Lo-Z Probe - Page 1
Forum section: electronics / projects

author: prenato
author_rank: Regular Contributor
timestamp: January 08, 2013, 10:57:35 pm
content: Sometimes also know as the "Howard Johnson" probe, though I'm not sure he was the first to come-up with the idea:http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=93:praise-for-the-lo-z-probe&catid=4:projects&Itemid=4Works surprisingly well...Paulo

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2013, 12:47:48 am
content: Nicely done!

author: prenato
author_rank: Regular Contributor
timestamp: January 09, 2013, 12:57:58 am
content: Thanks Rob!

author: notsob
author_rank: Frequent Contributor
timestamp: January 09, 2013, 01:25:37 am
content: And heres Doug Smiths versionhttp://www.emcesd.com/1ghzprob.htm

author: BravoV
author_rank: Super Contributor
timestamp: January 09, 2013, 01:37:28 am
content: Thanks for sharing !This popular cheap probe, its proven really good, another version by our fellow Jahonen (Janne) with its characteristic plotted using HP 8753D VNA from 30 kHz to 6 GHz and it's rising edge beats the 7 GHz Agilent Infiniimax probe->http://koti.mbnet.fi/jahonen/Electronics/DIY%201k%20probe/, this link was discussed here ->https://www.eevblog.com/forum/projects/cheap-wideband-121-oscilloscope-probe-for-logic-signal-applications/InfiniiMax Probe .. from the appearance it self, I'm just too scared to look for it's price ..

author: vk6zgo
author_rank: Super Contributor
timestamp: January 09, 2013, 02:56:37 am
content: Quote from: prenato on January 08, 2013, 10:57:35 pmI'm sorry,but after the preamble about measuring a 50 Ohm source,the obvious assumption is that his PCB also has an output Z of 50 Ohms.If so,why would he need a probe at all?A plain coax lead with an SMA one end & a BNC the other,plus a 50 Ohm termination,would be the normal way to do this.OK,there are spots where you do need to probe,but his example was a poor one.

author: saturation
author_rank: Super Contributor
timestamp: January 09, 2013, 04:42:38 pm
content: Nice. Here's details on HoJos probe by the man himself:http://www.sigcon.com/Pubs/straight/probes.htm

author: Unknown
author_rank: Unknown
timestamp: January 10, 2013, 11:06:35 pm
content: Quote from: vk6zgo on January 09, 2013, 02:56:37 amHis example of the 50 ohms source was to show the effect of capacitive loading of high-Z passive probes. This effect will be even worse, and the resistive probe will be more superior, at higher source impedances.Quote from: Mechatrommer on January 09, 2013, 02:35:51 amLo-Z probes have some advantages, but are much less versatile. Try subjecting one to mains voltages or using it in an audio circuit (usually fairly high impedances). A high-Z passive probe is much more complex to make, since you can't just use a simple transmission line. There is also the issue of mechanical integrity. How long is that resistor at the end of a coax cable going to last if it is used every day?

author: codeboy2k
author_rank: Super Contributor
timestamp: January 10, 2013, 11:20:27 pm
content: Quote from: alm on January 10, 2013, 11:06:35 pm... I'll be waiting for robrenz to apply his skills to making a low-Z probe like these ones, that is also mechanically sound!

author: PA4TIM
author_rank: Super Contributor
timestamp: January 11, 2013, 01:22:44 am
content: Quote from: alm on January 10, 2013, 11:06:35 pmA lo-Z probe is not a replacement for a passive general purpose probe. It. A way to lower capacitance without the disadvantage of fet probes ( limmited voltage ratings and psu need)It is used in high speed probing. Not for gerenal use. I played a bit with them too. They really perform very good. I used it to probe a 500 ps pulser, a 250MHz squarwave and it gave very good results. And as a compare: I have a 500 MHz HP FET probe, 1GHz philips and a Tektronix active probes, three commercial 1GHz hi-Z probes ( strange thing is they are called hi-Z but are made the same way, only with higher resistance so higher Z (100x probe) so where is the border you go from high to low-Z, is it not better to talk about high Resistive but low reactance probes ?In real high speed use they solder a probe adapter straight to the pcb and that holds the low-Xc probeThat is an other advantage of the DIY probe. Just solder the resistor to the pcb.By the way, I found that a small capacitor from coax centre to shield improved the performnce. Also the type of resistor. Very funny, tried a bunch off odd choises like a carbon composite that worked very well ( no guaranty they all do) and a wirewound with two trimmers was one of the best, but you had to tune it for other frequencys ;-)

author: vk6zgo
author_rank: Super Contributor
timestamp: January 11, 2013, 02:41:54 am
content: Quote from: alm on January 10, 2013, 11:06:35 pmThe resistor choice is fairly critical,too,as some film capacitors look like inductors at higher HF & VHF frequencies,although as PA4TIM has demonstrated,compensation is possible.

author: grenert
author_rank: Frequent Contributor
timestamp: January 11, 2013, 05:46:23 am
content: Quote from: PA4TIM on January 11, 2013, 01:22:44 amThat's interesting: There was an article in Nuts and Volts in November 2010 which had a recipe for a Lo-Z probe, and it specifically said to use a carbon resistor! I wonder what characteristics make for a good resistor for this application.

author: SeanB
author_rank: Super Contributor
timestamp: January 11, 2013, 06:57:48 am
content: Non inductive cut carbon or metal film works well, but they are expensive, as they are adjusted during manufacture by sandblasting the material away all around the unit or by using a flat substrate and eroding it across the whole surface before conformal coating it. Hand work that takes time. Capable of very good response at high frequencies.Carbon composition has tolerance problems, though you can adjust them a little with a hacksaw to get them higher in value closer to the desired value. Then seal with epoxy. They also have a limited frequency response and increase resistance with frequency as skin effect increases.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 11, 2013, 07:47:23 am
content: Carbon composite is not good for much but they have two interesting features.They are slightly capacitive, this lowers there resistance with increasing frequency. That is, according to literature, i have not read about skineffect in resistors. So I must measure for the total effect . But they have a voltage cooficient and terrible tempco so i will not advise them.They are good for pulsed loads and high voltage, but that is not interesting for high frequency low level probing.But the capasitive part is interesting maybe in some cases function as compensation.

author: tggzzz
author_rank: Super Contributor
timestamp: June 26, 2015, 08:30:47 am
content: Quote from: prenato on January 08, 2013, 10:57:35 pmHere's a Tektronix publication from 1969 describing a "Z0 probe", and it makes no claim for originality; seehttp://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdfand (for more general material)https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Certainly Tek and HP were suppling such probes back then.

author: w2aew
author_rank: Super Contributor
timestamp: June 26, 2015, 04:03:00 pm
content: Quote from: tggzzz on June 26, 2015, 08:30:47 amThe Tek P6150 (500 ohm input Z) and P6158 (1k ohm input Z) are still made today by Tek. Very useful - I've used them in several of my videos (the one that illustrates the filter functions of an RF up-converter comes to mind).

author: TimFox
author_rank: Super Contributor
timestamp: June 26, 2015, 07:40:24 pm
content: The problem with high-value carbon composition resistors at high frequencies is their internal construction.The internal material is a mixture of powdered carbon and powdered talc, with less carbon at high resistance values.The talc grains form tiny capacitors inside the resistor volume, which short out the carbon locally.Therefore, if you model the device as a resistor and capacitor in parallel, the parallel resistance falls at high frequencies.I measured this effect a long time ago with 1 megohm A-B traditional resistors at 5 to 20 MHz, and the effect was dramatic.

author: dadler
author_rank: Unknown
timestamp: June 26, 2015, 08:32:03 pm
content: So not DIY but I managed to get these on eBay for cheap, and for the convenience I think they are awesome.Maybe one can find a deal if they look around?

author: mazurov
author_rank: Frequent Contributor
timestamp: June 26, 2015, 08:53:07 pm
content: I usually solder 0603 resistors tombstone-style where I need to probe then solder a coax to the upper end of one of them and ground nearby.

author: Ivan7enych
author_rank: Regular Contributor
timestamp: November 16, 2015, 10:43:53 am
content: I've made this LoZ probe and checked how it works on spectrum analyzer + tracking gen (signal hound bb60c + TG44).1st attempt was made with simple 1\8 wt 1k resistor. The response rises high up to +12dB on 4GHz2nd attempt was done with 4 smd resistors, soldered on each other to minimize parasitic capacitance, 51 + 120 + 220 + 560 = 951omIt appears much better. But I don't like the 2dB ripple across all frequency range.I've added 20dB attenuator from minicircuits between probe and analyzer, it almost removes the ripple.It seems, that my analyzer has not exactly 50om termination and reflects some energy back to probe, but the probe end is not 50om matched at all, and reflects everything back again.I think, this probe requires very good 50om impedance matched input on oscilloscope (or analyzer) end to minimize the ripple.Another variant is to make probe end 50om matched (add 56om resistor between coax shield and center wire on probe end). I'll try to do this.

author: tggzzz
author_rank: Super Contributor
timestamp: November 16, 2015, 11:00:14 am
content: Quote from: Ivan7enych on November 16, 2015, 10:43:53 amWhich LoZ probe? Many different ones have been described, and the details matter! Would a photo give us any useful information?QuoteThanks for showing the results. Of course the 2dB imperfection has to be weighed against the alternatives.A quick spice simulation of the probe plus analyser input can reveal the theoretical response, and you can compare your practical circuit against that. Tweaking various values gives a feel into what is important.

author: Ivan7enych
author_rank: Regular Contributor
timestamp: November 16, 2015, 11:54:35 am
content: Quote from: tggzzz on November 16, 2015, 11:00:14 amThe simplest type,SMA connector + 70cm of rg316 cable + a resistor (1k or 950om as a sum of 4 smd resistors)

author: joeqsmith
author_rank: Super Contributor
timestamp: November 16, 2015, 06:02:54 pm
content: I tried to make some of these embedding the resistors inside of the coax. Not the best results.https://www.eevblog.com/forum/testgear/fifty-ohm-probes/15/

author: Gyro
author_rank: Super Contributor
timestamp: November 16, 2015, 09:05:30 pm
content: Just to add my build to the mix:The only novel feature they bring to the party (apart from a bit of gold plated bling) is that they include inserts from a turned pin IC socket which allows me to easily unplug and replace the tip resistors (normally 1k for ~20:1 with 50R through-terminators) for different impedances or damage. The ground contacts could probably be a bit shorter but apparently less of an issue with Zo probes.I don't make any claims for frequency flatness but they did allow me to view the outputs of some 100MHz crystal oscillators which simply refused to function with a Tek P6139A (8pF) passive probe!

author: joeqsmith
author_rank: Super Contributor
timestamp: November 17, 2015, 03:43:44 am
content: Looks good. TGZZZ posted that EDN article that I never even noticed until I reposted.Things get buried so fast on this site. I have never came up with one that would work very well over a GHz.I picked up some new old stock of LeCroy PP061 resistive probes that I have no documentation for. These were very inexpensive.

author: Mark_O
author_rank: Frequent Contributor
timestamp: November 17, 2015, 10:29:35 pm
content: Quote from: joeqsmith on November 17, 2015, 03:43:44 amThanks for posting the VNA tests (a year ago!). Interesting stuff, especially the "bumpy ride" in the amplitude domain. This response tends to be more typical of homemade devices, but considering how old these are, may have been the norm back then.(The first time you mentioned the probes in the video, you referred to them as PP066:"I also purchased some PP066 probes."Later on you did identify them as PP061 though.)Re: the PP061, all I can tell you is they were phased out by Feb'99, and replaced by the PP062 probes. Their characteristics don't seem to vary much from the story your VNA told you (500-ohm, 1.5pF, 1 GHz BW rating). See the attached chart...I noticed earlier in 1999, they still had the PP063 in their passive probe lineup, but it had been dropped by 2000. That one was also a 500-ohm unit, but had an 8 GHz BW, made possible by a <0.5 pF capacitive loading.[Archaeological digging courtesy of the Wayback Machine.]

author: mich41
author_rank: Regular Contributor
timestamp: November 17, 2015, 10:50:19 pm
content: I wonder how such setup deals with the capacitance of the scope input? With 1k resistor, even few pF translates to time constant of few ns, which doesn't look good. Am I missing something?

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 17, 2015, 11:43:19 pm
content: High speed probes of all types:http://www.ggb.com/And of course:https://www.cmicro.com/products/probes/rf-microwaveI know, not cheap and not home made. But the 50 ohm ones are pretty simple. Just basically a tiny length of coax.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 18, 2015, 02:56:10 am
content: Quote from: Mark_O on November 17, 2015, 10:29:35 pmIf I only make one mistake per video, I'm doing pretty good. In most cases, there are enough breadcrumbs to figure it out like the title in this case.The oldest LeCroy catalog I have is from 1990 and they are not listed there. They were brand new in the package so I couldn't pass them up for the price.

author: Gyro
author_rank: Super Contributor
timestamp: November 18, 2015, 10:31:20 am
content: QuoteThe 1k resistor is feeding directly into a 50 ohm cable, terminated at the scope by a 50 ohm terminator. The whole thing is a transmission line - edges propagate along the line at some fraction of the speed of light and are absorbed by the terminator at the scope end (google a reference on transmission line theory). As a result the signal at the 1k resistor never 'sees' the input capacitance of the scope (or the much higher lumped capacitance of the cable itself). It will of course see the capacitance at DC or slow moving signals, but the RC time constant has no consequence at those speeds.To get flattest HF response it is sometimes necessary to ADD a little parallel capacitance at the input end of the cable to compensate for the (tiny) parasitic capacitance of the 1k resistor itself. The resistor can be any value, within reason, limited only to zero at low end (50 ohm probe) to so high that the attenuation ratio becomes silly, or its parasitic capacitances becomes the dominant factor.

author: mich41
author_rank: Regular Contributor
timestamp: November 18, 2015, 01:04:45 pm
content: Oh, thank you for pointing me at the resistor "not seeing" this capacitance. I completely forgot that RC filter relies on R "knowing" the voltage across C in real timeThis being said, I know this capacitance simply must have some bad effect on performanceFlattened edges in one-shot transitions? Inverted reflections causing FR ripple? I think somebody reported the latter, blaming it on bad terminating resistor.edit:QuoteSo same thing as compensation of the garden variety probes, basically.

author: tggzzz
author_rank: Super Contributor
timestamp: November 18, 2015, 01:31:12 pm
content: Quote from: mich41 on November 18, 2015, 01:04:45 pmYes. The effects can be easily seen using a simple spice simulation (an ideal transmission line model will give a slightly pessimistic result), or via a Smith Chart, or via numerous online VSWR calculators. If the scope's input capacitance is 15pF, then model the termination as 50ohm//15pF

author: Gyro
author_rank: Super Contributor
timestamp: November 18, 2015, 02:13:02 pm
content: QuoteTheoretically there 'should' be no reflection to worry about. In practice nothing is perfect, I use a through-terminator, which is much better than a T connector and BNC terminator, buthopefullyworse than a scope with internal 50R termination (not sure if this is actually the case on modern low cost ones). There will always be some slight discontinuity in practice, but there is on a normal passive probe too. Remember at this point, the time constant you're talking about with the scope input capacitance is against 50R, not 1k (<1ns). There is no time constant associated with the cable distributed capacitance as this, coupled with distributed inductance is what makes up its characteristic impedance.The other imperfect area of course is the probe tip, where the resistor must couple into the end of the 50R coax. Parasitics both capacitive and inductive of the resistor and coupling. It is these factors that are the reason that joeqsmith and others (see the linked articles early in the tread) have measured ripple at high frequency, and tend to limit its use to 1-2GHz (where everything struggles anyway apart from a direct 50R connection).All that said, it still tends to work better than a passive probe at high frequencies. True, it has lower input impedance at low frequency, but at 100MHz the 8pF capacitance of the Tek P6139A (a 'good' 500MHz probe) brings its input impedance down to 200R, while the Zo probe is still up at nearly 1k.I tried both probes (the Zo and the Tek) at the same time on the output of a Williams type avalanche pulse generator, both give similar traces but if I disconnect one or the other, the P6129A tip is clearly distorting the pulse generator output a lot more than the Zo one, the same applies to logic outputs. As I say, I make no claims about the flatness of the ones that I built (others have certainly built and characterized better ones).Time for a question from me... In Douglas Smith's implementation,http://www.emcesd.com/1ghzprob.htmwhy does he include 50R termination (the 4x200R smd resistors) at the probe end of the cable? I realize it's a double parallel termination of the cable (presumably there's 50R termination at the scope end too), but what benefit does it give in this application, given that it seems to work pretty well without? One downside is obviously the increased attenuation for any given tip resistor. Confused.

author: macboy
author_rank: Super Contributor
timestamp: November 18, 2015, 06:06:11 pm
content: Quote from: Gyro on November 18, 2015, 02:13:02 pmI'm just thinking out loud here, but here is my take.You see that rippled pass-band frequency response in Ivan's post:https://www.eevblog.com/forum/projects/lo-z-probe/msg801477/#msg801477?That is caused (I assume) by a slight mismatch between the 50 ohm termination RTand the characteristic impedance of the cable Z0. Here is why: Any mismatch results in a bit of reflection. Let's say 5% of the energy is not terminated but is reflected (it is reflected either at the same polarity or opposite depending on whether RTis higher or lower than Z0). That reflected wave travels back to the tip, where it encounters almost no termination (the 1 kOhm or so is similar enough to an open cable end), so most (~95%) of the energy reflects again back to the scope side. This adds to the signal seen by the scope. For a pulse, you will see a time-delayed glitch (small step up or down in the top of the pulse depending on over- or under-termination), but for a repetitive signal, especially a sine wave, this adds at some phase that will either increase or decrease the amplitude. This causes the bumpy frequency response, as this reflected signal slides through in-phase to out-of-phase to in-phase again as frequency changes. The spacing of the bumps is determined by the length of the probe cable.Nowconsider the effect of adding that extra terminator at the probe end of the cable. Now the probe tip will absorb most of the reflected wave instead of reflecting most of it back again toward the scope. So at the scope end you might see 5% of 5% of the original wave as a total reflection, or .25% total. Without the extra terminator you might see 95% of 5%, or basically 5%. The double terminated case will have much less passband frequency response ripple.

author: Gyro
author_rank: Super Contributor
timestamp: November 18, 2015, 06:45:26 pm
content: Your aloud thinking seems entirely plausible, thanks. It does sound like the only reason to parallel terminate the source end. It's a shame that Douglas doesn't have any plots easily accessible on his site. I'm a bit surprised that he only 'specifies' his probe up to 1GHz given the improved termination, though he does talk about component limitations.A quandary now, I'm actually in a position to be able to do this on my probes, using 0603 resistors, It's a question of whether it's worth the trade-off of higher attenuation. A no-brainer I suppose for a SA, not as clear cut for a scope.

author: Ivan7enych
author_rank: Regular Contributor
timestamp: November 19, 2015, 08:29:32 am
content: Quote from: Gyro on November 18, 2015, 02:13:02 pmIdealy, both ends of coax cable should be properly terminated to minimize reflections on both ends of the cable.The scope end should have 50om input resistance.The probe end should be behave like an "ideal voltage source" + 50om resistance in series.If 50om termination of the scope is ideal in all freq range (no reflection at all), you don't need proper termination on probe end. But in my case it isn't true, my analyzer input resistance is not perfect and reflects some part of signal back to the cable.I've added 56om resistance on probe end of coax, so my probe tip is actualy a resistance divider, 560om + 56om, this divider output resistance is close to 50om. As a result, it almost removes any ripple in 0..1GHz range. (blue trace) Compare this trace with my previous one, with much higher ripple on lower range -https://www.eevblog.com/forum/projects/lo-z-probe/msg801477/#msg801477Adding a piece of metal foil around resistors partialy decreases sensitivity on >2GHz range to normal value. (pink trace)

author: Gyro
author_rank: Super Contributor
timestamp: November 19, 2015, 11:33:45 am
content: Thanks Ivan, that's an impressive ripple reduction! I'm surprised that the ripple extends right down to the lowest frequencies in your previous plot (I should have looked more closely), I was assuming it was only a high frequency problem for some reason.It's a shame there's no way of assessing the quality of the scope end termination without an analyser - I would assume that the termination in your analyser is actually pretty good in comparison, so some level of reflection is inevitable.It looks as if I may have to mess up my gold plating a bit then (though the added attenuation is still a concern).ChrisP.S. I'm impressed with the SMD construction, that must have hurt a bit!

author: Gyro
author_rank: Super Contributor
timestamp: November 20, 2015, 11:38:01 am
content: Really scraping the bottom of the barrel now in terms of keeping the attenuation down...I'm wondering if AC termination (ie. 50R + C) at the probe end would be sufficient to minimize the ripple. The reflection time should be directly related to the cable length, so calculating the minimum time constant isn't difficult. With careful tuning it ought to be possible to clean up the edges without reducing the overall attenuation ratio....maybeJust a thought anyway.

author: Mosaic
author_rank: Regular Contributor
timestamp: January 08, 2016, 11:35:28 pm
content: Shooting the breeze perhaps but:Why not build in that SMT attenuator onto the BNC connector crimp at the scope end. The result is a Pi pad configuration with a 56? SMT 'short' then a 560? series for attenuation and a 50? 'short' by the scope termination. Thus the probe source sees about a 51? end termination as does the TDR from the scope termination. Net 20X signal to sample ratio.Then just use the 50 ohm coax as the probe, thus as the probe end needs refreshing from soldering etc, you just trim.Now, since the ripple in play has to do with the leakage TDR beating on the signal, it's going to have less effect as the TDR induced ripple is gonna be at a high freq. and naturally attenuated (becomes not visible) by the scope or SA analyzer bandwidth up to perhaps 3Ghz.So instead of about a 205Mhz TDR beat freq over a 1M cable you'll have a 6.8Ghz TDR beat between the scope and the smt attenuator (30mm away). Thus the reflected attenuation of the beat interference is 30x more over the same time constant. (30dBmV down).

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2016, 11:57:49 pm
content: I've recently made a low-Z probe for my Chinese 'scopes (OWON and Rigol). Unfortunately 100MHz 'scopes, they don't have a 50R input impedance option, so I connected two 100R resistors in parallel, right next to the BNC connector on the 'scope's input. I didn't have a 450R resistor so used 270R & 180R in series at the probe end.It works quite well, although as it's just a 100MHz 'scope, I have no way of testing it at higher frequencies. Assuming the 'scope input is just 10pF and the 450R 50R divider has an output impedance of 45R, the -3dB point should be around 350MHz, which is more than good enough.I wonder if 75R co-axial cable and a 75R termination resistor near the 'scope would be any better? A 750R input impedance won't load the device under test so much but the -3dB point will be lower: probably around 236MHz, still good enough for a 100MHz 'scope.

author: Mosaic
author_rank: Regular Contributor
timestamp: January 09, 2016, 03:52:54 am
content: Well I think the purpose of a Lo Z probe IS to load the DUT with a matched impedance for HF work.Why would u need a Lo Z probe otherwise?

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2016, 04:01:53 am
content: Low capacitance.

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2016, 08:52:43 am
content: Quote from: Mosaic on January 09, 2016, 03:52:54 amNo, the purpose of a lo-Z probe is to load the device under test as little as possible: it may have a lower impedance at DC but it has a lower capacitance than a hi-Z probe, so will have a much lower impedance at higher frequencies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2016, 09:00:45 am
content: A "Lo-Z" probe is used when it has a higher Z than a conventional probe. A 10pF probe has less than 500 ohms reactance at only 30MHz.It's also used on naturally low impedance circuits, for example RF amplifiers with 50 ohm system impedances: a 500 ohm load diverts only 10% of the voltage, which is only -0.82dB, hardly noticeable.Tim

author: Gyro
author_rank: Super Contributor
timestamp: January 09, 2016, 09:51:16 am
content: Quote from: Hero999 on January 09, 2016, 08:52:43 amI think you have a typo there, I think you meant ".... , so will have a muchhigherimpedance at higher frequencies".

author: nctnico
author_rank: Super Contributor
timestamp: January 09, 2016, 11:02:49 am
content: Quote from: T3sl4co1l on January 09, 2016, 09:00:45 amStill in some cases 500 Ohms is too much of a load. In one of my current RF projects I added several 1:50 passive divider test points in the prototype in order to keep the load on the signal low. When measuring LVDS signals a 500 Ohm load is definitely too much so I also have 1:100 Lo-z probes but the attenuation is becoming troublesome. At some point you'll need active FET probes.

author: Mosaic
author_rank: Regular Contributor
timestamp: January 09, 2016, 05:58:51 pm
content: For RF work ,perhaps directional couplers (with attenuation for power RF) would be the better way to look at signals.

author: Mosaic
author_rank: Regular Contributor
timestamp: January 09, 2016, 06:07:21 pm
content: Quote from: T3sl4co1l on January 09, 2016, 09:00:45 am20log.9 = -0.91dB , not so?

author: Howardlong
author_rank: Super Contributor
timestamp: January 09, 2016, 06:56:07 pm
content: Quote from: nctnico on January 09, 2016, 11:02:49 amI am wondering, without wishing to tell grandma how to suck eggs ;-) did you try AC coupling? Certainly a 500 ohm dc coupling referenced to ground rather than the common mode offset voltage will be fairly nasty on LVDS.But I agree, a fet probe, even better a differential fet probe, is probably the instrument of choice in this situation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2016, 07:10:41 pm
content: Quote from: Mosaic on January 09, 2016, 06:07:21 pm500/550 = 0.9090..Although on a doubly matched 50 ohm line, the Thevenin is 25 ohms, so the ratio is 0.95ish.Quote from: nctnico on January 09, 2016, 11:02:49 amWell, I did specify 50 ohms...Inner networks of RF circuits frequently go into the kohms (e.g., many crystal, ceramic and SAW filters are around 1.5kohms), which of course aren't good points to probe. Often you'll need to add a buffer (which might be just an emitter follower) to get useful probing.Buffering in the circuit is even better than FET probes.Tim

author: Howardlong
author_rank: Super Contributor
timestamp: January 09, 2016, 09:31:02 pm
content: Quote from: T3sl4co1l on January 09, 2016, 07:10:41 pmBut then again, hindsight is such a wonderful thing

author: Mosaic
author_rank: Regular Contributor
timestamp: July 18, 2016, 01:01:56 am
content: These cheaper SA's etc ike the DSA 815 from Rigol appear to have frequency varying impedance at the input. Thus unless you're using the calibrated tracking gen as the input to your DUT you're going to get some dB variance from your true signal across the range. Thus if you have a signal generating device, VCO or whatever that you're characterizing, there's going to be an instrument accuracy issue at some frequencies.Which leads me to the concept of a device which will provide decent calibration for DSA's which implement correction tables.A device/VCO/sweep CW which connects directly (no coax) to the DSA and outputs a flat amplitude (+/- 0.1dB perhaps) across the required Freq span as driven by a USB control signal from a PC/laptop.this allows the user to implement & save the DSA correction table to get the flatness optimizations.Such a device can also drive a DIY Lo z probe and also allow for another correction table to compensate for BOTH the probes and the DSA VSWR issues.Interesting?

author: electrolust
author_rank: Unknown
timestamp: March 29, 2017, 09:16:12 pm
content: Quote from: Mosaic on July 18, 2016, 01:01:56 amOld post I know, but isn't that exactly what the DSA815 does? I thought I saw in the Dave video that it's not that the tracking gen is calibrated to the input for a combined flat response. I thought you applied the tracking gen (or other flat signal) and then you push a button on the SA to apply a software filter which compensates the input Z variation.Once you've calibrated the input response, you can apply any DUT signal and WYSIWYG.