Oscilloskope 50Ohm Input - Page 1
Forum section: electronics / beginners

author: Apochrom
author_rank: Contributor
timestamp: April 11, 2019, 05:18:19 pm
content: Hello everyone,I have a fundamental question about oscilloscopes. I have heard criticism several times when oscilloscopes do not have a 50 ohm input, or can not be switched to 50 ohms. For which specific case does one need the 50Ohm input?Thanks all

author: Moriambar
author_rank: Unknown
timestamp: April 11, 2019, 05:26:40 pm
content: Quote from: Apochrom on April 11, 2019, 05:18:19 pmI think it's to terminate 50 Ohm BNC cables

author: dietert1
author_rank: Super Contributor
timestamp: April 11, 2019, 05:49:25 pm
content: When measuring above 100 or 200 MHz you need to use a preamplifier, usually called "active probe". Or you have a RF port on the "device under test" to connect a coaxial cable. Anyway the signal is routed to the scope using a coaxial cable, usually 50 Ohm, that needs to be terminated. The best location for that termination is at the end of the coaxial transmission line, which is somewhere inside the scope.If a scope does not come with that built-in termination, the second best method is to use a feed through terminator on the input socket of the scope. Then only a very small piece of unterminated transmission line remains behind that, maybe some centimeters inside the scope. So this method works well up to several hundred MHz.Fast scopes above 2 GHz bandwidth often come with "50 Ohm only" inputs and with smaller input sockets. The usual passive probe can't be used with those scopes.Regards, Dieter

author: tggzzz
author_rank: Super Contributor
timestamp: April 11, 2019, 05:56:54 pm
content: Some outputs are defined to operate into a 50ohm load. If they don't have the specified load then:for low frequency signals the voltage will be higher than indicatedthe output could theoretically oscillate, but that shouldn't happen with any well designed circuitfor high frequency signals (think RF), connecting cables act as transmission lines. If transmission lines are not terminated in the characteristic impedance, then the amplitude will vary with frequency. Search terms: VSWR, transmission line.The last is the most important case.Some scopes have an internal 50ohm termination. In most but not all low-end scopes, a 50ohm resistor is simply connected in parallel with the 1Mohm//20pF input. That is imperfect and the source of a specification "VSWR<1.3".If there is no internal 50ohm termination, then it can be simulated by either a BNC t-piece plus 50ohm termination, or a through termination. That would still be in parallel with the 20pF, with VSWR consequences.

author: Apochrom
author_rank: Contributor
timestamp: April 11, 2019, 06:07:43 pm
content: Thank you very much. That'll help me a lot. So for high frequencies from about 200Mhz you need an active sample and for that you need a BNC 50Ohm connector.Many years ago I had difference active probes at the oscilloscopes in the training laboratory of Physics. However, we didn't only have them at high frequencies, but we could also measure with them safely without having to pay attention to the short-circuit problem.Thanks, Jürgen

author: tggzzz
author_rank: Super Contributor
timestamp: April 11, 2019, 06:17:22 pm
content: Quote from: Apochrom on April 11, 2019, 06:07:43 pmThe 200MHz is not a hard and fast number. Understand VSWR and how it might affect your measurements. The scope and probe are all part of the circuit being testedThere are "low" impedance Z0 resistive divider probes that are purely passive, and are good to several GHz. At UHF and above their input impedance ishigherthan that of standard "high" impedance *10 probes.

author: exe
author_rank: Unknown
timestamp: April 11, 2019, 06:34:53 pm
content: I strongly recommend watching this video:https://www.eevblog.com/forum/beginners/scope-input-vswr-figure-at-50-ohm-what-is-that/msg193640/#msg193640. Then this one:.I mean, you really, really should watch these

author: Apochrom
author_rank: Contributor
timestamp: April 11, 2019, 08:52:27 pm
content: Thank you very much. This is really a fantastic video!This guy makes a lot of very good videos, I think I will be busy for the next weeks... :-)Regards, Jürgen

author: rhb
author_rank: Super Contributor
timestamp: April 11, 2019, 09:15:43 pm
content: The attached screen shots show the difference. The first is with a 50 ohm 36 ps rise time square wave input going in to the 1 M ohm input. The 2nd is with a 50 ohm thru terminator. The pulse generator is one of Leo Bodnar's. There is an extensive thread on the forum:https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/That thru has no reflections up to about 2-3 GHz. It increase the rise time from Leo's pulser from 55 ps to 65 ps using a 20 GHz, 14 ps rise time SD-26 sampling head on my 11801. I think Leo used a 40 GHz SD-30 to make the step trace and rise time measurement that came with the unit. A 50 GHz SD-32 might show it as a bit faster. The spec for the Maxim 3949 laser diode driver is 21 ps.The BNC is a pretty dubious connector for scopes that go over 1 GHz. They can be made to work, but an N would be much more appropriate.My LeCroy DDA-125 has BNCs and 50 ohm inputs and goes to 1.5 GHz, but it's also got 20% overshoot to go with the <250 ps rise time. If I ever get around to hacking it to get a single 3 GHz channel I'll install N-F connectors along with proper anti-alias filters. 4x 750 MHz, 2x 1.5 GHz, 1x 3 GHz and SMA relay switching. It will merge all 4 channels to give an 8 MSa/s sample rate.

author: dietert1
author_rank: Super Contributor
timestamp: April 12, 2019, 06:56:42 am
content: At which frequencies a through terminator is useful depends on the scope.As i wrote above, it depends how long is the unterminated part of the transmission line inside the scope from its input socket to the sampler. In your SD26 measurement this appears to be about 12 mm (lambda/4 at 3 GHz and half the speed of light). If that distance is 50 mm, then reflections will show up at 500 to 700 MHz.Regards, Dieter

author: rhb
author_rank: Super Contributor
timestamp: April 12, 2019, 10:28:56 pm
content: A thru is *required* when using a 50 ohm source to get accurate results independent of frequency. For a 50 to 1 M ohm interface the reflection is |1e6-50|/(1e6+50) which is very nearly 1.0.The screen shots were from a 200 MHz Instek MSO2204EA. The SD-26 sampling head is 50 ohms only, with a 3 V damage level. It is not intended for use with probes.The frequency where the reflection gets large is easily measured by setting cursors over a range of one cycle. In general, dispersion results in the reflections being spread out.The strongest frequency is a function of the two way delay time between reflectors. So for reflectors 5 cm apart, the resonant frequency is 3e8/(2*0.5) = 3 GHz in air and 67% of that in typical cable.137 shows the basic setup with the calibrator feeding the 13 ps rise time SD-26. The calibrator is faster than the <35 ps specification. 138 shows the trace and the rise time more closely.139 & 140 show the setup with an SMA tee feeding the head with the other side of the tee unterminated producing a reflection from an open circuit.142 and 145 show the reflection with a 50 ohm terminator on the tee. The tee is about 2 cm long, so the extra path to the terminator and back is 1 cm x 2 and accounting for a 67% velocity factor we get 100 ps delay between the incident and reflected wave arrivals at the head. It's not a perfect match as the terminator reflection sees 25 ohms rather than 50 so there are 3 reflections instead of 1.146 shows the reflection off the open end of ~36 cm SMA-M to BNC-F jumper connected in place of the terminator. The precursor shown in 147 is the 200 ps period reflection at the ends of the tee. 148 shows the bounce in the tee after the step. It's ~300 ps as it's a round trip + 1/2.149 shows the reflection from a ~36 cm BNC-M to SMA-M with a SMA-F to SMA-F and the 50 ohm terminator. 150 shows the time of the approximate 2nd zero crossings giving a first order estimate of the BW of the BNC connectors of 3 GHz. I've tried to explain this a bit better in a later post.154 and 155 show the BNC thru reflection with the terminator removed.158 shows the reflection step off the terminator. 159 shows the cumulative effect of the mismatches,While it won't provide the resolution that the 20 GHz BW of the 11801 & SD-26 combination, the instek example shows that you can get very useful results if you have enough cable to get away from the ringing. In response to my inquiry, Leo tells me he can supply a <40 ps BNC pulser with a 1 MHz clock in place of the 10 MHz clock for an extra 10 quid. Thus one can easily determine the length and quality of a <45 m cable with a DSO and 1 MHz square wave generator to the BW limit of the DSO. With all the other uses for Leo's pulser, that's a really good deal.Edit: fixed blunders (Thanks Dieter!), tried to improve the explanation and corrected autorotation artifacts. It would be nice if software were consistent and displayed things without showing one thing and doing something else.

author: dietert1
author_rank: Super Contributor
timestamp: April 13, 2019, 07:10:43 am
content: rhb, please check your calculation. I think the result of 3E8 m/sec divided by (2 times 0.05 m) is 3E9 /sec, which is 3 GHz.In general traveling wave effects show up not when the delay is half the wavelength (maximum extinction) but already when it's a third or a quarter of the wavelength. That depends on how strict you are with your overshoot criteria.When you go from half wavelength to quarter wavelength 3 GHz => 1.5 GHz, if you account for the speed factor 0,67 this becomes 1 GHz. My estimates were somewhat lower because for simplicity i used a speed factor 0.5.Concerning your measurements: Did you disable the internal termination of the SD-26?To come back to the thread: A low budget 350 or 400 MHz scope that comes without active probe ports and without internal terminators can well be used up to its full bandwidth with pass-through termination on its input sockets. Internal terminators are essential only at higher bandwidth.And the missing active probe port isn't a real problem when measuring low impedance sources like differential digital signals. Often you don't need a preamplifier. The simplest high frequency probe in that case is a 450 Ohm resistor feeding a 50 Ohm coaxial cable. Together with the through terminator on the scope input socket this makes a reasonable 10:1 high frequency probe.Regards, Dieter

author: rhb
author_rank: Super Contributor
timestamp: April 13, 2019, 03:06:01 pm
content: Again, thanks Dieter for pointing out the blunders.With respect to the BW. The spacing of the zeros of a sinc(t) closest to T0 gives the BW which is 2/T. If you pick the 2nd zeros crossing, then the BW is 1/T. Out of habit I picked the 2nd zero crossings, but then explained it incorrectly. So I've gone back and fixed that. The waveform between the 2nd zeros is very similar to what is referred to as a Ricker wavelet in reflection seismology for which the BW is approximately 1/T.The 11810 is a sampling scope. It samples the input at 100 KSa/s. Each sample is taken at j*dT after the trigger input. They are not able to trigger on the input. To do so would require a 50 ns delay line. That would present so much capacitance that it would be impossible to get the fast rise time.It's not like a DSO. They are peculiar beasts designed to give very fine time resolution. The minimum interval, dT, using 5120 samples per sweep is 200 fs (yes, 0.2 ps). There is no internal termination. The input circuit is a 50 ohm hybrid module. Hence the 3 V damage level.The thru is sitting in a 50 ohm line. So the reflection shown in 149 consists of a BNC-F cable connector, the 50 ohm thru and the BNC-M cable connector in series. As shown in 142 and 145, there are multiple reflections caused by the tee. So the response of reflection response of the thru shown in 149 has been convolved with the reflection response of the tee to first order. In fact, it's convolved with the reflections from all the interfaces which precede it. That is infinitely long, though the amplitude dies off pretty quickly.In seismic work if one needs to properly handle the reflections that follow the first one, one uses the Z transform and evaluates the expression for the poles and zeros of the transfer function. That is complex enough to merit publication as a professional paper by a prominent seismologist while I was in grad school. As it happened, I had solved the same problem as a course exercise which I turned in shorty before the paper appeared.The bottom line is, that as Dieter pointed out, until you get well above 1 GHz, a 50 ohm thru terminator on a scope which only has a 1 M ohm input will provide accurate results. But you do need the 50 ohm thru to get accurate results if the source impedance is 50 ohms. Without it, the cable to scope interface is almost a perfect reflector. So there will be a series of reflections without it. What effect they have depends upon the timing of their arrival.I did all the work in the previous post for my own benefit. I've been using the 11801 to evaluate connectors, cables and terminators. In the process I discovered a bug in the 11801 FW that leads to spurious events until the scope is reset. I came *very* close to posting an elaborate interpretation of what proved to be FW glitches in the response. Very plausible, but utterly wrong.Have Fun!Reg

author: Apochrom
author_rank: Contributor
timestamp: April 13, 2019, 03:52:46 pm
content: Many thanks for all the information, even if I didn't understand everything completely.Since a few weeks I have a new beginner DSO (1Gs/s and 200Mhz bandwidth) which doesn't have a 50ohm connection. I have played a lot with this DSO in the last weeks. For fun I connected a very simple radio FM antenna to the oscilloscope and did a simple FFT analysis. It is not so easy to try out all possible settings in the DSO and it took me quite a while to reach a certain optimum of the display. An experienced "Oscillonaut" would certainly achieve even better results, but I am quite satisfied with the result.See appendix.If I see the contributions correctly, are these results at my 1MOhm input very uncertain?Jürgen

author: Apochrom
author_rank: Contributor
timestamp: April 13, 2019, 03:57:11 pm
content: Supplement,the antenna is something like this here:https://www.ebay.de/itm/UKW-Antenne-mit-Koaxialkupplung-FM-Dipol-Radio-Wurfantenne-Hersteller-goobay/252085799381?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

author: rhb
author_rank: Super Contributor
timestamp: April 13, 2019, 09:39:37 pm
content: That antenna is 300 ohms unless there is a balun in the connector, which is highly likely. In either case there is still a large reflection coefficient, so you'll get better results with a thru terminator.The screen dumps demonstrate the effect of the mismatch. This is the same setup as the previous Instek dumps expect that there is 2 m of cable from the pulser to the DSO instead of a direct connection.34 is the <40 ps pulser without the thru. 35 is with the thru. There is a 2 m RG58, 50 ohm cable between the pulser which is a 50 ohm source and the scope which only provides a 1 M ohm input.Because the pulser and cable are fairly well matched, the two way reflection from the DSO to the pulser and back is fairly small. The reflection coefficient of the cable to pulser match is proportional to the ratio of the voltage of the first and last halves of the positive portion of the square wave.As is clear from 34, the mismatch at the DSO input BNC to the cable seriously degrades the step response through reflections within the BNC connectors and the input of the DSO AFE. The mismatch increases the apparent rise time by a factor of ~2x as shown in my previous Instek screen shots.A comparison of 35 to the corresponding figure in my previous post using the Instek gives an indication of the effect of the cable. Measured with 11801 it goes from 55 ps to 341 ps as a result of the capacitive load presented by the RG58.

author: Apochrom
author_rank: Contributor
timestamp: April 13, 2019, 09:53:50 pm
content: Many thanks for the explanation and the two diagrams!I have measured the resistance of the antenna, it is only 3.8 Ohm.

author: tautech
author_rank: Super Contributor
timestamp: April 13, 2019, 10:33:18 pm
content: Quote from: Apochrom on April 13, 2019, 09:53:50 pmTip for new SDS1202X-E user:For screenshot Saves, use the blue Print button for direct to USB saves and in doing so the menu you have active still remains so we can see and advise about your settings.

author: rhb
author_rank: Super Contributor
timestamp: April 13, 2019, 10:41:00 pm
content: That's a transformer winding. What matters is the complex impedance. To measure that you need either a fast edge pulse generator to do Time Domain Reflectometry or a Vector Network Analyzer. The latter are rather pricey for a novice. You can do TDR with a VNA or VNA with a TDR via the Fourier transform if you have the data in machine readable form. Getting data off the 11801 is on my "To Do" list, but I've been having too much fun just messing around doing stuff like the sequence a few posts back.I plan to write software and a detailed description of how to do VNA using a DSO and a fast edge pulse. It's limited to the BW of the DSO, but for building HF radios it is quite adequate. You can get 100 dB dynamic range from a single 20 Mpt, 12 bit record from an Owon XDS2102A. You can also do it with an 8 bit DSO. It just requires multiple traces to sum together.

author: rhb
author_rank: Super Contributor
timestamp: April 13, 2019, 11:08:11 pm
content: Here are some spectral displays using the Instek MDO2000E series spectrum analyzer app. You can't buy a license to use it on a different model in the GDS2000E line, but you *can* hack it.The photo shows the antenna I'm using which is a 5 cm diameter H field probe I made. Except for a very thin slot at the top, the inner conductor is completely shielded from the electric field, so it just senses the magnetic field.The noisy display in the photo is because I had not yet set the averaging. I first did this a week or so ago when I made the H field probe out of a RG402 semirigid solid Cu cable jumper I cut in two. I'll make a probe with a smaller loop later.The following screen dumps are with the probe on a longer cable about 1 m above the DSO. I moved it there and when the noise on the display didn't clear up I realized setting the DSO at 1 mV/div was not a good idea. So I changed to 5 mV/div.37 is the spectrum using a 50 ohm thru to match the loop.38 is the spectrum without the thru.39 is the spectrum with the DSO set to 1 mV/div and the same averaging as 37. I checked all the peaks and they correspond to the US FM allocation spacing. I'm in a hilly, rural area, so there are a lot of stations 75 miles away that I cannot get because of distance and topography.My MSO2204EA is a 200 MHz BW DSO, so very comparable to your Siglent. Notice that the spectra are not the same. The mismatch loses most of the signal from the antenna. It also creates resonances which modify the amplitudes. The peak on the left in 38 missing from 37 may or may not be real.If you were trying to listen to a distant radio station it would matter a lot, though not as much as a comparable mismatch on a transmitter. Many transmitters, if keyed without an antenna will immediately destroy the output power amplifier stage because of the reflected energy |(1e6-0)/(1e6+0)| = 1.0. That is almost the same as the 50 ohm to 1 M ohm mismatch, 0.9999.The better ham transceivers have circuitry to detect an open antenna line and turn of the transmitter.I've added a couple more displays.41 shows the spectrum without the thru, using peak detect acquisition and 256 sample averaging in the spectrum. 40 shows te same thing, but with the BNC thru in place. Otherwise everything is the same so far as I know. It's raining here and close to sunset, so propagation may be changing. The length of the cable from the join of the loop to the BNC is ~1.7 m. It would require a good bit of experimenting to determine what's going on.Have Fun!Reg

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 08:22:55 am
content: Thank you very much for your answers!@tautech: I had already asked myself how to save the settings bar. With the print key, that goes super thank you very much for the tip!@Reg: Many thanks for the explanations and the interesting diagrams.Is your h-Probe similar to the one Dave Jones recently presented?https://www.youtube.com/watch?time_continue=3&v=nImoQcoqkuQI think I'll dare to take a look at it.So, by connecting my simple antenna, the results should be enjoyed with caution. However, with this simple wire I could make the local radio-transmitters visible without further amplification. In fact, the frequencies of the big peaks are the lokal radio frequencies.That was my main goal to make the radio frequencies visible, even if some of the client peaks can only be reflections.Here a few diagrams in appendix. The setting was average calculations with 20 samples.Jürgen

author: dietert1
author_rank: Super Contributor
timestamp: April 14, 2019, 09:31:09 am
content: When we try to understand your "measurement", your screen dump seems to indicate that you are using a 10x probe. Is this correct?Then if i read the 100 MHz peak at about -73 dBm, this would be 50 uV RF. If the antenna is connected directly to the scope input socket, the result is 5 uV instead. As far as i remember a usual FM receiver works above 1 uV.Regards, Dieter

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 10:14:27 am
content: Quote from: dietert1 on April 14, 2019, 09:31:09 amyou're right, it's not a real mesurment, I'm just playing around with my new oscilloscope, trying to find the right settings and interpret the results.And also correct, the antenna is directly connected to the Scope input.

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 10:19:02 am
content: QuoteThe measurement with the scope by means of curser shows me with the setting 10x probe approx. 100mV, thus with 1x 10mV for the largest peaks.

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2019, 10:20:19 am
content: Quote from: dietert1 on April 14, 2019, 09:31:09 amThat would be true with 50ohm matching everywhere, imho. The OP does some basic experiments without any care on an "impedance matching", what is the keyword in this topic..So the values of the spectral peaks could be anything.The antenna is a cheap joke, I doubt there is a balun inside the connector. He would get the same result with 1m long piece of wire put into the oscope's 50ohm input.

author: rhb
author_rank: Super Contributor
timestamp: April 14, 2019, 12:11:11 pm
content: Quote from: imo on April 14, 2019, 10:20:19 amA few points:The antenna is a standard VHF broadcast band folded dipole. It has a connector designed to mate with a 75 ohm F-F as is common on television sets. There have been millions of these sold with forked lugs to attach to the 300 ohm input of a TV. There have also been millions of 75 ohm F to 300 ohm baluns sold. So making a folded VHF dipole with the balun integrated into the antenna makes a huge amount of sense. Modern TVs don't have 300 ohm inputs.For the OP's experiment it is a completely sensible choice. There is still the mismatch issue, of course. Looking at the station density, the OP is in or near an urban area and a 1 m length of wire would give a similar picture.As for being a "joke" I had a 300 ohm dipole in my attic which I used to receive the only TV station in range. It may still be there. I need to take a look this afternoon as there is a program on the blues style in music I want to watch tonight. I made mine out of twinlead and did not fold it because I am rather distant from the tower. I can only get that one station and sometimes the propagation is such I cannot get it.The numerical values shown have no meaning. All that setting the input to 10x does is change the numbers shown on the screen. In an absolute sense they have *no* meaning. In a relative sense they show the relative strength of the stations.Finally, yes, the probe is exactly like the one in Dave's video. Aside from making one I strongly urge that you make a small 3-4 mm diameter loop of wire with close wound spring connections to slip over a probe tip with the grabber and ground lead removed. Do *NOT* use the probe as a form. Measure the diameter of the tip and the ground and use slightly smaller drill bits as forms to wind the springs. It will detect both E and H fields, but because of the small area is very insensitive. So it is extremely useful for locating sources of EMI. For a short range E field probe I made a heat shrink condom to fit over the tip so I could probe a PCB without risking a short. If you decide to do that be careful you don't melt the insulation on your probe as I did.Add a pair of 50 ohm thrus to your list of things to get close to the top and...Have Fun!RegOh, and thanks for giving me an opportunity to show what I'm up to my my new toy, the 11801.

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 12:28:22 pm
content: Quote from: imo on April 14, 2019, 10:20:19 amYou're right, that wasn't clever to mix the two themes "50ohm input" and and "I'm experimenting with the FFT of the Oszi and a wire loop" - sorry.

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 12:52:26 pm
content: Quote from: rhb on April 14, 2019, 12:11:11 pmYes, that's right and in fact I only experimented with a piece of wire before connecting the antenna.Quote from: rhb on April 14, 2019, 12:11:11 pmThank you for the explanation, because in this case I did not get along with the numbers.It is even worse if I use the Curser, I do not understand the data, but I will post an example.Quote from: rhb on April 14, 2019, 12:11:11 pmSorry, what ist the meaning of "thrus", is it the same like resistor?Quote from: rhb on April 14, 2019, 12:11:11 pmYou're welcome.regards, Jürgen

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2019, 12:56:06 pm
content: Quote from: Apochrom on April 14, 2019, 12:28:22 pmNope, the themes are closely related, and it is a good example why you would need a good impedance matching.In order to "measure" the field strengths of the FM radio station with your oscope, you would need a "standard antenna", usually a half-wave dipole (vertical for FM, 73ohm).The antenna feed impedance has to match your cable (as well as the connectors), and your oscilloscope input. Then you may start think about the signal levels in dBm or uVolts (considering the half-wave dipole gain).For example your folded dipole is around 300ohm balanced symmetrical (gain?), you need to transform it into 50ohm single ended (not easy), and feed the signal via a 50ohm cable into your oscope with 50ohm input.

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 01:13:48 pm
content: Thank you very much for the explanations.I don't think I will be able to do all that and I don't have a 50ohm input on my oscilloscope.For me this experiment was nevertheless a success, since I could represent the frequencies of my local radio stations in principle with a simple piece of wire. The visible frequencies on the oscilloscope correspond very exactly to the frequencies of the radio stations. That was already the success for me. I had never done an FFT analysis with an oscilloscope before and this was an opportunity to try out the possibilities of my new oscilloscope. It was not easy for me as a beginner to explore all possible settings and to try to achieve an optimum display.regards, Jürgen

author: dietert1
author_rank: Super Contributor
timestamp: April 14, 2019, 02:29:44 pm
content: Sorry for the confusion. Now i understand Jürgen's measurement was in the 2 mV/section range, and 5 uV for the 100 MHz signal strength is a reasonable result. I think it is absolutely possible to make it a quantitative measurement, few steps are missing.I remember logging the spectrum of the 1 KHz calibration signal of our old LeCroy 9354A and measuring the first peaks at f, 3f, 5f, 7f and so on. I compared the results to expectations (coefficients of fourier series of rectangular signal) and was impressed on how well this matched. Another exercise in FFT!Regards, Dieter

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 04:38:52 pm
content: I have another question that is not directly related to the topic, but to the numbers of the display of my oscilloscope.I have activated the cursor in the FFT diagram (see appendix). The cursor display then shows e.g. delta x = 144us, okay that corresponds to 6.944khz. This is correct but what does it have to do with the diagram?I would like the cursor to show the frequencies in the diagram, e.g. 100Mhz, of course I can estimate the value myself by counting the boxes, but why does it show me 152us, which corresponds to a frequency of 6578Hz, what does this frequency mean?regards, Jürgen

author: rf-loop
author_rank: Super Contributor
timestamp: April 14, 2019, 04:57:48 pm
content: Quote from: Apochrom on April 14, 2019, 04:38:52 pmYou need select what source your cursors use.Press "Cursors" button and bottom screen menu, push "Source" and select "Math" instead of Channel or others.After then your cursors are white as FFT trace and cursors data source is from FFT(in your image your cursors source is Ch1 YT acquisition. If you use Split screen, Full screen or Exclusive it make sense, and specially if you use FFT split screen together with YT zoomed so you can look FFT but also measure YT trace details on screen)

author: rf-loop
author_rank: Super Contributor
timestamp: April 14, 2019, 05:08:09 pm
content: Quote from: imo on April 14, 2019, 12:56:06 pmExample if have 73ohm or 300 ohm feed thru and set Siglent FFT input for used impedance it may give reasonable levels directly. Usually we use 50ohm but others can also use and there is adjust for it so that it display then right levels (user can set external input inpedance from 1ohm to 1Mohm in FFT "Units/Load" settings.

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 05:39:14 pm
content: Quote from: rf-loop on April 14, 2019, 04:57:48 pmGreat! Thank you very much, I've been looking for this for a long time!

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 06:04:33 pm
content: now the display works as desired!

author: rhb
author_rank: Super Contributor
timestamp: April 14, 2019, 06:08:58 pm
content: A "through" aka "thru" terminator is a BNC-M to BNC-F which has a resistor network so that the source sees the desired impedance looking towards the scope and the scope sees 1 M ohm looking at the source.Here's a top line example and price:https://www.amazon.com/Pomona-4119-50-Terminator-Feed-Through/dp/B00JJ4EWU2I bought 8 unbranded units on eBay like these:https://www.ebay.com/itm/P7001-2Pcs-50-Feed-Through-Terminator-BNC-Female-Seat-50KY-Device-Q9-Adapter/273271990269TDR testing with the 11801 show they are quiet satisfactory up to 2-3 GHz.I think I'll get one of these to test:https://www.ebay.com/itm/50-Ohm-Feed-Through-Terminator-BNC-to-BNC-50KY-Device-Q9-Adapter-Black/292743414241I also have a spectrum analyzer with a tracking generator and a vector network analyzer which go to 3 GHz, so I can do a very thorough evaluation.Be wary of used name brand BNC stuff on eBay. I've got a bunch (not from eBay) of Pomona 6 dB attenuators which are quite worn and have a lot of ringing if they are in the wrong position.

author: Apochrom
author_rank: Contributor
timestamp: April 14, 2019, 06:30:58 pm
content: Many thanks for the links!With my 200Mhz the e-bay offer should be good enough.I would order some immediately, but they are sold out at banggood as well as alieexpress... and the e-bay provider is quite expensive and gives as delivery date middle of July ...regards, Jürgen

author: rhb
author_rank: Super Contributor
timestamp: April 14, 2019, 07:32:43 pm
content: Have Fun!Reg

author: tomato
author_rank: Regular Contributor
timestamp: April 14, 2019, 08:13:10 pm
content: Quote from: rhb on April 14, 2019, 06:08:58 pmFeed thru terminators of the type you linked are bidirectional -- the scope "sees" 50Ω also, not 1MΩ.

author: rhb
author_rank: Super Contributor
timestamp: April 14, 2019, 08:44:07 pm
content: That's interesting. Perhaps you would show some evidence? It's contrary to what I would expect.

author: tomato
author_rank: Regular Contributor
timestamp: April 14, 2019, 09:04:39 pm
content: Quote from: rhb on April 14, 2019, 08:44:07 pmJust cut one of those Pomona terminators open; they are nothing more than a 50Ω resistor to ground.

author: dietert1
author_rank: Super Contributor
timestamp: April 14, 2019, 10:10:34 pm
content: The reference rhb made to the amazon pomona offer is interesting in that the pomona part has specs at 250 and at 500 MHz - which in that sense is the upper frequency limit.I also found a similar amazon rigol offer, that has several customer reviews which explain in detail what this thread is all about. Among them somebody ("Quality please" August 6, 2017) who presented a smith chart and recommends the part only below 50 MHz.Regards, Dieter

author: exe
author_rank: Unknown
timestamp: April 15, 2019, 03:10:49 am
content: I wonder why oscilloscopes do not display horizontal scale in fft mode. Isn't this a useful information?

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2019, 03:57:02 am
content: Quote from: exe on April 15, 2019, 03:10:49 amAh but they do, as in *Hz/div and simple maths can tell you what the Span is too.As in this screenshot from Apochrom and you have to have the FFT menu up for it to show:

author: dietert1
author_rank: Super Contributor
timestamp: April 15, 2019, 08:12:49 am
content: So once more the invalid screen dump with the false 10x probe assignment and without 50 Ohm pass through terminator?Can siglent dealers please stop flooding the forum with OT nonsense? Didn't we discuss how those budget scopes need careful alignment by the user in order to get any meaningful results in the 100 MHz region and above?That peak at 100 MHz looks somewhat suspicious to me and i'm not yet convinced it's an FM radio emission or an internal clock of that scope.Regards, Dieter

author: exe
author_rank: Unknown
timestamp: April 15, 2019, 08:23:35 am
content: Quote from: tautech on April 15, 2019, 03:57:02 amWhy not just put a ruler under the plot? Or at least start and stop frequency or something? I'm really puzzled with fft pictures like this one:https://www.eevblog.com/forum/testgear/rigol-ds1054z-fft-bug(s)/. Or here I have no clue what it shows:https://c1.staticflickr.com/3/2822/34283102786_58845da303_c.jpg.Not to say frequency measurement and navigation between peaks on fft...

author: Apochrom
author_rank: Contributor
timestamp: April 15, 2019, 08:38:03 am
content: Hello Dieter,If you refer to this picture in the appendix. Yes indeed, at exactly 100Mhz is one of the strongest radio stations. The other 5 large peaks are also exactly at the location of the strongest radio stations here in the region. So, no matter how suspect the measurement may be, the peaks definitely represent the local radio stations exactly.I have set 1x probe and 4ohm input impedance. I don't know if the measured values for voltage in the picture say something about it.regardsJürgen

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2019, 09:01:56 am
content: Quote from: dietert1 on April 15, 2019, 08:12:49 amWell then wise guy, you take the time to answer exe's question !QuoteYes of course it's user error and when Apochrom entered this thread he made it plain he was a scope novice and rf-loop explained how to compensate for that with input impedance adjustments and measurement source selection however Apochrom has a 200 MHz scope so a 100 MHz center frequency FFT with a 1 MHz/div+5 div span is well within its capabilities.Quote from: exe on April 15, 2019, 08:23:35 amWith some SA experience much can be read straight from the display like we did with CRO's however that doesn't help the scope novice much in FFT usage unless you have some sort of graticule scalinganda relevant FFT settings menu.QuoteThere are some you can do with the inbuilt measurements available however this is an entry level DSO compared to an entry level SA for 5x the price................Quote from: Apochrom on April 15, 2019, 08:38:03 am

author: iMo
author_rank: Super Contributor
timestamp: April 15, 2019, 09:32:33 am
content: When looking at the last picture - the sampling rate is 1GSample/sec. Therefore the FFT spans from 0 to 500MHz.I cannot see on the first (2nd, 3th..) glance what is the frequency scale, however. Would be good to add it such we, novices, can get the information.The amplitude of the signals (the peaks) will follow the BW of the oscilloscope, sure. Therefore from 200MHz up the amplitude will be lower and lower.PS: I've spent 5 minutes staring at the picture n. SDS00014.png trying to get an idea where the large peaks are positioned in the spectra, with little success..@Siglent: could you provide a hint how to get the large peak freqs from the data shown on the display?

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2019, 09:41:19 am
content: Quote from: imo on April 15, 2019, 09:32:33 amWith the right menu it's very clear what the Hz/div setting is yes this is always visible in a proper spectrum analyser.The real art to is to have the appropriate menu showing when doing FFT screenshots with a DSO.

author: Apochrom
author_rank: Contributor
timestamp: April 15, 2019, 09:50:13 am
content: Quote from: imo on April 15, 2019, 09:32:33 amhello imo,I don't know if I understood you right. The Mhz scale is always a bit dark, maybe you have to set your monitor a bit brighter. On picture 14 the center is 98.2Mhz and the right edge 108.2Mhz, the left edge 88.2Mhz. So one box is 2Mhz. I chose this because I was able to display the whole FM Radio Freuquenzband in Germany.regards, Jürgen

author: iMo
author_rank: Super Contributor
timestamp: April 15, 2019, 09:51:56 am
content: The spectrum analyzer with the Siglents is a nice feature (and a nice working one).Adding 2 cursors with an information on the freqs, amplitude and deltas would make a great tool from it. No need for an expensive SA then

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2019, 09:55:04 am
content: Quote from: Apochrom on April 15, 2019, 09:50:13 amDisplay/P2/Intensity and Graticule brightness.

author: Apochrom
author_rank: Contributor
timestamp: April 15, 2019, 09:56:06 am
content: Quote from: tautech on April 15, 2019, 09:41:19 amOkay, picture 7 might be better... ;-)The middle is exactly at 100Mhz and the pitch is 2Mhz/box.regards, Jürgen

author: iMo
author_rank: Super Contributor
timestamp: April 15, 2019, 09:57:56 am
content: @Juergen - sorry - my bad, I've started to play with my display's viewing angles and 've taken my glasses too - now I can see there is freq scale on the TOP.Thanks!

author: iMo
author_rank: Super Contributor
timestamp: April 15, 2019, 10:03:59 am
content: Quote from: imo on April 15, 2019, 09:51:56 amAgain, that would be a great feature.. (if not implemented already)

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2019, 10:06:44 am
content: Quote from: imo on April 15, 2019, 09:57:56 amThe graticules and scales are always darker in Siglent screenshots than on the live display.A moment in these menus can fix that:Quote

author: Apochrom
author_rank: Contributor
timestamp: April 15, 2019, 10:30:49 am
content: Quote from: tautech on April 15, 2019, 10:06:44 amI've toyed a lot with the brightness settings, but they don't do much. And with the raster setting nothing at all...Here are two examples with 1% raster and 99% raster in the appendix. The brightness of the screens only increases at the edge, neither inside nor at the inscription.I like the default color scheme of Siglent, but I didn't manage to change anything, the colors seem to be fixed?regards

author: iMo
author_rank: Super Contributor
timestamp: April 15, 2019, 01:11:56 pm
content: Back to your original question on 50ohm termination at the oscope's input.Below are 3 simulations with the LTspice.The cable is 1.7meters of RG58 50ohm coax.The first picture is 1V square wave of 10-300ohm source impedance (R+j0) into the 1Meg||22pF oscope input.The second picture is 1V square wave of 10-300ohm source impedance (R+j0) into 1Meg||22pF oscope input in parallel with a 50ohm termination.The third picture is 1V square wave of 50ohm source impedance (50+j0) into 50ohm termination.

author: rhb
author_rank: Super Contributor
timestamp: April 15, 2019, 01:53:24 pm
content: What's the frequency of the square wave? The figures I showed are measurements with a 100 KHz square wave. It would be very helpful if you did a simulation to match my measurements. Changing the square wave frequency is not possible.Also can you do anything about the display? Blue on black is especially hard to see.

author: iMo
author_rank: Super Contributor
timestamp: April 15, 2019, 04:53:33 pm
content: The frequency was 50MHz, 20ns period, with 1ns edges.I will do with different colors.

author: rhb
author_rank: Super Contributor
timestamp: April 15, 2019, 11:45:26 pm
content: You need to use a much lower frequency, <= 1 MHz to see what is going on. I'm going to get a custom 1 Mhz pulser from Leo Bodnar for exactly this reason. Even 10 MHz is too fast.

author: rhb
author_rank: Super Contributor
timestamp: April 17, 2019, 12:40:36 pm
content: FYI These just popped up in an Amazon adhttps://www.amazon.com/Copper-Adapter-38-5mm-Through-Terminator/dp/B07G566JC7

author: Apochrom
author_rank: Contributor
timestamp: April 17, 2019, 08:58:28 pm
content: Thank you very much for the link. Unfortunately they do not deliver to Germany.I had now ordered these two from my favourite dealer. (Straight adapters are currently not available in Germany...):https://www.reichelt.de/adapter-bnc-bnc-90-50-ohm-rnd-205-00479-p194674.html?https://www.reichelt.de/adapter-bnc-bnc-y-stueck-90-50-ohm-rnd-205-00482-p194677.html?Are those the kind you recommended?Both are indicated with 50ohm. In addition I still have a totally stupid beginner question.If I understood it correctly, the 50ohm only appear at high frequencies as impedance. If I measure the resistance normally with a Mulitmeter the Mulitmeter shows 0 ohm. Since the resistance of 50ohm only occurs at high frequencies, I can't measure it with a Mulitmeter, can I? How do I measure the impedance and at which frequency?regards, Jürgen

author: rhb
author_rank: Super Contributor
timestamp: April 17, 2019, 09:31:08 pm
content: Those are just adapters, not thru terminations. But essential items to have. I don't have a tee of that style. It looks as if it could be handy.With regard to measuring the impedance, stay tuned. I'm trying to work out a cheap way to do that by TDR.The impedance of a line in the limit as the frequency goes to infinity is sqrt(L/C) where L and C are the inductance and capacitance per unit length.

author: Apochrom
author_rank: Contributor
timestamp: April 17, 2019, 09:53:31 pm
content: Thank you very much,what is the meaning of "TDR"?

author: tautech
author_rank: Super Contributor
timestamp: April 17, 2019, 10:50:35 pm
content: Quote from: Apochrom on April 17, 2019, 09:53:31 pmTime Domain Reflectometryhttps://en.wikipedia.org/wiki/Time-domain_reflectometry

author: iMo
author_rank: Super Contributor
timestamp: April 18, 2019, 07:52:55 am
content: Quote from: Apochrom on April 17, 2019, 08:58:28 pmThe 50ohm (50+j0) appears at all frequencies, it is frequency independent.Your multimeter shall show 50ohm if the impedance is resistive (thus your termination will show 50ohm when measuring the mid pin against the gnd).Generally an "impedance" is a "complex value" R+jX or R-jX, based on the sign it could be capacitive or inductive (X is the reactance).When there is the reactance X the impedance is frequency dependent.For example your antenna impedance could be 180-j200 at 90MHz, or 340+j12 at 105MHz, etc.Your 50ohm BNC termination is theoretically 50+j0 (pure resistive, not dependent on frequency), in reality there is a capacitive or inductive portion of reactance based on parasitic capacitancies and/or inductancies. You may see the influence of the parasitic reactancies usually at higher frequencies, based on the manufacturing quality of the termination.Measuring impedance is not an easy exercise for a beginner, unless you have got the equipment and you know well what you are doing.PS:https://en.wikipedia.org/wiki/Electrical_impedance

author: iMo
author_rank: Super Contributor
timestamp: April 18, 2019, 09:18:23 am
content: Imagine an ideal signal Generator (50+j0 output impedance), lossy 2m long RG58 coax, and a cheapo 50ohm BNC Termination (an example only) put on Oscope's input.Below a simulation how the Generator "sees" the Oscope's impedance (the Generator "looks into the input connector" of the RG58 cable).The impedance is in "polar form" (impedance magnitude and phase, could be converted to R+jX at given frequency).

author: rhb
author_rank: Super Contributor
timestamp: April 18, 2019, 02:36:25 pm
content: Here is an example of measuring impedance by TDR.This is just an initial installment. I will have a *lot* more to say on the subject once I have written the software to do vector network analysis (VNA) by means of time domain reflectometery (TDR). I just ordered a couple of square wave pulsers from Leo Bodnar with 1 MHz instead of 10 MHz outputs so I can move the reflection event away from the ringing of the DSO AFE.The setup I'm using here is a square wave feeding a tee at the scope input with a ~35 cm SMA-M to BNC-F cable.167 shows the reflection with a 50 ohm terminator on the BNC-F connection168 shows the same setup with a 75 ohm terminator169 shows a 75 ohm cable in place of the terminator170 shows a 50 ohm cable in place of the terminator.Neither cable is terminated, but you can't see that as it is off the screen.171 shows the full step with the 75 ohm terminator The initial step, measured using the cursors is 168 mV. The 2nd step is 18 mV. That *appears* to be a reflection coefficient of |0.11| however, there is a factor of ~2 error as it is |75-50|/(50+75) = 0.20. This is almost certainly because I am using a tee rather than a two port splitter/combiner and the cable is not exactly the specified impedance. According to my 34401A using a banana plug to BNC-F, the 75 is 74.998 ohms. And the 11801 really isn't accurate enough to give 2 digits of accuracy even if I adjust the step to full scale. If I assume a correction factor of 2 then the measurements give the impedance of the terminator as 77 ohmsThe main point here is that TDR can give you a means of measuring impedance using a DSO. Accurate results require transferring the data to a PC and doing a bunch of windowing, FFTs, phase shifts and summing. But a 100 dB dynamic range using an 8 bit DSO is not an unreasonable expectation.

author: iMo
author_rank: Super Contributor
timestamp: April 18, 2019, 03:28:07 pm
content: I do assume your 35cm long coax is the RG174. What is your LeoB_pulser's amplitude (mine is 1V)?My first step is 3.6ns, btw.1st step 334mV2nd step 378mVdiff 44mV

author: rhb
author_rank: Super Contributor
timestamp: April 18, 2019, 04:01:19 pm
content: I'm using the <35 ps rise time 250 mV 100 KHz square wave calibrator output on the 11801. I didn't measure the rise time, but it's showing at the moment around 20 ps with the scope not warmed up. Generally I get a measurement of around 25-26 ps with it warmed up. The heads are rated at about 13 ps. So obviously the calibrator is faster than spec by a good bit.It took some looking, but it's Rohm RG174. The line from the calibrator to the tee is ~ 30 cm of RG402.

author: iMo
author_rank: Super Contributor
timestamp: April 18, 2019, 04:54:42 pm
content: I cannot find the lossy model for the RG402, using params of RG174. That is not good as the 402 is pretty different beast, but ok for this measurement.Below with 250mV generator/calibrator pulse amplitude.Also I've changed your oscope imped to 1Meg.Update: your sampling's head is 50ohm.First step is 124.9mV, the second 150mV.First step is 83.34mV, the second 94.48mV.PS: added sim with RG402's lossless td=4.8ns/m.Almost the same result (diffs of 0.1mV).

author: rhb
author_rank: Super Contributor
timestamp: April 18, 2019, 06:23:04 pm
content: The SD-26 is a 20 GHz, 13 ps rise time 50 ohm input sampling head. It *only* has 50 ohm inputs. The 11801 mainframe has a 50 GHz BW using an SD-32 7 ps rise time sampling head. This is *not* a DSO. Sampling scopes are completely different instruments.One can do similar measurements to the BW of a DSO with one of Leo's square wave generators, though obviously not to the extreme which the 20 GHz SD-26 will reach. The calibrator step is actually about 17 ps.The reflection from the terminator sees 25 ohms at the tee from the parallel combination of the 50 ohm sampling head input and the 50 ohm calibrator output. I've not done the algebra, but I'm pretty sure that is the source of the 2x factor.I'm using the 11801 because it happens to be sitting in the middle of my bench at the moment. Getting to another scope is a tad difficult as it's a very small bench area.

author: iMo
author_rank: Super Contributor
timestamp: April 19, 2019, 06:18:37 am
content: Updated above for your 50ohm sampling head.Btw, your calibrator output is most probably 500mV (not 250mV), such one can get your step's voltage numbers..

author: rhb
author_rank: Super Contributor
timestamp: April 19, 2019, 11:34:57 am
content: "The calibrator output is a 250 mV square wave with a rise time of approximately 20 ps and a period of approximately 10 microseconds."Tek 11801 User Reference Manual p 42part # 070-7037-00Revised May 1989

author: iMo
author_rank: Super Contributor
timestamp: April 19, 2019, 01:50:22 pm
content: Ok, 250mVpeak at the 50ohm output when loaded with 50ohm (internally 500mV).

author: rhb
author_rank: Super Contributor
timestamp: April 23, 2019, 02:39:14 am
content: FWIW I received four 50 ohm thrus today from:https://www.amazon.com/gp/product/B07G566JC7measured two wire DC resistances using a 34401A and a banana to BNC adapter:49.900, 49.937, 50.011 & 50.029TDR with my 11801 showed results on par with my other 50 ohm thrus.I don't currently have a 50 ohm splitter so there are reflections because of the mismatches, I have ordered 50 ohm resistive splitters. Once those arrive from China I will test them and post results both for the splitters and for the thrus.These are not Pomona or Pasternack quality, but they are better than I can measure at this point. I plan to improve my measurement abilities. At $5 each, very satisfactory. And probably indistinguishable from the high priced thrus at 200 MHz.Have Fun!Reg

author: NDani
author_rank: Contributor
timestamp: March 10, 2023, 09:03:29 am
content: Quote from: tggzzz on April 11, 2019, 05:56:54 pmHi all,Sorry if this has already been asked, it's quite a long thread to check all posts.The quoted post points out that on lower-end scopes the 50 ohm termination is achieved by switching a 50 ohm resistor in parallel with the high impedance input and that this can cause high VSWR at higher frequencies because of the scope's input capacitance. If I understand correctly, the situation would be similar when using a feedthrough BNC 50 ohm termination (but worse because of the unterminated stub after the feedthrough adapter).How is the situation different on higher-end scopes? I use a Keysight 350 MHz scope in our lab which has switchable internal 50 ohm termination and 14 pF input capacitance. Now at, say, 200 MHz that input capacitance would cause a problem since a 50 ohm resistor in parallel with a 14 pF capacitor would create a VSWR of about 0.75.My questions are the following:- Is the input capacitance problem eliminated in higher-end scopes in 50 ohm mode? If so, how does the input capacitance disappear (or decrease)?- Is it possible to eliminate the input capacitance problem with scopes that don't have an internal 50 ohm termination (and hence an external termination must be used)?Thanks,Daniel