Evaluating old CRO probes - Page 1
Forum section: electronics / beginners

author: jdutky
author_rank: Regular Contributor
timestamp: September 21, 2020, 06:06:55 am
content: I have two old Tek oscilloscopes that belonged to my father, one 475 that dates to at least 1975, and a 2213 from around 1982, both with the original passive voltage probes that he used for work (two P6075A probes for the 475, and two P6120 probes for the 2213, all x10 probes, non-switchable). One of the P6075A was marked "BAD" by my father some time prior to 1995, but, though he kept detailed notes, I have no other indication of what malfunction he observed. I have tried to evaluate the probes myself, but while I am not a rank amateur (I have a basic, college level understanding of electronics, understand Ohm's and Kerchhoff's laws, know how to handle a soldering iron, multi-meter, and even a bit about using an oscilloscope) I am barely a journeyman. I am likely to use the wrong terminology for some things, but I hope that my meaning is clear.My question is what should I be measuring about the probes to determine whether they are "good" or not? I have used a fairly nice multi-meter to measure the resistance and capacitance from several points: the probe tip ("tip"), the grounding clip ("clip") (with the GND REF button pressed and not), the BNC center pin ("center"), and the BNC ring ("ring"). I have measured continuity from tip to pin, from tip to clip, and clip to ring. I have measured resistance from tip to center, and tip to ring (with GND REF button pressed). And I have measured the capacitance from tip to clip. I have also compared my measurements against a newly purchased third party probe (cheap P2200 off Amazon), and a refurbished Tek probe (cheap P6075A off eBay).I have found that the contact between the ground clip and the probe shell/BNC ring has been unreliable on these old probes, but the probe that my father marked "BAD" seemed to be revived after I removed the ground wire and cleaned the spring clip that contacts the outer housing of the probe. Are there other measurements that I should be doing on a scope probe to determine if it is working?Thanks in advance

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 21, 2020, 08:05:38 am
content: I can see at least two possible reasons:1) There is some rupture in the wire, possibly intermittent.2) Some other problem which leads to a bad amplitude-frequency characteristics.If first - then it possibly is easier to repair. If second - you need a good enough functional generator and oscilloscope to check. I'm not sure if it is easy to repair (osc probes are a bit of art).P6120 is a 60 MHz probe (?) - it is not too wide range, so you may just buy another.P6075 is 200 MHz probe - so it hurts to loose it, and there is a sense to try to repair.

author: David Hess
author_rank: Super Contributor
timestamp: September 21, 2020, 08:33:37 am
content: What matters on an oscilloscope probe is transient response which is tested using a fast pulse generator with the probe connected to the 50 ohm termination so with a 25 ohm source. For this test to work, the signal must be connected to the probe tip coaxially without the ground lead; at 200 MHz, a BNC to probe tip adapter can be used.

author: jdutky
author_rank: Regular Contributor
timestamp: September 22, 2020, 09:58:05 am
content: So, when you say "fast pulse generator" what are we talking about? I have an inexpensive 0-3 MHz function generator that can make pretty ugly pulses that are about 200 ns wide at 3 MHz, but I'm guessing I'll need something that goes up to at least 100 MHz with a nice sharp rise and fall with a width of maybe 10-20 ns.

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2020, 02:17:46 am
content: Quote from: jdutky on September 22, 2020, 09:58:05 amYou need a nice sharp rise *or* fall with a transition time significantly faster than the bandwidth to be tested (rise or fall = 0.35/BW) and a minimum of pulse aberration. Tektronix recommended faster than 600 picoseconds for testing up to 100 MHz so figure 300 picoseconds for 200 MHz. These types of instruments are sometimes called reference level pulse generators. I use aTektronix PG506but it is marginal for 200 MHz.AC or LVC logic can achieve this directly with some care in construction. If you just want to buy something, thenLeo's pulse generatoris an excellent option.ABNC to probe tip adaptershould also be used. Some probes come with them.

author: jdutky
author_rank: Regular Contributor
timestamp: September 23, 2020, 03:04:16 am
content: wow, that's a couple orders of magnitude faster than I was thinking of. There's no way I'll be able to coax that kind of performance out of my cheap, old BK 4001.I have an old TM503 that I got for a song at the local university surplus outlet, and I've been looking at 500-series modules, including some of the PG50x modules, but everything I've seen on eBay (and listed as working) has been asking for real money. If I'm going to drop $500 on something it is going to be a modern digital scope.That Leo USB pulse generator looks like a nice piece of kit, though, and it's at least in petty cash range (if tending toward the high end). I was thinking about building my own, but that's pushing the limits of my current skill set. Of course, if you don't push your limits then how can you expect to learn anything?(also, I was wondering what those BNC dodads were. I got a couple of them with the cheap probes I bought, just in case the old P6075As were permanently dead. Thanks for clearing that up without requiring me to make any further displays of the depth of my ignorance)

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2020, 03:33:24 am
content: Quote from: jdutky on September 23, 2020, 03:04:16 amBefore I got the PG506, I modified the sync output on one of my function generators by replacing the original 74LS140 dual 4-input NAND line driver with the best performing replacement that I could find which was the 74S140, but it was still too slow for 100 MHz testing. The internal layout also prevented a clean edge so I abandoned further modifications.QuoteI just remembered another source for fast and high quality edges. The horizontal and vertical sync outputs on a VGA port are both fast and clean, if you can get the probe connected with a minimum of ground lead length.

author: srb1954
author_rank: Super Contributor
timestamp: September 23, 2020, 06:42:11 am
content: Quote from: jdutky on September 23, 2020, 03:04:16 amWhen looking for TM500 series modules also look out for the SG503 leveled output signal generator. This can be used to verify the bandwidth of the scope and probe or check for gain vs frequency variations that might indicate problems with probe compensation. The SG503 covers up to 250MHz so it is well suited for verification of the 2213 performance although it would be barely sufficient for checking out the ultimate performance of the 475.There is a higher frequency version SG504 that covers up to 1GHz but this requires a separate external leveling head which is often hard to come by.

author: jdutky
author_rank: Regular Contributor
timestamp: September 23, 2020, 03:31:10 pm
content: While I do have an immediate application for these two scopes, their main value is sentimental. I had considered getting he equipment to properly calibrate them, but the price, while lower than the sentimental value, far exceeds the price of a brand new digital oscilloscope (the Siglent 1204X-E is only $741 on Amazon!).I had also considered using micro-controller to generate the high speed pulses, but the idea of using a video signal is even better (though I'm not sure which is easier at this point: I have a couple Raspberry Pis lying around which I know work, but getting a known functional computer with a VGA port is going to be iffy). Sadly, it doesn't look like HDMI has H and V sync signals (or, if it does, they are encoded in the differential data signals).

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2020, 07:39:27 pm
content: Here is an example of the type of circuit that you could build using AC or LVC logic. As the video discusses, these types of pulse generators have other uses like time domain reflectometry. I often use mine to do quick tests of attenuators, coaxial cables, and oscilloscope probes; a VNA is even better for this sort of thing but I have fast oscilloscopes and no VNA.https://youtu.be/9cP6w2odGUcThe cleanliness of the pulse will depend on how well the circuit is constructed. Dead bug or Manhattan style construction over a ground plain is required and not particularly difficult.The photograph below shows how the fast rise and fall circuits of the PG506 are constructed. Note that the BNCs have build in 50 ohm termination resistors in the shape of a disc. It achieves 600 picoseconds or better with an exceptionally clean pulse.

author: jdutky
author_rank: Regular Contributor
timestamp: September 24, 2020, 05:39:53 am
content: So if I had a fast rise time circuit (similar to the one in the PG506) then I could drive it with my cheap signal generator and get a nice, crisp pulse to test my scope and probes with? That seems like it might be in my reach.I had a look at the schematics for my cheap signal generator, because I had already had to replace the main filter caps on the power supply (the 17 year old signal generator, a BK Precision 4001, that I bought for $100 in 2003 was dead from dried out electrolytic capacitors, but the 45 year old oscilloscope, which likely cost a couple thousand dollars when it was new, is still chugging along. I guess that really does show that you get what you pay for), and I was wondering if the sloppy high frequency square wave might be the result of other dried out caps elsewhere in the circuit. I focused on the MHz path, ignoring the common elements, because only the MHz path was showing real problems (probably not a good assumption), and did find that the capacitor used to select the MHz bandwidth regime was well below spec (should have been 220 pF but measured as only 135 pF). I replaced the dead 220 pF cap with a ceramic disc capacitor of the same value (and higher voltage spec), but the slow rise time persists. Looking at the schematic I see that there are a bunch of LM4588 op amps in the signal path (common to all bandwidth regimes) and from the datasheet (looking at the TI CR4558 datasheet, which is all the comes up in a Google search for LM4588, and looks like a similar component) it looks like this op amp tops out around 3 MHz (which is the stated upper bandwidth of the signal generator), so I'm guessing that the sloppy MHz peaks are a result of an inherent bandwidth limit in the signal generation circuit.Like you, I considered replacing the op amps (and assorted discrete transistors) with similar components that had higher specs (I assume that I will need parts that can handle up to 15-20 MHz in order to get a clean square wave at 3 MHz), but I abandoned that idea when you told me about the Leo Bodnar pulse generator (and as it became clear that I would have to replace a significant portion of the signal generator in order to get a much higher bandwidth out of it). If, however, I can just build the fast rise time circuit, and drive it from the signal generator's sync output, then I'd be perfectly happy to do that, if only as a learning exercise. I'm looking at the PG506 schematics now in order to get some idea of how to build a fast rise circuit.

author: jdutky
author_rank: Regular Contributor
timestamp: September 24, 2020, 03:51:15 pm
content: After studying the PG503 schematics, and reading a couple of articles about building fast rise time circuits, I'm wondering if I can just build a circuit with two op amps/comparators: one very vast comparator to generate the fast rise time output signal, and another (normal speed, e.g. 3MHz) comparator to condition the input signal into a voltage range suitable for the output fast comparator.I've see that there is a device from Analog Devices, the ADCMP580, that can deliver a 37 ps rise time on its output, and I can use (I think) just about any common op amp for the input signal conditioner.With the fast comparator costing $20 each this isn't going to be particularly cheap, but it does give me an excuse to play around with op amps.I also looked at a range of fast transistors, looking for anything with very fast output rise times, and didn't find anything that was even nearly as fast as the ADCMP580.(I got the AD part from this article:http://www.starlino.com/build-a-really-fast-pulse-generator-50ps-rise-time-using-an-ultra-fast-sige-comparator.html)(NM: I just found the EEVBlog discussion of the Jim Williams pulse generator circuit. All kinds of good information there)

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2020, 11:47:02 pm
content: Quote from: jdutky on September 24, 2020, 05:39:53 amThat is exactly right.Quote from: jdutky on September 24, 2020, 03:51:15 pmTake a look at how the reference level pulse generators were designed before the PG506, specifically the Tektronix type 106:http://w140.com/tekwiki/wiki/106http://w140.com/tekwiki/wiki/File:106_sidiode.pngThe transistor which does the switching is driven from its emitter while the base is grounded, which may be called emitter switching. The other transistor is an adjustable current source which sets the variable output level.The switched emitter could be driven through a resistor from the output of a fast CMOS logic gate; I have thought about using a 74LVC125 3-state buffer which is characterized for 3.3 volts but can operate at 5.0 volts.QuoteSpeed is limited by construction and packaging. The reason that the ADCMP580 can be so fast is that it implements an ECL or CML output internally with an internal termination so it operates as if it is at one end of a transmission line. Incidentally by my standards, the ADCMP580 output is not particularly clean, but it is better than average. At higher frequencies it displays too much overshoot to be a good source.If we could get dual emitter dual collector transistors like in the old days, then we could get much faster speeds out of a simple discrete implementation.

author: jdutky
author_rank: Regular Contributor
timestamp: October 01, 2020, 10:16:33 am
content: The Leo Bodnar fast pulse generator arrived (sooner than the web site led me to expect) and I hooked it up straight away. The bad news is that the probe that my father had marked as "bad" is indeed bad: there is some kind of intermittent fault about midway down the coax cable; I suspect the center conductor is broken. The good news is that the other probes, and the scope itself work wonderfully, and I was able to see a nice crisp square wave with a 3 ns rise time (which, I think, is what can be expected of the 475. If my math is correct that translates to a bandwidth of around 230 MHz). I still have some parts on order to try building my own Williams pulse generator, but that's just a learning exercise now.I have to say that I am still in awe of how these old scopes hold up. I'm sure that some of this specific scope's longevity is the result of careful treatment and maintenance by my father, but most of it must be a testament to the excellent work of Tektronix engineers. Just amazing.

author: Gyro
author_rank: Super Contributor
timestamp: October 01, 2020, 12:13:37 pm
content: Quote from: jdutky on October 01, 2020, 10:16:33 amYou can have no end of fun with that one, especially if you start air wiring it onto a piece of copperclad or directly on the back of a BNC socket. It's very instructive trying different resistor and capacitor constructions, lengths of different flavours of unterminated coax etc as the discharge cap.Leo's pulse generator is excellent but its rise time is a bit... err predictableIt will be a good exercise to see how closely you can approach its performance (within the limits of what is visible).

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2020, 05:41:29 pm
content: Quote from: jdutky on October 01, 2020, 10:16:33 amRise and fall time for a 200 MHz 475 should be 1.75 nanoseconds; 3 nanoseconds is way off and indicates a problem. Double check that the bandwidth limit switch is working properly.

author: jdutky
author_rank: Regular Contributor
timestamp: October 03, 2020, 12:21:48 am
content: The bandwidth limit switches (HF Rej and LF Rej positions on the trigger coupling switch) seem to be working correctly. I'm definitely seeing about a 3-4ns rise time on both channels. It's certainly likely that there is something wrong with this scope, as old as it is. In fact I have noticed a specific cross-talk issue between the two channels when either channel is receiving a square wave and is set below 0.5 volts per division; the other channel shows "noise" that is correlated to the rising and falling edges of the square wave on the other channel. The cross-talk is happening after the attenuation circuits because it doesn't change when you change the attenuation on the channel showing the noise. Are you sure that the rise time on the 475 should be 1.75 ns? That would imply a bandwidth upwards of 400MHz! I know that Tek is known for estimating their products bandwidths conservatively, but that's outrageous (and even if the scope can do that, the probes are only rated for 200 MHz, so the scope/probe system shouldn't show that kind of performance).UPDATE/MEA CULPA: I was doing stupid probe tricks that significantly reduced the quality of the signal. Now I have found my stash of BNC adapters and am going directly into the fast pulse generator with a F-F adapter and the BNC probe tip adapter: the square wave is significantly cleaner, and the rise time is down below 2ns. Wow. So the upper limit on this scope really is something north of twice what it was spec'd at by Tektronix. Wow.

author: David Hess
author_rank: Super Contributor
timestamp: October 03, 2020, 12:55:26 pm
content: Quote from: jdutky on October 03, 2020, 12:21:48 amTektronix gives the rise time in the specifications as 1.75 nanoseconds, which is consistent with a 200 MHz bandwidth. 100 MHz is 3.5 nanoseconds.QuoteYea, a BNC to probe tip adapter is required to get consistent results. Probes are tested in exactly that way and the probe's bandwidth specifications assume a directly connected source with 25 ohm impedance which comes from the termination. Directly connecting the probe to the pulse generator without a termination makes it 50 ohms but that is close enough to get an idea of what is happening.Tektronix specifies bandwidth of the combined oscilloscope and probe at the probe tip, assuming that a suitable probe is being used.

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 05:48:44 pm
content: Just a thought, youareusing the graticule correctly to measure the rise time? That is, using the V/div fine Var adjustment to set the low and high levels to the 0% and 100% graticule lines and then reading off the time between the trace crossing the 10% and 90% lines.As I say just a though if you're unfamiliar with analogue scopes.

author: jdutky
author_rank: Regular Contributor
timestamp: October 03, 2020, 05:55:30 pm
content: I've actually only used analog scopes, but it's been almost 30 years since I used one regularly (in college, when I still planned to major in EE). It's not quite like riding a bicycle.

author: jdutky
author_rank: Regular Contributor
timestamp: October 04, 2020, 11:22:51 pm
content: So here are the results of my tests of two probes: one of the P6075A that I got off eBay, and the other a cheap 1x/10x P2200 from Amazon. The first picture shows both probes connected to the internal calibration signal after I have adjusted them, the P2200 is on 10x but has no sense pin so the scope is indicating 1x. The calibrator or the time base may be a little off, but I've checked it against my BK signal generator and if it's off, it's not off by much. The second image shows channel 1 with the P6074A displaying the signal from the fast pulse generator with the input adjusted to fill five vertical divisions. The signal looks pretty clean, except for that stair step after the rise. Image number 3 shows the same signal at X10 magnification (1 ns per horizontal division) and the 90% rise time looks like 3.6 ns. The fourth and fifth images are the corresponding measurements on channel 2 with the P2200 showing a rise time of about 2.6 ns. I have swapped the probes between channels, and tried one of the other P6075A, all with similar results.I'm certainly willing to believe that a) this scope needs some adjustment, it has, after all, not been calibrated since 1979, and b) that the probes may be degraded (P6075A) or imprecise (P2200). In performing these tests I have had plenty of evidence that there are significant problems on channel 2 (the level is not centered correctly, many of the setting positions have dirty contacts, and there is some kind of crosstalk problem at high sensitivity ranges). While I have a specific task I'm trying to use this scope for, my real interest in it is nostalgic, as it belonged to my father. If all I were interested in were performing practical measurements I would spring for a Siglent 1204e-x and be done with it (I may yet do that: a good pair of probes from Cal Test Electronics will cost almost $200, and for that kind of money I could have a perfectly nice, modern Rigol 1054Z).I've got no fear of getting in the scope and doing some cleaning and restoration (okay, I'm a little scared of the of the HV section around the CRT, but I know enough to keep one hand in my back pocket), and I'm certainly learning a lot along the way (and enjoying the process).

author: james_s
author_rank: Super Contributor
timestamp: October 05, 2020, 05:09:58 am
content: That's a nice looking scope. The 475 was one of Tek's legendary analog scopes, they're still quite nice to use when the application is something that an analog scope is suited for.

author: jdutky
author_rank: Regular Contributor
timestamp: October 05, 2020, 03:55:59 pm
content: Quote from: james_s on October 05, 2020, 05:09:58 amThanks. It belonged to my father, who used it for about 8 years as a service engineer for a company that produced laboratory instruments and computers. He hauled that thing back and forth across the US, I think in checked baggage (hard to imagine, but maybe the treated it better than they do normal luggage), but even if he carried it on it's a bit of a marvel (that thing is HEAVY, and even carry on baggage takes a beating over time). He took excellent care of his tools, and this looks almost new.He bought a brand new 2213 when he went self-employed in 1980 or 81, which is a much lighter beast, but also much less capable than the 475. That scope was also taken very good care of (also hauled back and forth across the US, this time by car for, over a decade), but I'm concentrating on getting the 475 working because it's got the higher bandwidth and the dual delayed timebase.

author: james_s
author_rank: Super Contributor
timestamp: October 05, 2020, 08:03:28 pm
content: Those things cost a fortune in their day, I don't know exactly how much the 475 retailed for but the better Tek stuff was typically in the price range of a nice new car. It's the sort of thing that if you owned it personally it was a prized possession and you would take good care of it, especially if your livelihood depended on it.

author: jdutky
author_rank: Regular Contributor
timestamp: October 06, 2020, 09:21:11 am
content: Quote from: james_s on October 05, 2020, 08:03:28 pmThe stock 475 was about $2500, which isn't quite a fortune, only about the cost of a new, relatively inexpensive car. It was bought with company money, and bears a company sticker. I'm not entirely sure how my father wound up with it, even with 8 years of depreciation I imagine it was still rather expensive.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2020, 08:28:01 pm
content: The second probe shows a faster response because it is peaked.Test the oscilloscope itself without the probes by attaching the pulse generator directly to a vertical input. If you have to use a cable or adapters, then connect it through a 50 ohm feed-through adapter directly at the oscilloscope input.If you are going to be messing around with probes and testing oscilloscopes, then a feed-through adapter is a good investment anyway.https://www.amazon.com/Rigol-ADP0150BNC-Ohm-Impedance-Adapter/dp/B01C3EFUF0/https://www.amazon.com/Zyyini-Adapter-Through-Terminator-Device/dp/B07SD3TC51/

author: jdutky
author_rank: Regular Contributor
timestamp: October 28, 2020, 03:42:54 am
content: Quote from: David Hess on October 06, 2020, 08:28:01 pmIt took a while for the feed-through adapter to arrive, but now it has and I tested the scope with the pulse generator directly attached to the scope inputs. Both channels appear to show a rise time of something like 1.4-1.5 ns, which, if my math is correct, comes to something like 470 MHz. I also tested again with the various probes, with similar results (in the range of 1.5-1.6 ns).Attached is an image from one of the channels with the pulse generator directly attached through the terminator-adapter, with the timebase set at 0.01 us/div (10 ns/div) and using the 10x mag switch to give 1 ns/div. The other channel, and the images using the probes, look substantially similar (there's slightly more ringing with the probes in place, and the cheap P2200s actually seem to perform slightly better than the original Tek P6075As, but the rise times are basically the same to within 0.2 ns. At least I'll have no further hesitation about ordering cheap probes off Amazon).I've been using the scope for a micro-processor project for a couple weeks, and I'm getting more confident in its capabilities, as well as its shortcomings. The channel 2 selector switch needs a cleaning pretty badly, as do both vertical position POTs and the beam intensity POT, and there's some kind of crosstalk between the channels when one of them is set to the highest sensitivity. Also, I'm pretty sure that the timebase has drifted a bit since it was last calibrated (in Dec. 1979!), which only adds to my uncertainty about the actual rise time, but otherwise this old thing is a real trooper and a pleasure to use.I've ordered a "for parts" 475 off eBay, and it will arrive in a couple weeks. My intention is to keep that around for spare parts, but I'm going to open it up, try to diagnose it, and just get a feel for what it will be like refurbishing my father's old scope. I have hesitated to open up his scope without a pressing reason because I don't want to make any stupid mistakes with it, but a stranger's non-working scope is completely fair game.

author: srb1954
author_rank: Super Contributor
timestamp: October 28, 2020, 06:09:20 am
content: Quote from: jdutky on October 28, 2020, 03:42:54 amYour maths doesn't seem right. If your are measuring risetimes in the range 1.4-1.5ns this corresponds to bandwidths in the range 233-250MHz, which is about what you would expect for a 475.The formula for calculating bandwidth is BW = 0.35/TrThis formula applies to waveforms that have a smooth rise and don't overshoot too much.

author: jdutky
author_rank: Regular Contributor
timestamp: October 29, 2020, 01:11:55 am
content: Quote from: srb1954 on October 28, 2020, 06:09:20 amand that's what I get for doing cargo-cult math: I mangle the constant without noticing.So I looked up a definition of the bandwidth formula (https://www.tek.com/support/faqs/how-bandwidth-related-rise-time-oscilloscopes) and I really don't understand what they're talking about (because I'm bailed to computer science before hitting the real engineering courses in college). I don't really know what "This corresponds to a 1- or 2-pole filter roll-off in the frequency domain" means.Still, it's good to know that even with the correct math my scope seems to be performing as expected.

author: David Hess
author_rank: Super Contributor
timestamp: October 29, 2020, 02:37:56 am
content: The first picture below shows the transient response of a 465 (100 MHz) that I am working on; obviously it needs adjustment.The second picture shows the transient response of the 2232 (100 MHz) that I use regularly which in within specification.All aberrations are from the oscilloscopes; the PG506 used as a source and the RG-400 patch cable have been tested on a sampling oscilloscope and are completely clean with no aberration at all.

author: srb1954
author_rank: Super Contributor
timestamp: October 29, 2020, 03:31:19 am
content: Quote from: jdutky on October 29, 2020, 01:11:55 amThe reference to 1- or 2- pole filter roll-off comes from the mathmatics for representing the frequency response for a low-pass filter and implies a certain circuit configuration to produce such a response. The number of poles generally corresponds to the number of reactive elements i.e. capacitors or inductors, in the circuit.A 1-pole filter is the type of response produced by simple single-stage RC low-pass filter. This type This type of filter starts rolling off gradually above its cut-off frequency and eventually reaches a ultimate roll-off rate of -6dB/octave i.e. for every doubling of frequency the filter output reduces 6dB. The transient step response of a 1-pole filter is smooth and monotonic and there is no overshoot of the output waveform.A 2-pole filter contains 2 reactive elements and can be implemented with two cascaded RC filters or by an LC filter or by an active filter with two capacitors. This frequency response of a 2-pole filter rolls off more rapidly at -12dB/octave and the transient step response will generally have little (<5%) or no overshoot.

author: jdutky
author_rank: Regular Contributor
timestamp: October 29, 2020, 05:07:25 am
content: Quote from: David Hess on October 29, 2020, 02:37:56 amNice, I think my scope trace looked more like the 2232, before I engaged the 10x mag. I'll have to set up the test again and verify that memory. I will also have to work harder to get good camera images of the screen: your images are nice and sharp. Are you doing that by hand or do you have a rig to hold the camera?(I've considered buying one of the Tek camera rigs on eBay and using it to mount a digital camera, but that seems like the wrong way to be spending money when I could save it for a nice, modern digital scope)

author: jdutky
author_rank: Regular Contributor
timestamp: October 29, 2020, 05:09:24 am
content: Quote from: srb1954 on October 29, 2020, 03:31:19 amWow. That is a wonderfully clear and helpful explanation of a topic that I was despairing of understanding any time in the next few weeks.Thank you.

author: jdutky
author_rank: Regular Contributor
timestamp: October 29, 2020, 08:30:33 am
content: Sad news on the Tek 475. When I went to use it just now the horizontal sweep seems to have died, as has the second vertical channel. Also, when I went to pack it up, it looks like the bail has pulled out of the right side of the case.None of this is really surprising, the instrument is over 50 years old, and it spent its first 10 years being hauled back and forth across the country, so the mount points for the bail were probably badly fatigued all along. The 475 is pretty heavy, after all.Still, I'm sad about this. It really looked like it was in good repair and mostly functional, and I was just getting familiar with its features. Now I have had to fall back to my father's other scope, an only slightly newer 2213. This might the straw that forces me to join the 21st century and buy a nice new DSO (especially if the 2213 fails as well: then I'll need a new scope, even if it's just to diagnose and fix the old ones).Thank you, everyone, for all the help on this. I really appreciate it. I certainly learned a lot, and I'll be using it both to evaluate the 2213, and on whatever DSO I end up buying.

author: David Hess
author_rank: Super Contributor
timestamp: October 30, 2020, 01:30:34 am
content: Quote from: jdutky on October 29, 2020, 05:07:25 amI bought a Canon Powershot SX150 IS which uses 2 x AA batteries and has optical image stabilization for taking oscilloscope photographs and documenting other work. It works well enough freehand that I just do it that way and then clean up the photograph in Photoshop.QuoteA camera mount could also be made from copper clad board to fit in the mounting rails on the bezel.Quote from: jdutky on October 29, 2020, 05:07:25 amThat applies when the oscilloscope's bandwidth limiting is engaged however you might noticed from my screen shots that the response is *not* a common 1 or 2 pole response; it is actually closer to a Gaussian response from a number of cascaded amplifiers which gives it that nearly constant slope instead of an exponential curve which is what would be shown if the 20 MHz bandwidth limit was active.