Minimum Oscilloscope Bandwidth? - Page 1
Forum section: electronics / testgear

author: mc1822
author_rank: Contributor
timestamp: January 07, 2020, 08:06:10 pm
content: I'm a new guy here but I do have electrical and some electronic experience. (Worked as an electrician & built numerous electronic kits) This is my first post on this forum and I hope to learn from posters on this here!I want to start repairing shortwave receiver radios that I might pick up at garage sales, etc. What is the minimum bandwidth on an oscilloscope that I would need for working on receivers up to 30Mhz? My budget only allows me $275-$300 to spend. Would I be OK with a 100Mhz scope?

author: Grandchuck
author_rank: Super Contributor
timestamp: January 07, 2020, 08:08:22 pm
content: A 100 MHz scope should be good.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 07, 2020, 08:18:16 pm
content: Theoretically, you'd need a minimum of the top 30MHz channel plus the width of your upper sideband - but it's a relatively low requirement that should be covered by most modern scopes. A 50MHz or 70MHz base level scope would be sufficient bandwidth for that application, but it's generally not much more for 100MHz inputs (and is often software upgradable or hackable on digital scopes), and that will cover you up into reasonable speed digital buses and keep square wave signals looking like square waves - since you really want little attenuation at the 3rd and 5th harmonics for a square wave to look basically square, that would let you see 20MHz square/pwm signals without the rise/fall time of the edge being too much of an issue.There is a lot of discussion of budget scopes on this forum, so make sure to use that search button (the icon on the top right, not the box top left, it doesn't work as well) when you start thinking about a specific model. Your budget is just at or under the low entry level price from many manufacturers, but there are some deals to be had under that or used scopes in that pricepoint that would probably be great choices - just make sure to look at reviews or people discussing the unit... on the budget end there can definitely still be stinkers.

author: tggzzz
author_rank: Super Contributor
timestamp: January 07, 2020, 09:11:48 pm
content: Without knowing exactly what measurements you intend to make, I strongly suspect a spectrum analyser would be a more useful tool.Make sure you understand theconsequencesof a scope's vertical axis specifications.

author: bd139
author_rank: Super Contributor
timestamp: January 07, 2020, 09:23:58 pm
content: To fix short wave radios you don't really need a scope. You need an RF signal source, frequency counter and AC voltmeter. None of them have to be amazing specification so old second hand stuff is fine.As tggzzz points out, knowing the measurements is important. Typically in a simple SW receiver you go through several stages on alignment:1. stuff a low level RF signal into it (signal generator)2. Tune it in and check the LO frequency with a counter and adjust.3. Connect an AC voltmeter across the speaker.4. Peak all the IF filters.5. Adjust dial stringing if need be.To fix them the process is similar but when something doesn't work, a signal injector/tracer is a lot more useful!More complex radios, the radio test set comes in. Marconi 2955 does it all.Not a scope in sight.Not to say you can't use a scope but it's a jack of all trades instrument. If i had that use case in mind and budget I'd buy a junker 20MHz unit and the above tools as well.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 08, 2020, 12:20:07 am
content: Quote from: bd139 on January 07, 2020, 09:23:58 pmIt is often better (if less simple) to place a DC voltmeter across the receiver AGC line instead.If you do use an AC voltmeter, keep the RF input as low level as you can, so the AGC doesn't mask the peak.

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2020, 02:43:39 am
content: An oscilloscope is not generally used to directly view RF signals but it might be used to view an RF envelope. This requires either an oscilloscope of sufficient RF bandwidth or an RF demodulator probe which is simply an AM detector with a couple of diodes and capacitors.So with a demodulator probe, you could get by with a 20 MHz bandwidth which will also be sufficient for audio and power supply troubleshooting. In practice, you probably cannot find a good oscilloscope with less than 50 MHz of bandwidth.My suggestion is to get a less expensive instrument so that you can gain experience with it to know what you really need.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 08, 2020, 04:12:09 am
content: 100MHZ covers most generic projects.You can get cheap USB scopes but beware of sampling frequency vs bandwidth.Some have very unrealistic claims.A 100 million sample/second usb scope is only good for about 2-3MHZ bandwidth.I have seen some claim 30MHZ bandwidth with 100MHZ sampling !This means you get 3 dots/points on screen per sine wave cycle which is crap to say the least.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 08, 2020, 08:53:23 am
content: Seen quite many handheld mini usb scopes with 5 Mhz bandwidths on ebay and Amazon recently for sale. And sime 2 Mhz ones with self assembly scopes for abot 20 usd. Do they have any use?

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: January 08, 2020, 09:14:24 am
content: you could also start with a RF power meter, RF signal source and frequency counter. A scope is useful for time domain debug, but not really for RF circuits. But it could come handy, if an oscillator on the board needs to be checked.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2020, 11:13:19 am
content: Quote from: nigelwright7557 on January 08, 2020, 04:12:09 amNope. Look up sin x/x reconstruction and sampling theory in general. Usually digital scopes work well up to fs=2.5 * bandwidth. So 100Ms/s is good for 100/2.5=40Mhz. The theoretical limit is Nyquist but for practical purposes you'll need some headroom for the anti-aliasing filter.Anyway, for repairing receiver I'd also look for a signal source (RF generator) and a DMM.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 08, 2020, 11:32:40 am
content: Welcome to the forum.A 100MHz oscilloscope will be fine. This is a basic measurement tool for electronic work anyway, after the DMM (Digital Multi Meter).However, for RF work you might also need an SA (Spectrum Analyzer) for example to look for spurious RF signals or other similar measurements done in the frequency domain, and/or a VNA (Vector Network Analyzer) for example to tune antennas, check filters and so on.Expect either the SA or the VNA to be more expensive than an oscilloscope, in the range of $500 for used ones, unless you go for a poor's man SA or VNA (small USB only instruments).

author: RoGeorge
author_rank: Super Contributor
timestamp: January 08, 2020, 12:02:46 pm
content: Was not meant to offend, sorry.Small like in handheld small, tools like NanoVNA which is about $50 (no idea if it worths the money or not).

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2020, 12:59:48 pm
content: Quote from: RoGeorge on January 08, 2020, 12:02:46 pmI'm not sure whether a spectrum analyser or even a VNA is useful for fixing a receiver. These tools are more suitable for fixing / designing transmitters.

author: Fungus
author_rank: Super Contributor
timestamp: January 08, 2020, 01:10:32 pm
content: Quote from: vinlove on January 08, 2020, 08:53:23 amYou mean these?( starts at about 49:00 )They're not useless, but $20 is about right.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 08, 2020, 01:10:48 pm
content: Quote from: nctnico on January 08, 2020, 12:59:48 pmThat's true, for some reason I wrongly thought it was about repairing Rx/Tx radios.For vintage radios (receivers only), I don't even power the oscilloscope. A schematic and a DMM is usually more than enough to repair those. The most common faults are mechanical contacts in switches, dried out capacitors or audio power stages.

author: thinkfat
author_rank: Unknown
timestamp: January 08, 2020, 01:29:23 pm
content: Quote from: nctnico on January 08, 2020, 11:13:19 amIf youknowyour input is a sine wave (or generally, within the bandwidth limit), it's OK, yes. I think Dave published a video about DSO basics where he shows 4-times oversampling is the minimum for sin x/x reconstruction to work sensibly:

author: Fungus
author_rank: Super Contributor
timestamp: January 08, 2020, 01:51:14 pm
content: Quote from: blueskull on January 08, 2020, 01:36:11 pmie. Rigol DS1054Z.As noted above though: Most of the failures will be bad switches, dried up capacitors, crusty power supplies, etc.You may not even need an oscilloscope for most of the diagnosis, a DMM will do.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 08, 2020, 04:15:20 pm
content: Quote from: Fungus on January 08, 2020, 01:10:32 pmyeah, mine came ready made in nice black plastic case.I found it working well.Scopes are for reading voltages in time duration, so that is what I am using it for.I have one for 5 Mhz one too. It works even better. it was about 50 euros delivered.

author: james_s
author_rank: Super Contributor
timestamp: January 08, 2020, 05:00:00 pm
content: More bandwidth is always nice, but for most hobby stuff I would agree that 100MHz is more than enough. Until the last decade or so a hobbyist was fortunate to have even a 20MHz scope, we are really spoiled in recent years. Many of the engineers designing the classic shortwave receivers in the first place would have had access to maybe a 15-30MHz scope at work. 100MHz scopes like the Tek 465 didn't appear until the mid 70s and they cost as much as a nice car at the time.Regarding fixing radios, I don't think I've ever used my oscilloscope for that. Most radio faults are things like bad capacitors and cracked solder joints, dirty pots and drifted resistors. You should be able to accomplish most repairs with a multimeter and your eyeballs. Audio and RF test oscillators may occasionally be useful.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2020, 05:53:42 pm
content: Quote from: thinkfat on January 08, 2020, 01:29:23 pmMost probably sin x/x is broken on that Siglent scope from Dave's video (which is not an uncommon error on newly introduced oscilloscopes). On a decent DSO 2.5 samples per period is more than enough. DSOs have much steeper filters (far from a Gaussian roll-off) and most signals have harmonics which are multiples of the fundamental frequency so aliasing isn't a problem.I've watched Dave's video again(number 1213) and the sin x/x mode on the Siglent scope (SDS1202X-E) he uses is definitely broken!

author: mc1822
author_rank: Contributor
timestamp: January 08, 2020, 06:29:01 pm
content: Thanks to everyone that replied! I already have a digital multimeter, function generator, frequency counter and variable dc power supplies. I am curious if a audio generator or signal tracer would be of use?

author: james_s
author_rank: Super Contributor
timestamp: January 08, 2020, 08:30:19 pm
content: It can be. For signal tracing something as simple as a small audio amplifier can be sufficient for radio work. It sounds more like you are accumulating toys rather than picking up tools with a specific task in mind. All the tools in the world are not a replacement for knowledge and experience. Radios are relatively simple and should not generally require anything more than a multimeter and soldering iron to repair. More specialized stuff like transceivers can require more specialized tools but most repairs are going to be simple.The last radio I repaired had a bad speaker. The one before that had a cracked track in the volume pot. These were trivial to diagnose with a multimeter and a little intuition.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2020, 08:36:08 pm
content: Quote from: thinkfat on January 08, 2020, 01:29:23 pmEr, no. Arbitrary waveforms are fine, provided that the signal's highest component frequency is within the sampling limit.It is, of course, necessary to be clear about the definition of "signal". For example a Tek 1502 has ~25kS/s and a 4GHz front end; the hp54100a has 40MS/s and a 1GHz front end.

author: Fungus
author_rank: Super Contributor
timestamp: January 08, 2020, 09:35:11 pm
content: Quote from: mc1822 on January 08, 2020, 06:29:01 pmSounds like you need to look at the Analog Discovery:https://store.digilentinc.com/analog-discovery-2-pro-bundle/

author: mc1822
author_rank: Contributor
timestamp: January 08, 2020, 09:37:41 pm
content: james-s,I have accumulated some of this equipment over time in case I decide to go further in my electronics hobby. I do have some electronic knowledge.I know what capacitors, resistors, diodes, transistor, etc. are and what they do. I also am very experienced in soldering. I have read and am reading various books on electronics in general and how to diagnose and repair audio and radio equipment. I am also viewing videos on Youtube related to these subjects. My hope is that by joining various electronic forums I will be able to learn from more experienced posters! Besides, don't we all like "toys"?

author: Fungus
author_rank: Super Contributor
timestamp: January 08, 2020, 09:39:37 pm
content: Quote from: mc1822 on January 08, 2020, 09:37:41 pmYep,definitelya candidate for an Analog Discovery...

author: thinkfat
author_rank: Unknown
timestamp: January 08, 2020, 09:49:37 pm
content: Quote from: tggzzz on January 08, 2020, 08:36:08 pmThat's what I meant when I said "within the bandwidth limit". I should have said "sampling limit".The TEK1502 is a TDR, fidelity in waveform reconstruction is not key here, I'd say. The HP54100a is a sampling oscilloscope that explicitly exploits the aliasing caused by subsampling the input signal. Works for repetitive waveforms only.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2020, 10:40:43 pm
content: Quote from: thinkfat on January 08, 2020, 09:49:37 pmWaveform fidelityiskey when determining the location and value of impedance, e.g. where in a connector the impedance is 49ohms rather than 50ohms, or to what extent a cable has a regular impedance variation.QuoteOf course. That's the point! A point that many people think violates the sampling theorem.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 09, 2020, 01:04:21 am
content: Quote from: mc1822 on January 08, 2020, 06:29:01 pmA lot of people have dived into esoterica, way beyond what you would use.bd139has a more practical view, but has described an Oscilloscope as a "jack of all trades" instrument.With respect, that is its strength.A cheap, second or third, ornthhand analog 'scope can do the job of :-(a)A Signal tracer.(b)A large scale ac voltmeter.(c)A large scale centre zero DC voltmeter.In (a), if the radio is showing signs of life, but no audio output.you can look for the presence of the local oscillator, & if that is present, see if there is any IF output from the mixer, then check through the IF stages to the detector.If there is detected audio, you can then look at the audio output amp.In (b), if you are aligning the radio the "simple" way by just looking at the increase in audio output, you can observe the audio waveform.If the level increases, you are "peaking" the IF or whatever, just like an analog ac voltmeter.In (c), if you need to look at DC operating voltages, it is often easiest to "free-run" your scope ("auto trigger"), then watch the deflection of the trace in the positive or negative direction.Of course, you have to use your brain, as it doesn't "spoon feed" you like a DMM.If, instead of just looking at the audio output when aligning a Rx, you are monitoring the AGC line, you can again use the 'scope in this mode."Short wave " radios suggests a maximum frequency requirement of around 30MHz for conventional superhets, but for Wadley Loop & synthesised radios, you may be better with a 100MHz instrument.If you are lucky enough to pick up one of those two or more channel analog 'scopes which provide an output of one vertical channel, you can use the 'scope as a "buffer" or a preamplifier, allowing the use of a frequency counter, in places where the counter's low input impedance or lack of sensitivity may be problematical, if used alone.Another trick that can be used with most such 'scopes, is to use two vertical channels "in cascade" to obtain extra sensitivity.This is not usually a very accurate method, but sometimes, either the presence , or shape of a signal is more important than its absolute amplitude.DSOs can do most of these things, obviously excepting the last two, but usually have a reasonably good frequency counter built in.

author: David Hess
author_rank: Super Contributor
timestamp: January 09, 2020, 03:59:07 am
content: Quote from: james_s on January 08, 2020, 05:00:00 pmThat is my experience also when working on radio equipment. However I *have* used my oscilloscope as an impromptu RF voltmeter to verify signal levels. But mostly it gets used to trace audio circuits and verify proper operation of power supplies.In my most recent case, I used my oscilloscope to measure the transient response of the repaired regulated power supply which was for the RF power amplifier. I wanted to make sure that it was never applying excessive voltage to the output transistors. This turned up some flaws in the original design which Kenwood solved by adding a zener diode clamp to the output which is what had shorted out blowing up the regulator's pass transistors.I also used my DSO to measure the startup time of the various local oscillators which gives some indication of their design margin.

author: thinkfat
author_rank: Unknown
timestamp: January 09, 2020, 05:56:58 am
content: Quote from: tggzzz on January 08, 2020, 10:40:43 pmWell, it does "violate" the sampling theorem. If that was possible. But it isn't. The sampling theorem is not a law, it describes an effect. The sampling theorem just tells you where in the frequency domain the signal you're sampling is going to show up

author: Fungus
author_rank: Super Contributor
timestamp: January 09, 2020, 07:34:36 am
content: Quote from: tggzzz on January 08, 2020, 10:40:43 pm"Folding" is a well documented part of sampling theory:https://en.wikipedia.org/wiki/Aliasing#FoldingIt's why the Nyquist "limit" is also known as the "folding frequency":https://en.wikipedia.org/wiki/Nyquist_frequency

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 09, 2020, 07:59:15 am
content: Quote from: nctnico on January 08, 2020, 11:13:19 amExplain to me how 3 points on the screen represents a 40MHz sine wave ?I would suggest you need at least 30-40 points to give a half decent sine wave.

author: Fungus
author_rank: Super Contributor
timestamp: January 09, 2020, 08:13:01 am
content: Quote from: nigelwright7557 on January 09, 2020, 07:59:15 amNope. Sin(x)/x reconstruction only needs "more then two" points (less than three still works, eg. five points every two cycles is still plenty).The explanation: There's only one possible sine curve that fits through those points, it can be extrapolated from them.

author: bd139
author_rank: Super Contributor
timestamp: January 09, 2020, 08:18:55 am
content: Taylor series.Nothing wrong with repetitive signal sampling either. I rather like a 54600 scope. 20MS/s single shot. Aliasing? Just don’t be a dumbass and it’s fine.

author: thinkfat
author_rank: Unknown
timestamp: January 09, 2020, 09:09:06 am
content: Quote from: nigelwright7557 on January 09, 2020, 07:59:15 amIf your reconstruction filter was just a "connect the dots" interpolation, then, yes, you'd need a lot more points than 3.Of course that is not what is done. I've found the attached paper (an EETimes article) by some Agilent guy who explains how the interpolation works. Basically, you run the samples through a rectangular shaped filter with a stop frequency of Fs/2, or practically, through a FIR filter with enough coefficients to be a close approximation of this rectangular shape. This can be shown to exactly reconstruct the original waveformprovided it is properly bandlimited, i.e. has no frequency components above Fs/2.

author: tggzzz
author_rank: Super Contributor
timestamp: January 09, 2020, 09:12:31 am
content: Quote from: thinkfat on January 09, 2020, 05:56:58 amNo it doesn't.QuoteThe sampling theorem is a fundamental mathematical relationship that cannot be broken.QuoteFor a stationary unchangingsignali.e. with a bandwidth of ~0Hz, you can take as long as you want to take samples and then reconstruct it.To do that you need a trigger and sampler that operates sufficiently fast to resolve edges, but the samples can be taken arbitrarily infrequently.For another example, understand how Tayloe mixers work, as seen in SDR dongles and elsewhere.Key search term: subsampling.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 09, 2020, 12:00:12 pm
content: I don't know what type of radio you want to fix or experiment with, Shortwave? Ham? FM?For repair and other fun, I use an HP 8935 Service monitor. It combines spectrum analyzer with tracking or separate RF , AFgenerators and power meter. It is really a cell tower tester.It also has a scope , but is it really an audio scope and not really a good one. There is also a lot of cell phone stuff that is of no use to me.You can generate a signal ,modulate it with AM or FM and trace it. You can look for signal purity on the spec analyzer. This thing is more expensive, like about 1K USD, (I just sold a really good unit with a beat up case for $600) but it does lots of stuff that are quite helpful in fixing all sorts of radios.It is also a pretty good AM, SSB and FM receiver up to 1 GHz.If you are testing ham radios you can generate an audio signal to put into the mic input and see what you get in the ant output.http://www.amtronix.com/usequip1.htmhas a chart that lists some service monitors and what they do. Their prices are high in my opinion, but they stand behind their units.If you want a scope, may I suggest an old analog one. You can pick up an old analog scope at hamfests for cheap. The new digital scopes are getting much better and have caused the price on old analog scopes to come down quite a bit. You can get a nice analog one and learn what you want and need in a scope. In the meantime, the digital scopes will get better and better and more used ones will show up cheap.About bandwith... if you are working on old radios remember that some shortwave radios have IF above 50 MHz, even if they only receive up to 30 MHz. FM radios are higher.General rule with bandwith on old O scopes is that the MHz stated is where the signal that is put on the screen is 80% of what is should be in amplitude.Another thought: audiophools seem to love really old even vacuum tube scopes for their low frequency capabilities.As far as my O scopes go... I use an ancient 100 MHz HP scope. A HP 1980, sold for about $18K new, I bought it for $50. It is usable up to about 150 MHz. It is partially "digital"The thing that most folks do not take in consideration when using a scope is that the probes are at least as important as the scope at RF frequencies. You will need a 1X and 10X probe, probably two of each.I really have not used new digital scopes and am not a good source of info on them except they seem to be getting better and better for less and less money. I have used old digital scopes in the past but the technology is quite different.Wally KC9INK

author: vinlove
author_rank: Frequent Contributor
timestamp: January 09, 2020, 02:03:01 pm
content: A couple year ago I had a few different makes of old analogue o scopes, and all of them had some problems such as one channel not working, display problems etc. Most of them had some problems with dodgy contact on the rotary switches too.Like everything else, and all other electronic devices, with old age their limited lives were keep ending.I sold them all for parts, and got a brand new Rigol DSO and a couple of handheld o scopes.If you are lucky, you will get good perfect working old scopes, but also there are possibilties that you might be buying a bag of problems and restoration projects with little prospect of finding the replacement parts. Chances are more likely latter.

author: james_s
author_rank: Super Contributor
timestamp: January 09, 2020, 05:52:22 pm
content: Vacuum tube scopes are great for working with radio transmitters, tesla coils, other vacuum tube electronics and anything else where high voltages can be present. A tube scope has a much better chance of surviving abuse that would fry expensive semiconductors in a solid state scope. Other than that there is not much reason to want one beyond collecting an antique.

author: bd139
author_rank: Super Contributor
timestamp: January 09, 2020, 07:48:24 pm
content: Unless the front end is a nuvistor

author: David Hess
author_rank: Super Contributor
timestamp: January 09, 2020, 11:53:04 pm
content: Nuvistor's are also tougher than semiconductors.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 10, 2020, 12:45:35 am
content: Quote from: james_s on January 09, 2020, 05:52:22 pmSemiconductor based 'scopes have been used for 40+ years on vacuum tube & other high voltage equipment.(even a solid state Analog TV Horizontal output stage has some serious voltages)I've never seen one "fried", in a career of a similar duration.The number of times you have to connect a 'scope directly to the really high voltage parts of an energised transmitter are really vanishingly small.

author: 0culus
author_rank: Super Contributor
timestamp: January 10, 2020, 12:53:34 am
content: Someone blew up one channel of our very expensive Keysight scope at work last year. Never found out who did it; Keysight repaired it under warranty.If you use common sense and the right tools you won't blow it up.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 10, 2020, 12:56:29 am
content: Quote from: vinlove on January 09, 2020, 02:03:01 pmSo one channel doesn't work?You've still got another one that does.I've used 'scopes with one faulty channel at work, when, as often, they were the only one left.If I'd complained to the boss that it only had one channel, he would have replied: "Well how many channels are you using?"At home, I successfully fault found an ignition fault on a 1988 Ford, using an old BWD 'scope with faulty triggering.I just let it "free run" & watched the pulses from the EMM pop up & drift by.Faulty switches ---wiggle 'em!

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 10, 2020, 02:37:03 am
content: Quote from: Fungus on January 09, 2020, 08:13:01 amWhat happens if its not a sine wave ?

author: tautech
author_rank: Super Contributor
timestamp: January 10, 2020, 04:04:56 am
content: It gets it wrong and really noticable in Single shot mode.Ideally the more data points to reconstruct a waveform the better and scopes with a good amount can be used entirely in Dot mode where any interpolation results don't matter as damn as it's not used.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2020, 05:55:15 am
content: Quote from: nigelwright7557 on January 10, 2020, 02:37:03 amIf no aliasing is present, then there is only one solution and the sin(x)/x interpolation reproduces the input. If aliasing is present, then there are multiple solutions and interpolation does not produce an accurate result. Of course the anti-aliasing filter if present distorts the input anyway so it may come down to a case of choosing the lessor evil unless the sample rate can be increased.

author: Fungus
author_rank: Super Contributor
timestamp: January 10, 2020, 06:55:36 am
content: Quote from: 0culus on January 10, 2020, 12:53:34 amAnd a fixed 10x (or 100x) probe - none of that 1x rubbish.

author: 0culus
author_rank: Super Contributor
timestamp: January 10, 2020, 07:10:04 am
content: Quote from: blueskull on January 10, 2020, 01:25:35 amGrrrr, people can be so dumb. On the bright side, ours also got fixed for no charge AND proper use of the thing is going to be included in the mandatory training that will be required, no exceptions, for using the lab once they finish remodeling.Quote from: Fungus on January 10, 2020, 06:55:36 amYeah, and an HV probe too. I have a couple of Tektronix P6015s...sadly out of Freon but 1000x divider if you respect the frequency derating curve, and still quite useful with the derated spec when not filled with a dialectric fluid.

author: bd139
author_rank: Super Contributor
timestamp: January 10, 2020, 08:08:56 am
content: Ah that reminds me of university. We had two channel analogue scopes and had to play “which channel still works” every labThen they got the 54600’s in and we were not allowed to use them

author: nctnico
author_rank: Super Contributor
timestamp: January 10, 2020, 11:56:26 am
content: Quote from: Fungus on January 10, 2020, 06:55:36 am1x is necessary when looking at small signals. And I wonder how you can blow up a 1M Ohm input. Most are rated for 300V to 400V so they should be able to withstand mains.

author: Fungus
author_rank: Super Contributor
timestamp: January 10, 2020, 12:08:26 pm
content: Quote from: nctnico on January 10, 2020, 11:56:26 amSure, but I'd rather not be using something with a tiny switch on the side when I'm probing hundreds of volts.Quote from: nctnico on January 10, 2020, 11:56:26 amMains has transients, valve amplifiers have higher voltage then mains.But sure, go ahead. Risk your entire 'scope for the sake of a $10 probe.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 10, 2020, 12:28:08 pm
content: Quote from: nctnico on January 10, 2020, 11:56:26 amIn a school lab?I wouldn't be surprised if some students will have the initiative to visualize the waveform of a spark from a piezo lighter.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 10, 2020, 01:03:19 pm
content: On blowing up scopes:There are many old analog scopes around with one channel burned out. This is usually the dual FET on the input. A lot of old scopes have a dual six lead FET on input on each channel. The really old solid state ones have two separate FETs.Easy to fix. I have fixed a few. I have not worked on any of the newer scopes which are surface mount.I knew a guy who loved to buy these damaged scopes and fix them. He thought it was easy money.If you use a scope in DC mode or 50 ohm DC mode it is relatively easy to blow the input. FET amplifier and the 50 ohm circuit and the protection, if there is any, can easily be fried. Even in AC mode you can fry the input.Wally KC9INK

author: bd139
author_rank: Super Contributor
timestamp: January 10, 2020, 02:00:56 pm
content: Indeed.The most fragile bit is usually the 100X attenuators in a Tek 4xx as well. Seen many of them toasted!

author: vinlove
author_rank: Frequent Contributor
timestamp: January 10, 2020, 02:06:45 pm
content: I have seen some people using 10 khz max bandwidth tube scopes quite happily doing all sort of things, especially working on audio, amp, radio and curve tracing etc etc.

author: bd139
author_rank: Super Contributor
timestamp: January 10, 2020, 02:13:26 pm
content: Yep. Similar to signal tracing. Can use one with a demodulator probe.

author: tggzzz
author_rank: Super Contributor
timestamp: January 10, 2020, 02:47:55 pm
content: Quote from: nigelwright7557 on January 10, 2020, 02:37:03 amAs I'm sure you are aware, any signal can be decomposed into a number of sine waves.The statement applies to the sine wave with the highest frequency; all other sine waves will contain more points.

author: Fungus
author_rank: Super Contributor
timestamp: January 10, 2020, 03:43:39 pm
content: Quote from: nigelwright7557 on January 10, 2020, 02:37:03 amEverything is made of sine waves.So long as there's no sine wave above the Nyquist frequency in your signal then there's only one possible signal that goes through all your sample points.(and that's the whole of sampling theory in a nutshell)

author: bd139
author_rank: Super Contributor
timestamp: January 10, 2020, 03:50:31 pm
content: Any periodic function that is, not everything

author: Fungus
author_rank: Super Contributor
timestamp: January 10, 2020, 04:25:34 pm
content: Quote from: bd139 on January 10, 2020, 03:50:31 pmNo, everything.They'll come and go if the function is non-periodic but they're still sine waves.(and that's how audio compressors work)

author: gf
author_rank: Super Contributor
timestamp: January 10, 2020, 04:57:03 pm
content: Quote from: Fungus on January 09, 2020, 08:13:01 amAn exact sinc reconstruction of the original signal needs in fact aninfinitenumber of points, since a sin(x)/x kernel has anunlimited extent. I.e.eachsample influences thewhole(infinite) timeline of the reconstructed signal. Infinite is not practical, though, so in practice it is rather approximated by a truncated sinc kernel with a finite extent (with say a dozen of samples) - but strictly, it is no longer a "true" sinc interpolation then.

author: Fungus
author_rank: Super Contributor
timestamp: January 10, 2020, 05:00:57 pm
content: Quote from: gf on January 10, 2020, 04:57:03 pmLet me clarify that: "More than two points per cycle of the highest frequency sine wave in the signal"

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2020, 05:09:42 pm
content: Quote from: gf on January 10, 2020, 04:57:03 pmIf the displayed reconstruction goes through the original displayed sample points, then it is close enough.

author: gf
author_rank: Super Contributor
timestamp: January 10, 2020, 05:19:35 pm
content: Quote from: Fungus on January 10, 2020, 05:00:57 pmThat's correct.No, not nore than two points per cycle of the highest frequency in the signal, but anWith infinite I meant the total number of samples, i.e. an infinite number of samples preceeding the "time of interest" and an infinite number of samples succeeding the "time of interest" where we want to obtain the reconstructed value.Quote from: David Hess on January 10, 2020, 05:09:42 pmIn practice, truncation is fine, of course.

author: Fungus
author_rank: Super Contributor
timestamp: January 10, 2020, 05:38:51 pm
content: Quote from: gf on January 10, 2020, 05:19:35 pmAnd a minimum of two samples per cycle...

author: bd139
author_rank: Super Contributor
timestamp: January 10, 2020, 05:56:23 pm
content: Quote from: Fungus on January 10, 2020, 04:25:34 pmNope. It is within a fixed interval which is a stinking massive cheat. It’s periodic outside that interval.You can’t approximate a single pulse in infinite time with Fourier series. The domain of the transform is infinite if you don’t use a fixed interval and the sine function is periodic.Even if you could it wouldn’t be very practical unless you were Q or something.

author: Fungus
author_rank: Super Contributor
timestamp: January 10, 2020, 11:54:46 pm
content: Quote from: bd139 on January 10, 2020, 05:56:23 pmA pulse is a discontinuity in the signal.

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2020, 01:01:07 am
content: Quote from: blueskull on January 10, 2020, 01:25:35 amThat's super annoying, I think I would have gone to the police about something like that. I mean it's one thing to damage someone's personal property but it's quite another thing to flee the scene without saying anything. Not to mention it's really disrespectful to the person who owns the gear.As far as blowing up scopes, I've never done it myself, but I've certainly seen plenty of damaged scopes. The problem is that people *aren't* sensible about what they're doing and in some cases you can have much higher voltages than you realize. Even though I have a proper Tek HV differential probe I don't let my scope near my HV experiments, it's not worth the risk of possibly damaging expensive gear.

author: gf
author_rank: Super Contributor
timestamp: January 11, 2020, 08:50:02 am
content: Quote from: Fungus on January 10, 2020, 05:38:51 pmSorry, the phrasing of my last post does not say what I actually wanted to say. I have corrected my post.The Nyquist limit at fs/2 applies of course, and the condition is "<" and not "<=", i.e. fmax < fs/2 (or fs > 2*fmax). So your statement "More than two points per cycle of the highest frequency sine wave in the signal" was indeed correct. I.e. a sine wave of say 49.999999999 MHz sampled at 100MSPS can be theoretically reconstructed exactly (but you you need an infinite number number of samples). When doing an approximate reconstruction via truncated sinc interpolation from a finite number of neighbor samples, fmax must not be that close to fs/2, but more headroom is required.