50 Ohm AC input for Oscilloscopes - Page 1
Forum section: electronics / testgear

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: July 31, 2017, 01:51:30 pm
content: A good friend of mine who makes products development has recently sent me a data sheet (see the attachment) regarding an idea to measure RF or fast transient pulse signals on an 1M? oscilloscope input. He has made some samples already.Here is some details from the attached data sheet:Features- 50? Input Impedance for RF/Pulse Measurement up to 2GHz • DC-Blocking (50V)- 8W continuous input Power (20W @ 1 sec.)- 1:1 Voltage Divider on 1M? Oscilloscope Input- 1:10 Voltage Divider on 50? Oscilloscope Input- 20 dB High Power Attenuator for Spectrum Analyser- Small and handy Design (65 x 25 x 25 mm)Application- Measuring DC coupled RF/Pulsed Signals- Oscilloscope RF Measurements- High Power Attenuator for Spectrum AnalyserDo you think this product can be interesting? How would you use it?If you need more info, please ask.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 31, 2017, 03:42:28 pm
content: Quote from: simone.pignatti on July 31, 2017, 01:51:30 pmSo which oscilloscope with sufficient bandwidth to appropriately capture fast pulses doesn't have an 50 ohms input, either fixed or switchable?And to see some RF on a low-end scope with 1M inputs, why wouldn't a feedthrough terminator not be sufficient?QuoteSpectrum Analyzers usually have 50 ohms (or 75 ohms) inputs.QuoteInteresting, yes. Useful, err, no.To me this looks like a solution in search of a problem.

author: ebastler
author_rank: Super Contributor
timestamp: July 31, 2017, 03:53:43 pm
content: I agree with Wuerstchenhund here. Assuming that the scope or spectrum analyzer already has a 50 Ohm input, the proposed product is just a (medium power) attenuator, right? And these are readily available.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: July 31, 2017, 05:08:39 pm
content: here we have some replies:- Which oscilloscope with sufficient bandwidth to appropriately capture fast pulses doesn't have an 50 ohms input, either fixed or switchable?e.g. Rigol 1000, 2000 series- Which oscilloscope with sufficient bandwidth to appropriately capture fast pulses doesn't have an 50 ohms input, either fixed or switchable?Yes, they have no DC-Block. So you can't connect it on a DC/DC converter directly to measure transient pulses- Spectrum Analyzers usually have 50 ohms (or 75 ohms) inputs.Yes, on a 50 Ohm input you get a 20 dB attenuator which copes with 8W continues or 20W peak power. No spectrum analyser can do this without external attenuator.

author: Rbastler
author_rank: Frequent Contributor
timestamp: July 31, 2017, 05:18:48 pm
content: Quote from: simone.pignatti on July 31, 2017, 05:08:39 pmYou just can buy decent feedthrough terminators, DC block adapters und attenuators. Therefor I think there is no market for your friends product. Safe him from a failure.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 31, 2017, 05:41:21 pm
content: Quote from: simone.pignatti on July 31, 2017, 05:08:39 pmEven when we just ignore for a moment that we clearly have different opinions what consitutes a "fast pulse" (something which none of the Rigols would see), the fact remains that for such low BW simple scopes cheap feedthrough terminators are already readily available.QuoteFair enough. So why not use one of the many inexpensive DC blocks that are available for pretty much all bandwidths?QuoteAgain, why would one not use any of the many available attenuators and DC blocks?I still think this is a solution in search of a problem. Also, your friend's device will be more expensive than a high quality terminator, an attenuator and two DC blocks (one for the scope, one for the SA), while the latter will offer a better BW than just 2GHz.I'm sorry but I have to agree with Rbastler here, save your friend from wasting his money. This is one of the ideas that only sound great if you don't know much about test instruments and didn't bother to do some basic research.

author: exe
author_rank: Unknown
timestamp: July 31, 2017, 07:48:15 pm
content: Quote from: simone.pignatti on July 31, 2017, 05:08:39 pmWhat's the difference between AC coupling and DC block?

author: alm
author_rank: Super Contributor
timestamp: July 31, 2017, 08:18:56 pm
content: The LF cut-off point? Although AC coupling in 50 Ohm can also easily have a cut-off frequency of a few kHz (much higher than in 1 MOhm mode).

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2017, 10:34:48 pm
content: Quote from: alm on July 31, 2017, 08:18:56 pmExactly and this is why it is rare for an oscilloscope to support AC coupling in 50 ohm mode and if it does, then the AC coupling occurs *after* the 50 ohm termination. In addition, some sources which an oscilloscope would typically measure require a 50 ohm DC termination.Quote from: simone.pignatti on July 31, 2017, 05:08:39 pmStandard x1 or x10 probes work for this and with them, the built in AC input coupling works fine down to low frequencies. The low impedance of the source makes the loading of a x1 probe irrelevant and the probe will deliver it maximum bandwidth or better.

author: nctnico
author_rank: Super Contributor
timestamp: August 01, 2017, 06:14:23 am
content: Quote from: alm on July 31, 2017, 08:18:56 pmWith a 50 Ohm feedthrough (or 50 OHM terminator on a tee) you can use AC coupling in 1M Ohm mode and you get best of both.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: August 01, 2017, 09:34:56 am
content: here we go:- On "You just can buy decent feedthrough terminators, DC block adapters und attenuators":Yes, correct. Technically it would work but practical you come up with a length of 10cm or longer if you connect DC-Block and Feedthrough-Termination or Attenuator together. I don't want to load this weight on my oscilloscope BNC input connector. Also this setup is only safe up to 1W RF and not 8W.- On "a solution in search of a problem."One problem we had was to measure output ripple above 10 MHz from a DC/DC converter after the output filter avoiding direct coupling from converter into probe line. To avoid such coupling you need an impedance controlled wireing (e.g. 50 Ohm Coax) and termination close to your oscilloscope. Yes, we ended up in stacking DC-Block, Feedthrough-Termination in front of our oscilloscope and nearly break the input BNC connector.The other problem was to check the output power of an Amplifier. We damaged a feedthrough termination by forgetting the DC-Block. We damaged a second feedthrough termination by putting too much power on it, not to mention RF-Transistors we had to replace as well.- On "With a 50 Ohm feedthrough (or 50 OHM terminator on a tee) you can use AC coupling in 1M Ohm mode and you get best of both."Don't do this! The DC-Bias may damage your feedthrough as it is not DC-Blocked. 1W is equal to 7V DC. Connecting you probe on a 12V line will blow you feedthrough (3W). You need a DC-Block on top.Standard passive probes have a bandwidth of 35 MHz (350 MHz on a well trimmed 1/10 divider). This is too low for fast transients.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 01, 2017, 12:06:05 pm
content: Quote from: simone.pignatti on August 01, 2017, 09:34:56 amMost BNC 50 ohms pass throughs we have are 4cm, the BNC DC blocks we have are 5cm which makes 9cm, weighing around 35 grams each. If your scope's BNC can't hold a weight of 70 grams then it's a piece of crap and then you have a lot more problems than the lenght and weight of a BNC terminator and DC blockAlso, BNC isn't the only format, you could use SMA componts which are even lighter and shorter.And most SAs use N connectors which are even more massive and mechanically robust than BNC.BTW, what's the weight of your friend's device?QuoteThen I'm sorry but either your scope is crap or you're doing something very wrong. Also, you could have easily used SMA components if the mechanical load worried you.I also wonder why you'd want to use a low impedance connection to measure ripple from a DC/DC converter.QuoteWhy the hell are you measuring amplifier output power through a scope connected with a low impedance line?QuoteDecent standard passive x10 probes are fine for up to 500Mhz, which is pretty much the limit for the class of scopes (Rigol DS1000/DS2000) you mentioned.If you have a better scope with more BW then this will very likely also have an active probe interface for higher BW probes.

author: alm
author_rank: Super Contributor
timestamp: August 01, 2017, 01:35:26 pm
content: Quote from: Wuerstchenhund on August 01, 2017, 12:06:05 pmTo be fair they are not the only ones, see attached screenshot fromone of Dave's videos(at 31:25, not sure how to make a link to a specific time without embedding). That was after using a Hi-Z 1x scope probe with a 50 Ohm terminator.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: August 01, 2017, 02:10:26 pm
content: - On BNC isn't the only format: Sure I prefer also SMA or N-Type but even R&S RTO has BNC input connector.- On "your scope is crap or you're doing something very wrong": Please have a look onhttp://www.analog.com/media/en/technical-documentation/application-notes/AN-1144.pdf.On figure 6 you see the standard measurement and the problem related. Figure 12/13 shows the solution we target.- On "the hell are you measuring amplifier output power": An oscilloscope can show you the reasons for non linearity by showing you the cause of clipping. A spectrum analyzer shows you only the (scalar) result. Btw. this amplifier worked on 40.65 MHz. Easy to analyze with a 200 MHz oscilloscope.- On "Decent standard passive x10 probes are fine for up to 500MHz"Have a look onhttps://beyondmeasure.rigoltech.com/acton/attachment/1579/f-06be/1/-/-/-/-/PVP2150%20User%27s%20Guide.pdfat page 2. 1/1 = 35 MHz, 1/10 = 150 MHzhttps://beyondmeasure.rigoltech.com/acton/attachment/1579/f-0750/1/-/-/-/-/PVP2350.pdfat page 2 1/1 = 35 MHz, 1/10 = 350 MHz for 59€http://beyondmeasure.rigoltech.com/acton/attachment/1579/f-0671/1/-/-/-/-/RP5600A%20User%27s%20Guide.pdfreaches 600 MHz on fixed 1/10 for 356€

author: alm
author_rank: Super Contributor
timestamp: August 01, 2017, 02:37:33 pm
content: Quote from: simone.pignatti on August 01, 2017, 02:10:26 pmYou could also use a female-female BNC coupler and a short piece of BNC-BNC coax to avoid the DC block and attenuator hanging off the front input. Obviously you would still locate the terminator (if external) directly at the scope input. Should not affect the performance at this relatively low bandwidth (for BNC).Quote from: simone.pignatti on August 01, 2017, 02:10:26 pmThe link does not work unless you remove the trailing period (likethis).

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 01, 2017, 06:58:13 pm
content: Quote from: alm on August 01, 2017, 01:35:26 pmI can see why he did this, but even Dave said that to do these measurements properly one should use a proper active differential probe.Quote from: simone.pignatti on August 01, 2017, 02:10:26 pmYes, but adapters do exist for a reason.Also, as someone else mentioned, you could have simply moved the DC block to a different position if you're really concerned about the weight.QuoteI didn't mean the measurement was wrong (although it begs the question why you don't use a differential probe instead), I meant that you must be handling your equipment wrong if you break a scope's BNC port just by attaching a 50ohm feedthrough and a DC block.This isn't a problem so I can't see the point why I'd want to use your friend's device instead.QuoteSimilar as before, my comment didn't refer to the use of an oscilloscope but your probing solution (together with a scope with 8bit ADC). Besides using a proper probe I'd want a bit more vertical resolution. But anyhow...QuoteYou can often get 500Mhz passive probes made by HP/Agilent/Keysight, LeCroy or Tek for less than the Rigol probes, and if you can live with used ones actually for a lot less.Anyways, back to the topic: now what's the weight of your friend's device?

author: nctnico
author_rank: Super Contributor
timestamp: August 01, 2017, 07:27:47 pm
content: Quote from: simone.pignatti on August 01, 2017, 02:10:26 pmThey are not. You have to realise that passive divider probes are tested/specified for a 25 Ohm source impedance and don't care about how bad the signal is affected. In a realistic scenario there isn't a way you can probe a 500MHz signal with a high impedance passive probe. The signal will be disturbed so what you get on screen is far from the reality.

author: alm
author_rank: Super Contributor
timestamp: August 01, 2017, 07:39:23 pm
content: Quote from: nctnico on August 01, 2017, 07:27:47 pmThey do care. The signal may affected by at most -3 dB within the pass band. And are you really complaining about the input impedance for a power supply rail or a power amplifier? And the proposed alternative is a 50 Ohm input?

author: ebastler
author_rank: Super Contributor
timestamp: August 01, 2017, 07:48:28 pm
content: @simone: While we have not reached a conclusion yet whether the proposed product is useful or not, I think it is fair to say at this point that it will be a tough sell... ;-)

author: nctnico
author_rank: Super Contributor
timestamp: August 01, 2017, 08:12:25 pm
content: Quote from: alm on August 01, 2017, 07:39:23 pmIt depends greatly on what you are trying to measure. For many circuits a short piece of coax directly to the scope will do using AC coupling in Hi-z mode but I recall a thread where someone used a >1GHz diff. probe to look at some resonances in a power supply.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 01, 2017, 10:04:18 pm
content: Quote from: nctnico on August 01, 2017, 07:27:47 pmI once tested some of my x10 500/600Mhz passive probes that came with my scopes on a high Z output of a signal generator, and at least as the signal shape is concerned there weren't many issues once the probes were properly matched.And if you believe Keysight, they state thatpassive x10 probes can go up to 700MHz.The main issue with x10 passive probes and high frequencies is one of matching, as the relatively high capacitive loading makes them unsuitable for higher frequencies.Also, the OP is talking about measuring ripple on a DC converter or clipping on an amplifier on a 200MHz Rigol scope. At least in regards to BW a 500MHz passive probe won't be the limiting factor here.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: August 02, 2017, 07:20:42 am
content: Quote from: ebastler on August 01, 2017, 07:48:28 pmYes, it is a possibility. However it seems to be a good topic from which I'm learning several things thanks to everybody posting its own comment, the power of the forum!from the data sheet I can read 100gsize 65x25x25mm

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: August 04, 2017, 09:35:00 am
content: Quote from: alm on August 01, 2017, 02:37:33 pmThank's to the hint. You are right.

author: nctnico
author_rank: Super Contributor
timestamp: August 04, 2017, 08:03:55 pm
content: Another option could be this low cost AC coupled 1.5GHz FET probe:http://www.ebay.com/itm/RF-Active-Probe-0-1-1500-MHz-1-5-GHz-HF-VHF-UHF-SHF-analyzer-oscilloscope/291998278881The problem with measuring ripple in power supplies using regular probes is (like any HF measurement) the resonance caused by the tip capacitance and the inductance in the ground lead. Reduce either and the ringing will be pushed up to higher frequencies.Either way there are several options available already.Edit: Ebay link

author: alm
author_rank: Super Contributor
timestamp: August 04, 2017, 08:31:04 pm
content: The best way would be a differential FET probe. I believe the the ATX standard pretty much specified a Tek P6046 for noise and ripple measurement. Something like a 100 MHz differential probe that goes down to 1 mV/div with a CMRR of 10k:1. Not sure what the modern replacement would be, do any of the low voltage 1x differential probe have AC coupling? I guess you could use a clean voltage source to provide an external offset.

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2017, 10:08:41 pm
content: Quote from: alm on August 04, 2017, 08:31:04 pmThe ATX standard for power supply testing specified a P6046 and a couple days ago I posted long post explanation as to why and what could be done with more modern equipment but the forum silently deleted it. I am not sure if that is a forum bug or if it was do to moderation.