oscilloscope preamp and splitter - Page 1
Forum section: electronics / testgear

author: justanothername
author_rank: Regular Contributor
timestamp: September 08, 2020, 11:41:43 am
content: Dear experts!I recently wanted to measure an output stage like the attached picture. It is supposed to go into a microcontroller pin.It was no problem for my oscilloscope with a 10x probe but my frequency counter hat its problems with this because of the 1M input and the long coaxial cable.So I asked myself if there is something like an oscilloscope preamp with multiple outputs for this? I guess I can build one myself, but I'd rather buy one for convenience. Since I could not find a device that does exactly this I'm now asking you all.I often had the need to connect a single probe signal to multiple measurment devices at the same time and splitting with a coax splitter wont work if it's not a low impedance source.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2020, 08:36:57 pm
content: 10x probes for oscilloscopes can be used with frequency counter 1 megohm inputs.

author: srb1954
author_rank: Super Contributor
timestamp: September 08, 2020, 10:22:05 pm
content: Quote from: David Hess on September 08, 2020, 08:36:57 pmYes, but then you have the problem of adjusting the x10 probe frequency compensation to match the frequency counter input capacitance. The speed of this waveform is such that you would definitely need to compensate the probe to get accurate results. Unfortunately, most counters don't have any provision for observing waveform after the input stage, which you need to have to be able to correctly adjust the probe compensation.A better option would be to use a FET input scope probe with the counter as this would not require compensation to match the counter input capacitance. The lower input capacitance (typically 1-3pF) of a FET probe would also be an advantage: the 10-15pF input capacitance of a typical x10 probe would significantly affect the measurements of rise time and pulse width with this circuit.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2020, 11:31:24 pm
content: Quote from: srb1954 on September 08, 2020, 10:22:05 pmI know of some counters that do provide a way to adjust the probe compensation but in practice, it usually does not matter. (1) If you do want to compensate the probe, then one way is to watch the point immediately after the high impedance buffer with your oscilloscope so that you can adjust the probe compensation for a square waveform. Tektronix did this for calibration of some of their frequency counters which used input buffers based on oscilloscope vertical inputs.(1) Oscilloscopes with largely separate vertical and trigger signal paths usually suffer from poor compensation (and offset, and gain, and frequency repsonse) on the trigger signal path but it is never noticeable. Examples of this include the Tektronix 2230 and many 7000 series mainframes. Often the trigger signal path completely lacks high frequency equalization.QuoteI agree but active probes are relatively expensive and uncommon. Many counter applications are slow enough that they can make use of a low performance home made active probe like that shown below.

author: srb1954
author_rank: Super Contributor
timestamp: September 09, 2020, 08:00:02 am
content: Quote from: David Hess on September 08, 2020, 11:31:24 pmCompensation of the probe doesn't matter much if you are just measuring frequencies as you can always adjust the counter sensitivity to suit. Compensation does matter if are trying to measure rise and fall times or short pulse widths. If your probe is not properly compensated timing measurements on signals shorter than 100us will become increasingly inaccurate as time scales reduce.The only counter that I possess that has a probe compensation monitor output is my Philips PM6681, which is capable of precision measurement of pulse widths and rise and fall times down to 3ns. To get full performance, when using a x10 probe, compensation of that probe is essential. However, it is PITA to use as the monitor output as it is on the rear panel and I have to remove the counter from its rack every time I want to use this feature.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 09, 2020, 09:05:46 am
content: With a RC time constant of around your 10us, it's slow enough for any regular opamp.Making an active FET probe is also not too difficult if you want reasonable performance.Part of the expensiveness of active probes is that it's a small market, and part because they want "super high performance" instead of some reasonable capabilities.Higher price makes more people to skip it, which lowers numbers of units sold in an ever going circle, until the only customers left are the ones who do not care if the thing costs EUR1000, as long as it gets the job done in their mega buck lab setup.

author: SilverSolder
author_rank: Super Contributor
timestamp: September 09, 2020, 01:44:55 pm
content: Quote from: srb1954 on September 09, 2020, 08:00:02 amI sometimes leave a BNC pig-tail going from the back to the front laying next to the face plate, so it is easier to get access to back panel signals that are occasionally needed. Obviously not worth doing for all the ports on all equipment, but a few strategic ones... big hassle saver! Remember to label them with your Dymo or similar labeler...I never regretted building my equipment shelf with a narrow walkway behind it. It is surprising how often you need to "go behind"! But even that is more hassle than leaving a pig-tail in place.

author: justanothername
author_rank: Regular Contributor
timestamp: September 09, 2020, 08:32:25 pm
content: Those relatively cheap active fet probes are not going down to DC or at least I did not find one. Any recommandations are highly apreciated.Beeing able to measure DC or sub-Hz oscillations is necessary for me. In my image in the first post this pulse is triggered every second or so.

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2020, 12:45:34 am
content: Quote from: justanothername on September 09, 2020, 08:32:25 pmI do not know what you were looking at but the active probes we are discussing work fine down to DC.

author: med6753
author_rank: Super Contributor
timestamp: September 10, 2020, 05:03:36 am
content: Another alternative: Many scopes, like this 7904, have a "Signal Out" or a "Channel 1 Output" which can be utilized by other equipment such as a frequency counter. The impedance of that output is 50 ohms so if you're measuring lower frequencies the input to the counter should incorporate a 50 ohm feed thru terminator or else you may get erroneous readings.

author: srb1954
author_rank: Super Contributor
timestamp: September 10, 2020, 12:49:16 pm
content: Quote from: justanothername on September 09, 2020, 08:32:25 pmCheap FET probes probably have poor DC offset and DC drift characteristics so they just add AC coupling capacitors to hide those problems.If you look at circuits for high grade DC coupled FET probes quite a bit of the extra circuitry included is for correcting drift and offset problems. This all adds considerably to the cost and difficulty of manufacture for a high-end FET probe.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 11, 2020, 01:44:36 am
content: oQuote from: med6753 on September 10, 2020, 05:03:36 amI was going to make the same suggestion, as I do this all the time with my 7613, then I thought, "Probably the OP has a DSO".The trouble with that explanation is that DSOs usually have a frequency readout, anyway.

author: justanothername
author_rank: Regular Contributor
timestamp: September 11, 2020, 07:25:22 pm
content: In my case the frequency measurement of my dso is not good enough for these 10us pulses coming at a rate of 0,5Hz. I often find that an external counter is necessary for some acurate measurements.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 11, 2020, 11:45:15 pm
content: Quote from: justanothername on September 11, 2020, 07:25:22 pmIf your initial picture is drawn to scale, the pulse isn't all that hard to handle.Analog sync pulses are around 5 us wide, & (again if the drawing is to scale), have a much faster rise time.All of this is routinely handled by video amplifiers which either go down to DC, or employ DC restoration techniques.Pulse repetition frequency shouldn't really be a big factor.It should be possible to build a fairly simple preamp/splitter to do the job.

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2020, 04:39:05 pm
content: Quote from: srb1954 on September 10, 2020, 12:49:16 pmI am not sure I have ever seen either a cheap or an inexpensive FET probe.QuoteMost of the extra circuitry is for remote adjustment of the DC offset. It is quite practical to make an accurate and low drift DC FET probe if matched FETs are available with one being used to temperature compensate the other, like in the example I showed. Below is the full schematic for a relatively simple 500 MHz active probe which could not be manufactured today because of lack of suitable parts (Q22).

author: 2N3055
author_rank: Super Contributor
timestamp: September 12, 2020, 06:28:10 pm
content: Quote from: David Hess on September 12, 2020, 04:39:05 pmWhat is special about Q2 if you don't mind me asking. Thanks!

author: vk6zgo
author_rank: Super Contributor
timestamp: September 13, 2020, 12:30:21 am
content: Quote from: 2N3055 on September 12, 2020, 06:28:10 pmJust looks like a bog standard PNP silicon .I thought, maybe germanium, but the voltages don't support that hypothesis.In any case, it still wouldn't help the OP, as it only has one output, so for their case, it would need something like a surplus video distribution amplifier (VDA) to follow it.(they usually have multiple outputs)Not all VDAs are DC coupled, though, some use DC restoration.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2020, 02:02:15 am
content: Quote from: 2N3055 on September 12, 2020, 06:28:10 pmIt is a 2 GHz RF PNP transistor and nobody makes RF PNP transistors anymore, at least that I know of. NXP discontinued theirs not too long ago.

author: srb1954
author_rank: Super Contributor
timestamp: September 13, 2020, 04:34:20 am
content: Quote from: 2N3055 on September 12, 2020, 06:28:10 pmI think the more critical part is the dual FET, Q16a & Q16b. This is a custom Tektronix part with the two FET die closely thermally coupled to minimise thermal induced drift. It was probably also carefully selected for minimum offset voltage. Such exotic parts would be pretty much unobtanium these days.Even with these exotic parts this probe doesn't achieve particularly outstanding offset drift figures. The heat from your hand while holding this probe would produce significant observable offset drift.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2020, 11:09:06 am
content: Quote from: srb1954 on September 13, 2020, 04:34:20 amThere are completely adequate substitutes for the dual JFET using matched individual JFETs which are thermally coupled. Offset voltage and offset voltage drift will not be limitations and matched individual JFETs even have advantages over dual parts due to less electrical coupling between the transistors. But there are no substitutes for the PNP RF transistor, or at least none which are generally available.

author: 2N3055
author_rank: Super Contributor
timestamp: September 13, 2020, 12:44:40 pm
content: Quote from: David Hess on September 13, 2020, 11:09:06 amNTE2403 ?

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2020, 12:41:55 am
content: Quote from: 2N3055 on September 13, 2020, 12:44:40 pmThat may work but has half the collector current. I try to avoid anything from NTE except for repairing old equipment, and wow, check out that price, but they are apparently available.

author: justanothername
author_rank: Regular Contributor
timestamp: September 14, 2020, 12:23:50 pm
content: Quote from: vk6zgo on September 13, 2020, 12:30:21 amIt would help, since the output is way lower impedance than it was with the probe. I could use a t-splitter and a termination through at the end.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 14, 2020, 02:49:23 pm
content: Quote from: justanothername on September 14, 2020, 12:23:50 pmIn that case, I agree with you,------------that would be pretty much the same configuration as the output of a VDA.

author: srb1954
author_rank: Super Contributor
timestamp: September 15, 2020, 08:20:02 am
content: Quote from: 2N3055 on September 13, 2020, 12:44:40 pmNo need to go searching for exotic PNP transistors unless you are trying to restore a vintage Tek probe.If you were building a probe like this nowadays you would substitute the two emitter followers with a current feedback amplifier and get better performance in just about every department. You would still need the fancy matched dual input FETs though as CFAs have relatively high input bias currents and too low input impedance.

author: 2N3055
author_rank: Super Contributor
timestamp: September 15, 2020, 09:09:44 am
content: Quote from: srb1954 on September 15, 2020, 08:20:02 amOf course you would do that. I simply pointed out, that, if need be, there still ARE RF transistors out there...You could even get one of high speed FET input opamps (those usually used for transimpedance amps), and just use that, no discrete JFETS.. like for instance OPA858...or OPA818

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2020, 01:33:07 pm
content: Quote from: srb1954 on September 15, 2020, 08:20:02 amThere is a reason feedback amplifiers are not used in circuits like this; they have worse overload recovery and worse transient response. It can be done, but it requires even higher Ft transistors and better construction for the same performance. At lower frequencies, feedback amplifiers can be acceptable if input bandwidth is limited.The faster 900 MHz P6201 from the same era does exactly what you suggest with two emitter followers of the same polarity, but they are PNP emitter followers! A pair of NPN emitter followers would add to the Vgs of an n-channel JFET making for a difficult operating point so either two PNP emitter followers are used or a p-channel JFET, and RF p-channel JFETs are even less common than RF PNP transistors.Quote from: 2N3055 on September 15, 2020, 09:09:44 amDifferential pairs with external feedback are especially problematical for the reasons given above.

author: justanothername
author_rank: Regular Contributor
timestamp: September 24, 2020, 02:39:42 pm
content: Just to conclude this thread: I got an old Tektronix P6202 with PSU from Ebay for this and it works as expected.Nice probe but a little smelly idk why.