Leo Bodnar LBE-1420 square wave problem [solved] - Page 1
Forum section: electronics / testgear

author: hp3310a
author_rank: Regular Contributor
timestamp: March 28, 2025, 11:35:52 am
content: Hi,in order to test the accuracy of my homegrown frequency counter I got a GPSDO from Leo Bodnar, model LBE-1420. Really nice tiny thing.My counter had some problems between 20Hz and 33Hz, doubling the frequency. So I put the LBE-1420 output in my scope and I'm not sure what I see is how it is supposed to be. The specs say:QuoteSo the 1MΩ scope input should give me about 0-3.3V square wave. However, I get a square with a huge spike-like overshoot to 3.7V and the "rest" is square but less than 3.1V. I'm feeding the output via SMA cable directly into the scope (1x), I don't think the spike comes from my setup (photo at the end).At 10MHz the spike's form becomes apparent and the shorter the puls the shorter the square portion. At 40MHz there's only the spike left.More screen shots in the next post.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 28, 2025, 11:41:20 am
content: More screenshots and the setup.The device runs with firmware version 1.05, my request for support with installing the latest firmware from a non-Windows-PC computer was ignored (so far). Not sure if a firmware update would be able to influence the waveform though.So, the question is: Is this spike normal?

author: blackdog
author_rank: Frequent Contributor
timestamp: March 28, 2025, 11:59:25 am
content: Hi hp3310a, ;-)For measuring to fast pulse signals miss it necessarily that you do this with a 50 Ohm load.Your scope channel is at 1Meg input impedance, which gives a lot of abberations in the pulse response.Kind regards,Bram

author: hp3310a
author_rank: Regular Contributor
timestamp: March 28, 2025, 12:00:45 pm
content: For fun I tried putting a 50Ω terminator at the end of the line and the spike is only a very tiny one. Output is at 1.8V better than spec'd. But of course that doesn't fix my problem with high impedance output.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 28, 2025, 12:03:48 pm
content: Quote from: blackdog on March 28, 2025, 11:59:25 amSo how does one measure a high-impedance input scenario? My frequency counter is high input impedance by design because were it is attached to it can't draw any load.

author: tggzzz
author_rank: Super Contributor
timestamp: March 28, 2025, 12:30:41 pm
content: Quote from: hp3310a on March 28, 2025, 12:03:48 pmYou need to understand transmission line theory and practice. Physics wins, every timeWhat frequencies/periods will be input to your counter?If not a sine wave, what will thetransition timebe? The rise/fall time determines the highest frequency present in the signal; the period is irrelevant. For a little theory and some practical measurements, seehttps://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/A scope's "high impedance" input - and almost certainly your counter's input - isn't 1Mohm. It is 1Mohm//17pF. I strongly suggest you calculate the impedance of the 17pF capacitor at the highest frequency in your signal. For a digital signal, it will be muchmuchless than 1Mohm! Then determine whether that loading will be a problem in your system.Quote from: hp3310a on March 28, 2025, 11:35:52 amThat problem is related to either noise on the signal or to a seriously defective counter front end.

author: Grandchuck
author_rank: Super Contributor
timestamp: March 28, 2025, 12:44:28 pm
content: Try the 20 MHz bandwidth limit setting. It might give you a better feel for what is going on. Not saying that is thecorrectsetting, BTW.

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 01:35:19 pm
content: Most scopes will have some series damping resistance at the 1 Meg input.There might still be 2pF of capacitance in the BNC connector body right at the scope input but this will usually be followed by some series damping resistance of maybe a few tens of ohms.The overshoot in that Siglent scope does look to be quite significant as it is about 10ns wide on some of the plots. Maybe there is an issue with the setup especially if a long (and basic low cost) input cable was used.I've still got a Leo Bodnar LBE-1421 here although it is currently packed away ready to go back to work on Monday. I probably won't be able to borrow it again at least in the short/medium term. I've also got various scopes here but they are all quite old and none of them can measure a 300ps risetime. I can unpack the Bodnar unit and measure it on my fastest scope if that helps? I have done this already a couple of weeks ago so I know it looks OK on this scope. It has a 1 GHz BW. I can also try and measure the Bodnar unit on a 40GSa/s (13 GHz BW) scope at work but this won't be for about a week.

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 02:19:15 pm
content: Here's a plot of the LBE-1421 with my scope set to 1 Meg input and 40ns /div and this can be compared to your 50 ns/div plot with a 1 Meg input.It's generally a bit risky to use the 1 Meg input for stuff that has fast edges but this scope coped OK I think. There isn't much in the way of overshoot or ringing. Maybe try your measurement again without the coax cable? feed the LBE direct into the scope?With the input set to 50 ohms I see a rise time of about 450ns but this is limited by the bandwidth of this scope. I think the rise time of the LBE-1421 is more like 300ps.

author: tggzzz
author_rank: Super Contributor
timestamp: March 28, 2025, 02:22:39 pm
content: Quote from: G0HZU on March 28, 2025, 01:35:19 pmThe scopes' front panel states 17pF.

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 03:17:27 pm
content: Quote from: tggzzz on March 28, 2025, 02:22:39 pmI guess you can look at the scope input in the time domain or measure the impedance in steady state in the frequency domain with something like a VNA.In steady state, the 1 Meg input of a typical 100-200 MHz scope will only look like 1Meg (in parallel with 17pF) up to maybe a few hundred kHz. By 10 MHz it might look like 10k ohm in parallel with 17pF. By 100 MHz it might look like 300 ohms in parallel with 14pF.By 200 MHz it might look like 100 ohms in parallel with 3pF and at frequencies above this it may start to look like a shunt resistance of maybe 60 ohms with little or no capacitance at all. This is because of the impact of the series damping resistance at the input.When feeding it (very) fast edges, the series resistance will act a bit like a termination or damper. I'm not a scope guru/designer but I have a lot of experience using them and looking inside them and modelling how they behave. However, the ideal person to answer all this is a scope designer from one of the big name companies.I have no idea what Rigol and Siglent do with the front end design of their scopes. Maybe they don't have much in the way of damping at the input at all?

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 04:18:48 pm
content: Here's the Bodnar LBE-1421 measured at the 1Meg input of my old Tek TDS2012. I rarely use this scope but I dumped a screenshot out of it over GPIB.This is on 50ns/div and you can see there is no overshoot. This is direct into the 1 Meg input of the TDS2012 via a decent (Suhner) SMA to BNC adaptor.I set the output of the LBE-1421 to 20 Hz and also measured it with a couple of reciprocal frequency counters I have here. Both coped fine with the 20 Hz input with no need to limit the bandwidth or use an attenuator at the counter input. The counters both have a 1 Meg input.Both counters have the option to select a low frequency lowpass filter at the input but they coped fine without it. Both read 20.000 000 0 Hz when set to 9 digits.

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 04:42:23 pm
content: Looking at the 50ns/div plots from the Siglent scope, I'm still struggling to see how it could look this bad. See the plot below to see what I mean. I doubt it is the scope.If I assume the LBE-1420 output driver is similar to the LBE1421 then I don't think it's an issue with the LBE-1420.Maybe it is the coax cable that you used? In your plot below it looks to me like you may have a cable that isn't a genuine 50 ohm cable. If it was a 75R cable that was about 4ns long (2.5 feet long?) then I think it would produce a response like your scope plot below.Can you try another cable? Find one you know is a good quality 50R cable with decent connectors at each end.

author: David Hess
author_rank: Super Contributor
timestamp: March 28, 2025, 05:25:49 pm
content: Quote from: G0HZU on March 28, 2025, 04:42:23 pmIt looks like the reflection from a series terminated 2.5 foot coaxial cable.

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 05:41:28 pm
content: Quote from: David Hess on March 28, 2025, 05:25:49 pmI don't exactly know you mean but the OP said this:QuoteIf the coax cable was 75R and it had a delay of 4ns (2.5 ft long?) and the EMF from the 50R source was 3.1V then I'd expect to see a pulse at the scope end that initially starts with a spike at about 3.72V. This spike would last for about 8 nanoseconds before dropping to just under 3V for another 8 ns. It would eventually settle at 3.1V for the rest of the pulse duration.This assumes a perfect high impedance scope. A real scope will have some resistance and capacitance at the input so the waveform spikes will be a bit more rounded than this.But a 75 ohm cable would (in theory at least) give the scope waveform seen by the OP on the 1 Meg input of a healthy scope driven by a pulsed waveform from a genuine 50 ohm source.

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 05:54:29 pm
content: Something like the quick and dirty excel sheet below? This shows what happens when a 50R source is fed into a 75R cable with no termination at the far end. The voltage levels at the end of the 75R cable are the ones coloured in red and orange and yellow. End voltage 1 is the one that causes the 3.72V spike.This does match the symptoms described by the OP. I can try and find some 75R cable and show you this in practice? I already know what it will look like as I've been doing basic RF stuff like this for decadesHowever, maybe this isn't what is happening in this case and maybe there is another reason and there is a mismatch somewhere else in the system, I can only guess based on what I see in the plots supplied by the OPSorry but the spreadsheet below isn't very complete. The Zo cell is the coax cable Zo. The 50R cell is the source impedance and the 3.1V cell is the source EMF. I've updated it now...

author: G0HZU
author_rank: Super Contributor
timestamp: March 28, 2025, 06:43:40 pm
content: Of course, you would get similar symptoms with a 3.72V leading edge spike if the Bondar LBE-1420 source impedance was about 33.3 ohms rather than 50 ohms and a genuine 50 ohm cable was being used. But as the dual output LBE-1421 appears to have a 50 ohm source impedance this seems unlikely.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 01:04:19 am
content: I had some free time this evening so tried replicating this issue after finding some old 75R TV coax in the garage.I cut a piece about 3 feet long and fed it with the Bodnar LBE-1421 set to a 10 MHz output. At the far end of the coax I placed a high impedance /10 probe made by Marconi. This has fairly low input capacitance so the waveform at the far end of the cable will still look fairly square at each change in level.The EMF from this unit is about 3.25V so the first spike should be at 3.9V with the 75 R coax and after the waveform settles it should be at 3.25V. You can see that this is roughly what happens but all voltages are divided by 10 by the probe. The probe does cause some distortion of the waveform but it still shows the expected rectangular 'staircase' voltage levels.I then fed the far end of the coax directly into the Tek scope set to a 1 Meg input. You can now see that the rectangular nature of the spikes have become more rounded due to the input resistance and capacitance of the scope. This does now look fairly similar to the OPs plot when set to 50ns/div. So it looks like something is mis-terminated in his case. Either the Bodnar unit doesn't have a 50R source (unlikely but possible) or the Bodnar unit does have a 50R output but the cable isn't a 50 ohm cable? It might look closer to a 75R cable?

author: hp3310a
author_rank: Regular Contributor
timestamp: March 29, 2025, 08:05:59 am
content: First of all thanks a lot for trying to help me understand what is going on. It appears that my assumption this being unrelated to my setup is completely wrong. As usual.My cable is self made 80cm long from 50Ω RG174-5 with male SMA connectors on both ends. See picture.I did another test run with BNC cables, first 1m long second 55cm. Not sure about what quality they are, I believe the short one came with the UTG962 function generator, the long one was ordered somewhere. The 1m is definitely worse than the RG174, the shorter one is about the same. In both cases directly connected to the 1M scope input.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 29, 2025, 08:15:31 am
content: Here's a comparison between BNC 55cm and RG174 80cm at 50MHz.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 09:22:31 am
content: I’m away from my bench and on an iPad but I think I can see enough info now. I should have spotted this from your earlier plot with the 50r termination. You had 1.8V pkpk then but only 3.1V into an open.So it looks like the LBE-1420 has a slightly lower source impedance than 50 ohms. Maybe 35 ohms? This assumes your 50r termination is an accurate 50r.I’ll show you what I mean later

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 10:35:11 am
content: The 1m cable looks a bit wild. I'm not sure what is going on there.However, I simulated a 35R source impedance and a 55cm long 50R cable at 10 MHz. This does seem to agree with your plot quite well as below. I think you need to double check everything and maybe try testing again with a 50 ohm 20dB attenuator at the output of the Bodnar LBE-1420. See if the attenuator cleans up the source impedance and improves the waveform although it will shrink to about a tenth of the amplitude.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 10:42:44 am
content: To try and match your wild result with the 1m cable I ended up having a 35R source for the Bodnar unit and a 75 ohm cable 1m long. This is getting a bit unlikely so maybe there is something else going on with your setup?See below to see what I mean.

author: raherh
author_rank: Newbie
timestamp: March 29, 2025, 11:38:58 am
content: I've acquired a Siglent 812X and LBE-1420 recently. And coincidently was also checking a frequency counter kit with 10MHz to set its TCXO.So I measured the waveform at 10MHz to make a comparison with your measurement.First I connected the GPSDO to the scope via 20cm SMA cable. Spike was 80mV, rising edge about 8ns, the 50 ohms termination or not - values similar.The results with 50cm BNC cable were same.Usually I use some attenuation to reduce the voltages, the results with 26dB attenuator were same. Also it's better to use shorter cables and to terminate the generator with its output impedance.On a sidenote (don't know in which thread to add), that's my first DSO, I commonly measure wafeform parameters as well, Pk-Pk, RMS and so on. So I enable this meaurement but don't get why the scope displays values for instance 103.4567mV with last five or six digits fastly changing. I couldn't find out how to round off or remove the useless ones or to slow down the sampling. It's disturbing.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 29, 2025, 11:54:38 am
content: I tried the BNC cables because you suggested that the first cable might be/cause the "problem". I'm not sure how to interpret any of the wave forms I see, though.If there are spikes like those in the initial posting, is there any way to determine if the spike is being produced by the source (the LBE-1420)? How could it be distinguished from some other effect the fast edge causes in the wire under the given circumstances?Theoretically I could plug the LBE-1420 directly into the scope and do away with almost all cable length (only one adapter with 25mm length between BNC connector and device connector). The device would be held by the SMA connector only, which isn't made for that. So I'll try a very short cable first to see if it changes anything.Also, I don't want to use 50Ω termination. Although the wave form looks very nice (see posting abovehttps://www.eevblog.com/forum/testgear/leo-bodnar-lbe-1420-square-wave-problem/msg5863495/#msg5863495), it is not my use case in this particular instance.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 12:36:26 pm
content: Probably the first thing to do is use the shortest cable you have (or connect directly to the scope) and set the frequency fairly low at maybe 1 MHz. Then measure the voltage of the flattest part of the waveform after the spike.You might see 3.1V in your case as this is what you saw previously.Then double check that your 50 ohm terminator really is 50 ohms (or very close) and then repeat with the 50 ohm terminator at the scope. If you end up seeing 1.8V with the 50R termination attached then the source impedance will be about 36 ohms as in the spreadsheet attached below.If the source resistance of the LBE-1420 really is 36 ohms then the spiky waveforms you are seeing on the scope (with the 55cm and 80cm 50R cables) are due to the 36 ohm source impedance and they are what I'd expect to see with a 36 ohm source impedance.This test treats the system as a simple potential divider. If the source impedance was 50 ohms and the terminator is 50 ohms then the voltage with the terminator ought to be half the 3.1V or 1.55V. But it appears to be more like 1.8V which implies the source impedance is more like 36 ohms.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 29, 2025, 12:50:52 pm
content: Hi,It's not important what you like....This is important, and not because i say it.What matters is what physics wants to teach you.And that is the following, if the source impedance is 50 Ohms, then the coaxial cable will also have to be 50 Ohms and last... the termination impedance will also have to be 50 Ohms.If you do not meet all the conditions, then there is little to be said about the voltage level at the end of the cable.There are too many variables that come into play and yes that can all be calculated if you know all the impedances well.In practice almost nobody does that, fast signals, then always terminate the cable properly!Kind regards,Bram

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 12:52:21 pm
content: When I do the test on my LBE-1421 I see 3.30V on the scope when unterminated and a tiny bit more than 1.6V when terminated with the accurate 50R load inside the scope. So I think the source Z of this LBE-1421 is very close to 50 ohms because I see almost exactly half the voltage when it is terminated in 50 ohms.I can't explain why the LBE-1420 appears to be different in your tests?

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 12:59:31 pm
content: Quote from: blackdog on March 29, 2025, 12:50:52 pmOK but it stands out to me like bollocks on a dog that the cable isn't matched to the source impedance. Those spikes in the initial posts are the right duration for a 2.5 foot cable that has a Zo that is maybe 50% higher than the source impedance of the LBE-1420.Because the LBE-1421 I have here does appear to have a 50 ohm source impedance, I'm a bit confused why the LBE-1420 would be so different.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 01:45:02 pm
content: I don't know if this helps but here is a simulation showing what to expect if you fed a pulse generator into a typical 1 Meg scope via a 1 metre long 50 ohm cable (left unterminated at the scope end) and you then varied the source impedance of the pulse generator across 35R, 45R, 50R, 55R and 60R.You can see that with a 35R source there is a spike. In all cases the pulse shape has a slower than expected rise time but not by much. This is caused by the input capacitance of the (unterminated) scope.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 29, 2025, 02:37:52 pm
content: I don't know anything about your Bollocks :-)Among other things, this could be going on:The Leo Bodnar pulser may be faulty.Connectors may have an abnormality.Coax cable may have an abnormality.But again, a coax cable is supposed to be properly driven and terminated at both ends, and here that is 50 Ohms.Your frequency counter may not indicate the correct frequency if the coax is not properly terminated with 50 Ohms.With the counter I have here, I have for up to 160MHz input 50Ohm or 1Meg.But that 1Meg input almost certainly has 15 to 30pF in parallel.This is part of what I indicated regarding the many variables.Another variable is that if you do not terminate the cable with 50 Ohms, the input amplifier of your frequency Counter can overload with the overshoot that occurs when the cable is not terminated.Then again, this may depend on the measurement frequency under test.Now do you understand what I mean by many variables?Here three pictures of 10MHz out the older dual version from Leo Bornar GPS Reference.The first picture is of the Bodnar pulser connected via a 0.5 Meter reasonably good coaxial cable, I have better but this is chosen this way on purpose.This shows very clearly what happens if all the conditions for good transmission over a piece of cableare not met.This signal is very difficult for a frequency counter to measure properly due to the oscillation as a result of not properly driving/ terminating the cable used..This is with the same cable, but here the scope is set to 50 Ohm terminating impedance.Also note the signal in the orange box which is the refelction due to misadjustment of the cable and all other variables..And here there is no Coax cable used, but a BNC-BNC connector, which is about 3cm long.There is now almost no reflection in the signal..The rise time shown here is that of the GPS reference,if I use my Leo Bodnar BNC pulser, this scope shows 660nsec.The shorter the rise time seen by the cable and the rest, the bigger the problems become if the whole thing does not meet the entire 50 Ohm transmission line.I mean all this among other things:It's not important what you like.... but what isThe whole thing is complex and I also had a fairly long learning curve before I got the hang of it.When I learned this there was no Internet, no affordable good broadband scoops, no spectrum analyzers etc and certainly no fancy Lea Bodnar measuring instruments.Not to mention connector higiene.Keep your connectors clean, tighten your SMA connectors the right way, tighten the nut and not the cable.Above 500MHz if you want to make accurate measurements whit SMA or better use a torque wrench.Kind regards,Bram

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 03:06:56 pm
content: OK thanks. Your first plot looks like the low impedance driver chip (inside the older Bodnar unit) was fed directly to its output such that the source impedance is only about 10 ohms or so. This is unusual to see (for me at least). Normally, a series resistor would be fitted internally to better define the source match so it is closer to 50 ohms. Otherwise you end up with that awful response on a scope when left unterminated.If the source match is well defined at 50 ohms you can get away with having an open (or a 1 Meg scope) at the far end of a cable as long as you don't mind a slight reduction in the rise time. It won't be as good as having the transmission line correctly terminated at both ends but it should be fine for general stuff like feeding a counter or scope via coax. But the source end has to be properly managed so it is close to 50 ohms

author: hp3310a
author_rank: Regular Contributor
timestamp: March 29, 2025, 03:09:14 pm
content: This look a lot better using a short cable. The screenshots below were all made with a 15cm RG174 cable, direct connection to the scope. From 100Hz to 30MHz the spike and the drop off values (3.45V and 3.05V) are exactly the same. 50MHz looks much more like a square wave.A little background for all of this: My frequency counter is designed to work on a much higher input voltage level of 16Vpp. It has only one purpose to be used in one specific instrument which ideally shouldn't become aware of its presence. Whether or not one usually doesn't do that (high impedance input) is besides the point, it is what it is. The input of my counter will not be 50Ω.The test with the LBE-1420 was done to determine how accurate the counter is. In previous tests it worked with input voltage as low as 4Vpp with a sine waveform output from my function generator (which it turned out has terrible accuracy). So I was not sure if it will work at all. Turns out, it does all the way up to 36MHz, which is 7 times the desired maximum frequency at only 1/5 of the designed input voltage.Since my counter is not a reciprocal one (so it counts impulses against a timer clock), it is not terribly accurate at lower frequencies (in terms of ppm). However at 10MHz, it is only 5Hz off, which is fantastic.Quote from: raherh on March 29, 2025, 11:38:58 amThis can't be right, the rising edge should be much faster and if you direct-connect the LBE-1420 to the scope the signal will have a much higher amplitude than when 50Ω-terminating it (3.3V vs 1.65V according to specs). Maybe your scope input isn't set to full bandwidth?

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 03:19:34 pm
content: To compare against the older Bodnar units here's the LBE-1421 connected up to an unterminated scope via a 3 foot long 50R coax cable. I've set it to 20 MHz and to 10MHz below.This isn't perfect by any means but it shows what to expect if the source match is quite well defined so it is close to 50 ohms. Only the source end of the cable is correctly terminated in the example below. The far end goes into a 1 Meg scope.There's no spikes, no ripple and this would be fine for a counter

author: hp3310a
author_rank: Regular Contributor
timestamp: March 29, 2025, 03:41:49 pm
content: Interesting, but what is the input impedance of the scope? How is it connected exactly? I have a hard time imagining what your are describing. Cable from LBE-1421 into a T-BNC connector with one end open and the T connected to the scope?

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 03:54:43 pm
content: Quote from: hp3310a on March 29, 2025, 03:41:49 pmIt's a classic 1 Meg scope input. LBE-1421 port 2 to an SMA to SMA RF cable 3 ft long and then into a decent SMA to BNC adaptor and then into the scope CH1 set to 1 Meg input. The scope does have an internal 50R resistor but this isn't selected. You can see the voltage is still at 3.25 Vpkpk. If I enable the internal 50R resistor the voltage will drop to about 1.6V pkpk.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 29, 2025, 04:26:02 pm
content: So basically exactly the same setup as mine in the original post. I really don't understand why the result is so different.

author: bdunham7
author_rank: Super Contributor
timestamp: March 29, 2025, 04:27:31 pm
content: Quote from: hp3310a on March 29, 2025, 11:54:38 amYou really can't say that the spike is "produced" by one thing or another, it is produced by the entire system which includes the LBE-1420, your cables/connectors and the scope.I would definitely follow up on the suggestion about the possibiity that the output impedance of the LBE-1420 is not 50R. If so, that would seem to be a defect?Also, I'm not clear on why this issue would cause your frequency counter to double count at very low frequencies.

author: David Hess
author_rank: Super Contributor
timestamp: March 29, 2025, 04:29:18 pm
content: Quote from: G0HZU on March 28, 2025, 05:41:28 pmThinking about it now, I have to take that back.A proper series termination would not have shown the overshoot if the series termination matched the cable impedance, but *would* produce that spike if it did not. So either the series termination is bad, or the cable is the wrong impedance.With a proper series termination, the unterminated end of the cable should should the full voltage without any step, and any point along the cable will show the step at 1/2 of the voltage.Quote from: bdunham7 on March 29, 2025, 04:27:31 pmThe frequency counter may be seeing an extra edge if the trigger level is not properly set.

author: bdunham7
author_rank: Super Contributor
timestamp: March 29, 2025, 04:53:54 pm
content: Quote from: G0HZU on March 28, 2025, 03:17:27 pmOK, I've long since figured out that scope inputs are more complex than R with parallel C, but I'm not clear on how what you describe there can possibly work with various 10x probes?QuoteSeems like a good excuse to pull out the NanoVNA! I can tell you that with an LCR @ 100kHz, my Siglent does not look like 1M in parallel with 17pF, it looks a bit different. I'll pay closer attention next time I look at it. However, I don't think the OP has any issues with his scope here as I'm sure you'll agree.

author: David Hess
author_rank: Super Contributor
timestamp: March 29, 2025, 05:14:59 pm
content: Quote from: G0HZU on March 28, 2025, 03:17:27 pmDave's reverse engineering schematic shows a 75 ohm resistor in series with the BNC, and a 33 ohm capacitor shunt to ground on the high impedance attenuator input.Various dampening circuits have to be added to quash unintended resonances. Part of the old Tektronix instruction for oscilloscope calibration included sweeping the input with an RF signal generator to verify the absence of resonances.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 29, 2025, 05:17:07 pm
content: Quote from: bdunham7 on March 29, 2025, 04:27:31 pmOK, I will have to life with that nagging feeling that something is wrong somewhere. I don't like it.Quote from: bdunham7 on March 29, 2025, 04:27:31 pmHow? Stick a Ohm-meter into the output?Quote from: bdunham7 on March 29, 2025, 04:27:31 pmI don't know the exact reason. The whole system works "by accident" with the input voltage being so low. The comparator which drives the counter has its switching hysteresis set to between 0 and 0.3 volts (about). Since the input voltage is so low (much too low) and gets attenuated 1:8 the comparator barely makes its switching point. Normally (with proper input voltage) the duty cycle of the comparator output is almost 50%. With the weak signal it switches only for 24ns, just enough for the counter to register a pulse. I assume that in that special range between 20 and 33Hz there is more jitter that lets the comparator switch back and forth. In the end it is not important but it made me curious enough to look at the input signal and that started this whole thread.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 05:37:59 pm
content: I don't think there's anything significantly wrong even if the source impedance does turn out to be about 35 ohms. It isn't far out enough to cause major issues but I'm surprised it isn't the same as the LBE-1421 I have here?What I can say for certain is that test gear does not always have to have 50 ohms at both ends of a coaxial cable. If you take the case of a classic lab pulse generator it will typically have a 50 ohm source impedance.In days gone by this might be used to clock a logic gate or a comparator with a 0-5V logic level.I've got a couple of old pulse generators here and one is an ancient HP 8002A. It is a bit slow and the rise time is a few nanoseconds but it is designed such that it has a quite well controlled 50R source impedance. This means that the operator can connect a fairly long 50 ohm coax cable to it, eg one metre or several metres long and then connect the far end to a scope to look at the waveform before feeding it to a comparator or logic gate etc.This sort of thing has been happening for many decades across thousands of electronics labs. The pulse at the unterminated end of the cable should be twice the amplitude compared to the case when it is terminated correctly in 50 ohms. This is only valid if the source impedance of the generator is a well controlled 50 ohms.Here's an example below using a 1 metre 50 ohm cable. There is no ringing or significant artefacts in the response in the unterminated case with the HP 8002A? You can see the amplitude drops by a factor of two when correctly terminated. The unterminated case isn't perfect, but it is easily good enough and also this generator is almost as old as me and it has seen a few repairs to the output stages over the years. It may have been better than this when new.If the source impedance of the pulse generator was 35 ohms then the result would not look as nice. There would be a spike at the leading edge just as in your case.

author: bdunham7
author_rank: Super Contributor
timestamp: March 29, 2025, 05:39:40 pm
content: Quote from: hp3310a on March 29, 2025, 05:17:07 pmMeasure your terminator and verify that it is 50R. Set the LBE-1420 for some low frequency, like 100Hz or 1kHz. Connect it to your scope with as short a connection as possible in a way that allows you to put in or take out the terminator easily. A BNC tee might be used in a way that allows you to install or remove the terminator without moving anything else. If you have no way to do this without using a cable, then use your ohmmeter to verify that the DCR of the core and the shield are each less than 1 ohm end to and and the resistance from core to shield is an open circuit. Note the peak-to-peak voltage (ignoring any spikes) both ways and if the voltage with the terminator is not very near half of the open voltage, then the output impedance of the LBE-1420 is not 50R. And that's a problem.

author: bdunham7
author_rank: Super Contributor
timestamp: March 29, 2025, 05:43:12 pm
content: Quote from: G0HZU on March 29, 2025, 05:37:59 pmAs long as one end of the cable is terminated by something with 50R, then things are often OK. But 35R on one end and a capacitor on the other isn't good. The LBE-1420 is specified as a 50R output, if it is 35R I'd think Leo Bodnar would want to know about it. Sometimes it might not matter but other times it will.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 05:48:53 pm
content: Quote from: bdunham7 on March 29, 2025, 05:39:40 pmThe OP has effectively done this already in an earlier post and saw 3.1V and 1.8V in each case. This implies 36R for the source impedance. I've already told him to do it again after checking the accuracy of the 50 ohm terminator but I don't think he's done it yet.

author: bdunham7
author_rank: Super Contributor
timestamp: March 29, 2025, 06:07:45 pm
content: Quote from: G0HZU on March 29, 2025, 05:48:53 pmYes, that's what I was telling him to revisit and concentrate on, but...He's using an unknown cable and there are possibly other experimental errors that should be eliminated. I'm not clear on what would cause the 3.3V output to be low so I'm suspecting a bit more odd is happening here. IDK, it's one of those things that I think we'd both have figured out pretty quick if it were on our benches, but it's difficult to instruct someone else do it when you don't know exactly what they're working with. IDK what the output circuitry of the LBE-1420 looks like, but on some generators there is a matching pad made of a number of resistors and if one of those was wrong, omitted or misplaced you might get some strange result like this.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 06:10:17 pm
content: If it helps I can deliberately screw up the (50 ohm) source match of this LBE-1421 by adding an SMA T piece and a tiny 50R terminator on it. This will reduce the source impedance to just 25 ohms. The simulation below predicts what the output will look like on this scope as I have a basic model here for its 1Meg input. This is for an unterminated scope via a 3 foot run of good quality 50 ohm cable.You can see how much worse it is with a change to 25 ohms and an unterminated scope.See also the result for real with the source forced to be just 25 ohms. It is the essentially the same.This was a very well controlled experiment because I used an 18 GHz rated 3 ft long 50 ohm RF cable with SMA connectors and an 18GHz SMA load connected to a high quality SMA T piece. I used a Suhner SMA to BNC adaptor at the scope input. I already know that the LBE-1421 output is very close to a true 50 ohm output so adding a decent 50 ohm termination in parallel will produce a 25 ohm source impedance.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 06:30:21 pm
content: I just had a play with the low power option on the GUI for this LBE-1421 and this causes issues with the source match. It definitely isn't 50 ohms anymore in low power mode. It also looks like the source match is different for both logic 1 and logic 0 as part of the low power mode. In this case the only way to get a decent square wave is to terminate the output of the cable.

author: fourfathom
author_rank: Super Contributor
timestamp: March 29, 2025, 06:41:22 pm
content: FWIW, I have a Bodnar "mini GPS" and I just did the unloaded/loaded measurement. With very short connections (no cables, just an SMA Tee and a SMA/BNC adaptor to a Scope) I calculate a 47 Ohm output impedance. My termination measures 50.0 Ohms, and the scope trace looks flat (except for the slow rise-time due to the scope response.) I tested at 10 MHz.The Bodnar is set for 32 mA output. When I switch it to the "8 mA" setting the output impedance rises to 82 Ohms.I suspect that the newer Bodnar has a similar output stage but I suppose I could be wrong.

author: G0HZU
author_rank: Super Contributor
timestamp: March 29, 2025, 07:07:10 pm
content: I guess the other thing to bear in mind is that this is a very versatile device in terms of the frequency range it can operate over. It may be that the source impedance of the LBE-1420 is optimised for use up in the GHz region where it may be closer to 50 ohms up at those frequencies. The source impedance will probably creep up at higher frequencies. There may also be firmware related tweaks that could alter the output stage.This LBE-1421 is running FW 1.07 if that is of any relevance?I don't see anything to be concerned about.

author: tautech
author_rank: Super Contributor
timestamp: March 29, 2025, 08:06:09 pm
content: Quote from: raherh on March 29, 2025, 11:38:58 amWelcome to the forum.Measurements stability can be adjusted by engaging Averages in the Math menu.Further, using Advanced Measure with Statistics ON will provide quite stable #'s for the Min and Max values.See screenshot.

author: bdunham7
author_rank: Super Contributor
timestamp: March 29, 2025, 10:15:44 pm
content: Quote from: G0HZU on March 29, 2025, 06:30:21 pmIs there an actual manual for these products or do you just have to pick your way through the config software and discover things like this at random? I think you've found the issue here!

author: fourfathom
author_rank: Super Contributor
timestamp: March 29, 2025, 11:38:06 pm
content: Quote from: bdunham7 on March 29, 2025, 10:15:44 pmhttps://leobodnar.com/files/datasheets/LBE-1420-Datasheet-v1.1.pdfThis doesn't give a lot of information about the output impedance, just the output voltage into 50 Ohms and into high impedance. I suppose this is an interesting puzzle, but for its intended use the Bodnar units are quite adequate. In any case, I would never intend to use these without a proper termination.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 30, 2025, 07:52:34 am
content: So I went ahead an took the requested measurements.First, the SMA-terminator is 49,76Ω according to my Brymen 789.I tested 4 setups:Direct connection using SMA-T-ConnectorDirect connection using BNC-T-ConnectorCable connection using SMA-T-ConnectorDirect connectionAll tested with the LBE-1420 set to 100Hz output.No matter which setup, the readings are all the same every time. With termination: 1.78V, without termination 3.04V.The spike is there, no matter what setup, I never get a clear, spikeless curve like GOHZU does.I'm attaching images of setup and readings intermixed.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 30, 2025, 07:54:46 am
content: More documentation.At the end there's one screenshot for the SMA-terminated setup at 10MHz.

author: gf
author_rank: Super Contributor
timestamp: March 30, 2025, 09:09:45 am
content: Even if you manage to cope with the reflections and to feed an ideal square wave into the scope input, keep in mind that the step response of your scope's frontend will still overshoot. A maximaly flat frequency response and a step response w/o overshoot are mutually exclusive. Many modern scopes prefer a maximally flat frequency response over a Gaussian-like frequency response. Also note that GOHZU has a faster scope.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: March 30, 2025, 01:49:41 pm
content: My background is in mobile radio systems, where we used off air standards, frequency counters and oscilloscopes pretty much every day.I find the discussion over terminating a frequency reference bizarre. Instruments can behave strangely if not properly terminated. Using a BNC “T” piece with a 50 Ohm terminator on one connection is not a good termination. 50 Ohm oscilloscope terminators and decent 50 Ohm coax is what is needed.I have a couple of Leo Bodnar GPSDOs and they work just fine.Pico make suitable low cost terminators for oscilloscopes.https://www.picotech.com/accessories/bnc-cables-and-adaptors/50r-terminator-bnc] https://www.picotech.com/accessories/bnc-cables-and-adaptors/50r-terminator-bncSJ

author: G0HZU
author_rank: Super Contributor
timestamp: March 30, 2025, 02:35:35 pm
content: Quote from: Solder_Junkie on March 30, 2025, 01:49:41 pmTo be fair (to me at least) hp3310a wanted to know 'why' the scope trace looked so bad when fed into the scope unterminated. All he seemed to be getting in reply was 'you have to terminate it' but without any explanation about the misbehaving waveform.That's fine, but I tried to explain what was causing the spike.To me the plots look like a mismatch between the LBE-1420 source impedance and the coax cable. The spike is about 8ns wide and 600mV tall for the 80cm cable for example and this gives a big clue. It's usually possible to know (from experience) how long the cable is and how bad the mismatch is from just looking at the waveform on the scope. I've tried to explain this and show demonstrations. If it helps I could try and set up a harmonic balance simulation of that setup and this would show the impact of changes in the cable length and the source Zo in real time on a brief video capture of the simulator. This might prove to be more intuitive for hp3310A? I can do this stuff in my head from experience because I've been doing RF design all my career but maybe this stuff is less obvious to people who haven't explored this stuff before?Of course, there might be another explanation but the voltage tests with and without the termination do suggest that the source impedance of the LBE-1421 is somewhere around 35-37 ohms and if this is the case, the spike in the waveform is inevitable when fed into (unterminated) 50 ohm coax cable

author: tggzzz
author_rank: Super Contributor
timestamp: March 30, 2025, 02:52:12 pm
content: Quote from: G0HZU on March 30, 2025, 02:35:35 pmI suspect that for people who aren't used to RF, a Spice time domain simulation would be easier to understand - w.r.t. both the schematic and the plot.The OP could learn a lot from creating his own 5 component schematic (voltage source, source resistor, transmission line, load resistor, load capacitor).

author: G0HZU
author_rank: Super Contributor
timestamp: March 30, 2025, 03:14:07 pm
content: Quote from: tggzzz on March 30, 2025, 02:52:12 pmThe HB simulator has the advantage of being interactive in real time. I put together a quick screen capture lasting a few seconds and dumped it on youtube.I have poor presentation skills and I don't have a great speaking voice plus I'm no good at video editing but hopefully the video below will show what happens if the source match isn't 50 ohms and the far end of the cable is fed into a scope ::

author: hp3310a
author_rank: Regular Contributor
timestamp: March 30, 2025, 03:22:20 pm
content: Quote from: G0HZU on March 30, 2025, 02:35:35 pmYes, that is the only question. I would like to determine if the device is maybe defective or my equipment is just not up to the task of measuring it.Quote from: G0HZU on March 30, 2025, 02:35:35 pmI think I do understand why it is happening in theory (not on a mathematical level) but some of the explanations are really confusing.For example when you talk about this happening with my 80cm cable (the initial test in the first posting), because of reflection, issues with cable and connectors, etc: Why does it seemingly make no difference at all whether the cable is 80cm, 15cm or 2,5cm long (the last test with direct connection to the scope). The spike keeps appearing and its shape is identical. If the cable is the issue, shouldn't there be a detectable change somewhere?For me, this indicates that the scope can't do better and the other components are not as relevant in this case. Is this wrong?Yes, I think it is.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 30, 2025, 03:52:32 pm
content: Quote from: hp3310a on March 28, 2025, 12:03:48 pmReading the thread, I wasn't sure if you are trying to get your home made counter to work, or are just playing around with trying to measure a squarewave with your scope.The idea of making any direct measurement with a scope or counter without adding any load seems impossible.

author: hp3310a
author_rank: Regular Contributor
timestamp: March 30, 2025, 04:22:17 pm
content: Quote from: G0HZU on March 30, 2025, 03:14:07 pmThanks a lot for putting this together. I put the simulation to the test and replicated it (for 20cm cable length). I noticed that simulation and real life agree more on the negative spike below 0V. "My" spike in the positive side protracts longer than the simulation.It's also clear that my previous statement about the spike not changing with cable length is complete nonsense. It changes a lot, not quite like the simulation though.

author: G0HZU
author_rank: Super Contributor
timestamp: March 30, 2025, 04:54:41 pm
content: Yes, the size of the spike is a function of the mismatch between the source impedance and the coax impedance. The lower the source goes below 50 ohms, the bigger the spike amplitude. The duration of the spike is a function of the length of the coax cable.Maybe the easiest way to start out is to consider a really short pulse of duration that is short compared to the delay time of the coax. Maybe just 3 ns wide. This can then be used to see how the spike gets created at the leading edge of the wider pulses from the LBE-1420.If the source Z of the pulse generator (Zs) is 35 ohms and the cable Z is 50 ohms (Zcoax) and the source EMF is 3.1V as in your case, then the amplitude of this very narrow pulse when it arrives at the far end of the cable is quite easy to predict.The first thing to calculate is the voltage at the input side of the coax the moment the pulse starts.This should be EMF*(Zcoax/(Zcoax+Zs)) in the same way you would work out the voltage from a potential divider. This should be 3.1*(50/(50+35)) = 1.824V. Easy stuff.This initial pulse of amplitude 1.824V will travel down the coax cable until it reaches the far end as shown in the hasty diagram below. If we take the simplest case where we treat the scope loading as insignificant, then once the pulse reaches the far end, it will have nowhere to go. As we know that energy can't be created or destroyed, the energy in the pulse reflects back towards the source and it will do it in phase with the forward pulse.This means that the voltage seen at the far end will double to 1.824 *2 = 3.648V and the amplitude of this narrow pulse at the far end of the cable is the same amplitude as your spike voltage. The lower the source impedance away from 50 ohms, the bigger the spike will be. You can see this in my youtube video. If the source impedance was reduced to 25R then the spike would be at 4.133V.If we then go back to using the Bodnar unit then the spike duration will last for twice the delay time of the cable. With an 80cm cable that has a 4ns delay it will mean the spike lasts for twice this at 8ns.So for your 80cm cable with its 4ns delay, the spike will last for about 8ns before it is affected by other reflections taking place between the 50R cable and the 35R source. It will then be reduced in small chunks until it all settles at 3.1V until the pulse finally ceases as shown in the diagram below.In reality, your scope has input capacitance and some resistive loading and this will tend to round off the spike so it looks more pointy rather than looking like a flat top pulse 8ns wide.

author: raherh
author_rank: Newbie
timestamp: March 31, 2025, 05:11:26 pm
content: QuoteCorrection to my post and apology:The rise time was including the spike time, so 10%-90% time is around 2ns, and spike voltage values are proportionally same (twice less with the 50termination). My intention was to put an example with the LBE-1420. At the time of my writing there was only post with LBE-1421.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2025, 05:19:56 pm
content: Quote from: raherh on March 31, 2025, 05:11:26 pmWas that an automatic measurement? Transition time does not include preshoot or overshoot.If the transition time (or bandwidth) changes with signal amplitude, then the full power bandwidth of the oscilloscope front end is not sufficient for the claimed bandwidth. Rigol has this problem but maybe some others do also.

author: raherh
author_rank: Newbie
timestamp: April 01, 2025, 07:55:26 am
content: Originally I erroneously claimed rise time 8ns, the right value is 2ns, without overshoot. The value is from Measurement menu and checked with cursors.

author: hp3310a
author_rank: Regular Contributor
timestamp: April 08, 2025, 04:31:10 pm
content: I had a little back and forth with the man himself at Leo Bodnar Electronics and they gave me a beta firmware update tool for macOS (can't use the PC version) and new firmware.Bla bla bla, long story short, with version 1.07 that it runs now, the LBE-1420 outputs 3.3V into the 1MΩ scope input at 10MHz, the overshoot/spike also went down about 200mV and that with the same cable I used in the first post.Excellent!

author: tggzzz
author_rank: Super Contributor
timestamp: April 08, 2025, 04:38:54 pm
content: Quote from: hp3310a on April 08, 2025, 04:31:10 pmWhat does the 100ns/div trace look like if you double or halve the length of the cable?

author: 2N3055
author_rank: Super Contributor
timestamp: April 08, 2025, 06:50:29 pm
content: Quote from: hp3310a on April 08, 2025, 04:31:10 pmAs a side note, there is an automated measurement ROV / FOV that will automatically measure overshoot on rising /falling edge.

author: hp3310a
author_rank: Regular Contributor
timestamp: April 09, 2025, 02:29:47 pm
content: Quote from: tggzzz on April 08, 2025, 04:38:54 pmSee the comparison attached. The long cable is 80cm, the short one is 20cm (I have neither 40cm nor 160cm length available at the moment).

author: tggzzz
author_rank: Super Contributor
timestamp: April 09, 2025, 04:32:15 pm
content: Without being able to see the details in the transition and "ringing", it looks like you are seeing the consequences of an unterminated cable and limited scope bandwidth.That's independent of the quality of the source, and is under your control.

author: G0HZU
author_rank: Super Contributor
timestamp: April 12, 2025, 09:54:35 pm
content: To me the original issue looks like a source problem. The fact that changing the firmware improves it kind of proves this.If the source impedance is a genuine 50 ohms then you can usually leave the 50 ohm cable unterminated at the other end if you are using a regular 70MHz -200 MHz scope with a 1Meg input. You should see a reduction in rise-time because you are effectively driving the scope input (capacitance) from a 50 ohm source and not the 25 ohms you would get with a terminated cable driving the scope. But there shouldn't be that 8ns wide fat spike at the leading edge of the waveform when using an 80cm cable. However, that can happen if the source impedance is lower than 50 ohms and hopefully I've proved this.It looks like the 1420 is improved with the firmware upgrade.I don't have the LBE-1421 anymore so I can't do any more tests with it.

author: hp3310a
author_rank: Regular Contributor
timestamp: April 13, 2025, 07:10:57 pm
content: It definitely improved the high impedance output. I don't know how this device works, I was surprised the output voltage is something that can be changed via firmware update. The output impedance is probably something rather unchangable. But it works great for my little experiments with my own counter and as reference input for the real thing, a 53181a frequency counter.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 09:37:27 pm
content: Quote from: hp3310a on April 13, 2025, 07:10:57 pmMy recollection is that the output is generated by a fast CML (current mode logic) comparator from Analog Devices with internal termination, which would explain why the pulse is not particularly clean.