed rippling around the zero baseline level (that you positioned with the "ground" setting) at a good magnification. So in this case even though you are looking at a mostly DC signal, the AC-coupled setting is the appropriate one to use to view the ripple. You use DC-coupled to see the overall voltage level, and AC-coupled to see the tiny ripple on top.

Second, the AC-coupled setting takes the _average_ of the displayed signal and moves it up or down to the channel's zero baseline level that you set using the "ground" switch setting and the vertical position control. Obviously, you then lose the absolute voltage values of the signal when this happens. Peak-to-Peak value remains the same and is accurate, but you no longer know what the actual _peak_ values are because the trace has been moved down or up by some unknown amount to place the _average_ on the zero baseline. So even if you are measuring a truly "AC" signal, if it is not symmetrical about the zero value, the scope will make it _look_ like it is, and sometimes this will be false.

All scopes, whether the latest digital one with all bells and whistles, or an ancient analog clunker, accomplish this miracle in the same way: they simply place a capacitor in series with the probe when "AC-coupled" is selected. Analog scopes do this with a simple switch, digital ones use a software-controlled relay.

Most of the time you will use the DC-coupled setting for almost everything you do with the scope, whether measuring "AC" or "DC" signals. The most often-used reason for using the AC-coupled setting is what I mentioned above: Viewing small ripples on top of larger DC offset signals.



That Interstate F74 sweep FG that I linked earlier is a _good buy_ at 100 dollars. It has functionality that you will not find on many modern FGs for three or four times the price. 20 MHz range, a relatively fast pulse generator feature, a high-amplitude output, the sweep function, ability to float the signal ground, etc. The only drawback to these old Interstate models is the frequency setting knob. It is a vernier control, but it is still a little hard to set a precise frequency on it. I generally run my F43 into a hardware frequency counter when I need to set a precise frequency and this pretty much solves that problem. It is also kind of hard to find manuals for the Interstate line. But they are constructed with the same "generation" of technology as your 465 scope: mostly discrete components, only a few ICs on a robust double-sided circuit board with plenty of room to work.

[Vortex1]

No separate soundcard is needed. Just use the existing soundcard in the pc.

Stereo line or headphone out is the 2 channel FG output. Stereo line in is the 2 channel scope input.

This applies to the PC based scope I referenced earlier.

I don't know if this software supports a separate dedicated soundcard. A manual is available, it mjght address that issue.

[MarkE]

Ok, so what I have done here is this:
I have a standard 3.5mm stereo audio cable ran to a jack that I pulled off a board.  I figured out what was left, right, and ground, and soldered red, white, black to it.  I'm about to connect another jack into the setup so I can listen in to the signal.
Do I need to be on AC or DC? This is what I am unsure of as far as when I connect to something... what qualifies for an AC signal and what qualifies as DC? and what do I use ground for? Is that ground setting also to be referred to as "reference"? meaning that, on that setting, I would be seeing what my AC or DC signal would be in reference to?

Also, I would only need a limiter protection box if I were running my signal to an amp and/or using it for anything other than putting the signal to the scope, right?

Brian

AC coupling inserts a series capacitor between the signal that you are measuring and the vertical amplifier.  This applies a controlled amount of distortion to the measured signal that:

Removes all DC information.
Attenuates AC information in varying degrees as the frequency falls below about 1/(C_COUPLING*1E6) or typically about 1% attenuation at 40Hz, and shifts the phase of the measured signal beginning below about 40Hz.

AC coupling is useful when:  There is a large and stable DC offset on a signal, and there is little or no content between DC and at least ~40Hz or higher.  For example:  If you want to see the ripple on top of a power supply rail, then AC coupling lets you crank up the gain and see that ripple.

AC coupling is of course useless for DC signals, or signals where the DC content matters (power measurements for instance), or where there is signal content that matters between DC and ~40Hz.

Scope settings for novices:

Probe set to 10X.
Input set to DC.
Gain set to display the entire signal on the display, including the extent of any spikes.
Persistance set to OFF
Averaging set to OFF
20MHz filter set to OFF
Trigger mode set to AUTO
Trigger coupling set to DC
Trigger level autoset to 50% once signal is connected(Tektronix scope feature, may not be available on other brands)
Trigger position set to 0 (Tektronix scope feature, may not be available on other brands)
Change trigger mode from AUTO to NORMAL

[Brian516]

TK, MarkE,

Thanks for the clarification.  I am going to mentally tag them "Direct" and "Average" so as to remember that DC is for Directly measuring the actual signal, and AC zeroes out the Average signal in order to show the ripple.
This is all starting to come together and make sense to me now. I'll know how to use this scope in almost no time at all!    It even seems like a good place to start with scopes in general, since the digital ones have sooo many more functions I will need to learn when I one day scoop one up.  This one will do just fine for quite a while.

Vortex,

Thanks for pointing me to that program. Seems like a decent all around FG/scope when it comes to free stuff.  I have a mono mic input on my combo jack so I would only be able to use 1 channel, but for now I'll only be using the FG part since I already have this 465 to figure out.


I would also just like to note that putting the case back on the scope made a HUGE difference on the stability of the trace and functionality of the controls. I would never have imagined that something as simple as that would have such a big impact.  They really designed these scopes down to the tiniest detail, didn't they??

Brian

[Brian516]

Went to check the compensation on the new probes and the old probes, and noticed that the waveform I am seeing differs a little from the one depicted in the old scope manuals.  Just want to make sure that is what it should look like, so I uploaded another vid. I went thru various settings with the direct lines still hooked up to get the hang of things, and am now checking the probes and then putting some signals from the FG into it to see what I get.  Getting pretty tired so I'll probably save a bit of the good stuff for tomorrow.
I noticed a little issue on the ALT setting with it not wanting to show the CH1 waveform in it's full width on the top section of the screen, and did the same with CH2 on top, and it seemed that adjusting the B trigger level made it a little better.  I decided that I would try it again with the probes instead of direct links and see what happens. If I'm still having issues with it and can't figure it out tomorrow, I'll shoot another vid to show you all what's going on. Hopefully it's nothing, though.

https://www.youtube.com/watch?v=FDZ2ZhdMle4

Brian