Why are Siglent channel logic module adaptors so expensive? - Page 1
Forum section: electronics / testgear

author: Myrv
author_rank: Contributor
timestamp: January 25, 2017, 05:29:48 pm
content: Short Version:Do the Siglent channel logic module adapters (SPL2016) have some kind of active logic to them? Like is there a logic level convertor or some kind of protection in the adapter?Because at US$369 (http://www.saelig.com/product/spl2016.htm), even more in my far northern reaches, that seems like an awfully expensive cable (and that doesn't even include the software license).Are there any 3rd party versions? Is it possible to hack up your own?Thanks for any insights.Long'ish Version:I have been bouncing back and forth between a Rigol DS1054Z or a Siglent SDS1202X for a couple weeks now (4ch versus 200MHz, interface, ....). I was starting to lean toward the 1054z as 500Msa/s for 2 channels just wasn't enough for 200MHz operation) but then I noticed the Siglent promotion for the SDS2204X. 4 channels, 200 MHz, and 2GSa/s for"only"4x the price (ya I know).Anyway, the SDS2204X is also a MSO and they have a further deal if you buy the probes and software you can get the serial decoder and AWG for "free". Then I looked at the price of the channel logic module adapters:US$369For what looks like a nothing more than a bunch of wires (not even shielded, i think).As asked above. Is there something special about this adapters?Is it even worth picking up this function for the SDS2024X? I've been getting differing opinions on whether logic analyzer abilities of the SDS2000X are useful (some say it only decodes what's on the screen?)Again thanks for any insights.

author: nctnico
author_rank: Super Contributor
timestamp: January 25, 2017, 09:28:40 pm
content: I'm not sure whether having digital channels in an oscilloscope is very useful if you have a seperate logic analyser with deep memory. Two/three years ago I insisted on getting an MSO and I did but so far I have not used the digital channels at all. For more demanding jobs involving digital signals I just use my logic analyser. In the $400 range you can buy a decent used logic analyser from HP or Tektronix (TLA700 series) because there is very little interest in these older machines or go for a new USB based one.Edit: it is hard to tell whether the SPL2016 pods are active or not. In the frequency range of a typical MSO the cables can use passive divider probing to get reasonably high input impedance with good high frequency response and 50 or 75 Ohms towards the MSO inputs to match the probe cable impedance.

author: Myrv
author_rank: Contributor
timestamp: January 26, 2017, 06:38:53 pm
content: Quote from: nctnico on January 25, 2017, 09:28:40 pmI have recently picked up a DSLogic Pro (USB Logic Analyzer). On paper it seems to have good specs (for the price point) but I haven't had a chance to play with it much. My guess is the SDS2000 wouldn't be any better (it's actually quite hard to dig up any real information on the Siglent LA functionality. The manual doesn't really cover it and the spec sheet is a little light on details).Quote from: nctnico on January 25, 2017, 09:28:40 pmYa, I don't really see any need for anything special for the Siglent probes. That's why I was asking the forum. If I go the SDS2000X route I'll likely just forgo the digital channel functionality (seems like a waste but for the money the want there are better options as you have pointed out).Thanks for commenting. Now I just have to decide if 200MHz, 2GSa/s, and 4 channels is really worth it

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2017, 06:52:49 pm
content: Did you read the other thread about the SDS2204X (https://www.eevblog.com/forum/testgear/siglent-sds2204x-any-good/)? There is quite a bit of information in there on pros/cons and alternatives.Still an MSO can show the digital signals in realtime which a logic analyser typically can't do. Then again: if you have 4 analog channels then you can get a whole bunch of traces already. Sometimes you can get the best of both worlds. In my Tektronix TLA715 logic analyser I have a TLA7AA4 acquisition module (these modules and the probes usually sell for little money on Ebay). The TLA7AA4 module has 4 analog outputs which can be fed from any logic analyser input. This means you can look at the signals in both analog and digital domain using the logic analyser's inputs. If you are planning on diving deep into digital designs then this could be a setup to consider.

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2017, 07:14:40 pm
content: Logic analyser pods include signal conditioning circuitry in the form of compensated input attenuators and fast comparators which then drive the low impedance cable back to the instrument. This is certainly the case with the SPL2016 do to its 100k input resistance but I wonder why its input capacitance is so high at 18pF.

author: rf-loop
author_rank: Super Contributor
timestamp: January 26, 2017, 09:41:56 pm
content: Quote from: David Hess on January 26, 2017, 07:14:40 pmPerhaps capasitance is wrong.SPL1016, is 100kOhm and 8pF (also it reads on the probe what is now on my hand, not only data sheet.SPL2016, 100kOhm ?pFSPL3016 (For SDS3000) 100kOhm 5pF

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2017, 11:07:25 pm
content: Quote from: rf-loop on January 26, 2017, 09:41:56 pmThe datasheet quality is pretty poor so maybe it is wrong.As far as whether they are expensive, I looked into the feasibility of designing a modern version of an old Tektronix high input impedance logic analyser probe and while the design and concept is simple, execution is difficult even with or maybe especially with modern parts. I have not done an exhaustive search but suitable high input impedance analog comparators do not seem to exist and while in theory a fast line receiver should work, they all have high input bias currents and they are not specified for this type of application. Old designs use an FET input stage followed by a line receiver and commonly used hybrid construction.

author: Unknown
author_rank: Unknown
timestamp: January 27, 2017, 02:01:48 pm
content: I thought I would jump in here to try clarify.The SPL2016 Logic probe uses a true shielded coaxial cable for each channel. The SPL2008 used on the older SDS2000 series of scopes did not.Also, each channel has a 5-element RC impedance matching circuit on each channel built into the remote head.

author: Fungus
author_rank: Super Contributor
timestamp: January 27, 2017, 03:06:08 pm
content: Quote from: Myrv on January 25, 2017, 05:29:48 pmThat's a fairly typical price for those things. The usual excuse is that they sell you the cableanda software license.If you look inside they're full of chips, eg. they have comparators so you can set the voltage trigger levels for 5V/3.3V logic (or whatever).It's far from a simple cable but they could probably sell them for $50 if they really wanted to.

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2017, 03:52:08 pm
content: Quote from: Siglent America on January 27, 2017, 02:01:48 pmI've seen the inside of the SPL2008 and it is an active pod which sends the signals amplified (or already compared) over differential pairs. It is an entirely different beast. With an active pod the signals between the pod and the inputs are much less sensitive to noise and with differential signalling you can get away with even using low cost flatcable.@Fungus: Others already pointed out the SPL2016 probe is a passive divider low-z probe which is more than adequate for it's purpose.

author: Myrv
author_rank: Contributor
timestamp: January 27, 2017, 03:53:10 pm
content: Quote from: Siglent America on January 27, 2017, 02:01:48 pmThanks for the clarification. Much appreciated.It would be great if this kind of information was on the website.

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2017, 04:04:22 pm
content: Quote from: nctnico on January 27, 2017, 03:52:08 pmI guess that makes sense if the logic analyser inputs are 10k so the probe operates with x10 attenuation and it neatly explains why the input capacitance is so high. The input capacitance looks like a good way to distinguish them.I do not remember ever seeing a passive logic analyser probe before but it is not something I have watched for.Based on the specifications and photographs then:SPL1008 Ribbon Cable PassiveSPL2016 Coaxial PassiveSPL3016 Coaxial Active

author: Fungus
author_rank: Super Contributor
timestamp: January 27, 2017, 04:24:22 pm
content: Quote from: nctnico on January 27, 2017, 03:52:08 pmI was talking more generally.$300 isn't an unusual price, even Rigol DS1000Z probes cost that much.http://www.rigol-uk.co.uk/Rigol-MSO1000Z-Upgrade-Kit-p/mso1000z-upgrade.htmAlso Hameg HMO:https://www.reichelt.com/Oscilloscopes-Spectrum-Analyser/HO-3508/3/index.html?ACTION=3&GROUPID=4044&ARTICLE=110074If Siglent is charging $300 for a bunch of resistors then it's a tiny bit more of a ripoff, maybe 20% more (I'm sure Rigol/Hameg could sell them for under $50 and make money).PS: If those cables include a software license then you can get a cheap replacement cable if you break yours, right?

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2017, 04:41:37 pm
content: Quote from: Fungus on January 27, 2017, 04:24:22 pmYou are a funny man. If the cable breaks, then the software license breaks.

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2017, 04:48:40 pm
content: Quote from: David Hess on January 27, 2017, 04:04:22 pmAFAIK all Keysight MSO cables are passive. Tektronix also has lots of passive probes on their logic analysers.

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2017, 05:24:34 pm
content: Quote from: nctnico on January 27, 2017, 04:48:40 pmThe Keysight 2000, 3000, 4000, and 6000 specifications all say 8pF for the logic probe inputs which is low for a passive probe unless the attenuation is higher but I expect Keysight is good enough to design a passive probe that good.

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2017, 05:46:15 pm
content: 8pf is very high but these are not passive probes like the regular high-z probes you use on an oscilloscope. The passive probes on a logic analyser are typically low-Z probes which have an impedance towards the logic analyser which I assume is somewhere between 50 and 100 Ohm (depending on the design) instead of 1M Ohm. This means you can achieve much lower probe tip capacitances and higher bandwidths. The Tektronix P6417 logic analyser probe (passive) for example has an input impedance of 2pf parallel with 20k Ohms. This page has some more details:http://www.emcesd.com/1ghzprob.htmThis also answers the question why the probes are relatively expensive: there has to be a small circuit for each input somewhere in the (molded) cable.

author: Myrv
author_rank: Contributor
timestamp: January 27, 2017, 05:57:16 pm
content: Quote from: nctnico on January 26, 2017, 06:52:49 pmYes, I saw that thread. It's the thread that brought to my attention the current promotion on the sds2000x series. From what I've read you seem to be a strong advocate for the GW Instek scopes and made some strong arguments about their post processing capabilities (fft, decoding, etc). rf-loop is a strong advocate of the Siglent scopes. Both of you (not to slight anyone else in this thread) have consistently contributed valuable posts to this board so take your opinions seriously. Right now the SDS2204X is the same price as the GDS-2204E and offers double the sample rate as the Instek and MSO capabilities so I am leaning towards the Siglent (without the promotion the Instek would probably be the preferred choice). That all said, if I'm honest with myself a (hacked) 1054Z is all I need for my immediate plans but reaching a little higher now does introduce some future proofing (however this may just be false economy). Again thanks for your comments. Appreciated as always.

author: Myrv
author_rank: Contributor
timestamp: January 27, 2017, 06:08:39 pm
content: Quote from: David Hess on January 27, 2017, 04:41:37 pmThis was partially my concern as well. I could almost make sense of it if the software license was $369 and the cable was $188. But with the cable at the higher price point I'm scared of losing it (or breaking it). It's scary that I can buy a 4 channel 50MHz scope for about the same price as one of these adapters.Of course, not being an RF designer I might be missing something obvious about these cables that makes them more expensive than I would assume (ergo this thread).Anyways, I'm learning lots in this thread. I appreciated every ones comments. Thanks.

author: rf-loop
author_rank: Super Contributor
timestamp: January 27, 2017, 06:11:04 pm
content: Quote from: David Hess on January 27, 2017, 04:04:22 pmI have many ancient Hewlett-Packard passive "real logic analyzer"("real logic analyzer" = state and timing analyzing) pods/probes.Attached description(HP 16550A manual)

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2017, 07:25:55 pm
content: What gives? The probes on my 1999 Agilent E9340A logic anlyser passive probes are spec'd at 100k with 1.5pF parasitic tip capacitance.(Edit: this seems to be much lower capacitance than everyone else is quoting).

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2017, 09:41:52 pm
content: Quote from: Gyro on January 27, 2017, 07:25:55 pmThat is what I was wondering but we have resolved the issue. Unlike your Agilent E9340A, the logic analyzers we are discussing use high impedance passive probes similar in construction to a high impedance oscilloscope probe. Part of my confusion was the claim that they are low-Z probes but that is not the case.rf-loop's link to the description of the HP 16550A gives the description I was expecting. The input resistance at the instrument is 10k (bipolar input?) instead of the 1M for an oscilloscope input so the probe's 90k series resistance yields 100k total and divides the cable's input capacitance down to about 8pF. Note that nothing special is required in the cable for a high input impedance passive probe; it may be coaxial or ribbon cable and if the best pulse fidelity at the highest bandwidth is not required, it does not need controlled distributed resistance.A low-z probe would have a input impedance at the instrument input which matches the cable resistance but this would yield a typical probe input resistance somewhere between 500 and 5000 ohms depending on details like the attenuation factor which is not friendly with all common logic types. Tektronix once made a low-z probe (P6048) for use with high input impedance oscilloscope inputs; it included a built in 50 ohm termination at the BNC connector. This might seem counter productive but it had a much lower input capacitance than even the best 500 MHz high impedance passive probe made today.Bipolar input active probe designs can achieve a 10k input resistance (Tektronix P6203) and would be suitable for a low cost low input capacitance logic probe design but the only active probe designs I have studied use FETs.So despite what I initially though, you are indeed playing $369 for a Siglent logic analyser probe which contains no active devices.

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2017, 09:57:44 pm
content: Sure David, but the E9340Adoesuse passive probes and still achieves that input impedance (100k / 1.5pF). The analyser spec is 100MHz state, 250MHz timing btw.The old Agilent Probing Solutions datasheet provides a good reference...http://cp.literature.agilent.com/litweb/pdf/5968-4632E.pdf

author: David Hess
author_rank: Super Contributor
timestamp: January 28, 2017, 12:11:02 am
content: Quote from: Gyro on January 27, 2017, 09:57:44 pmBased on the equivalent schematics, that looks like specmanship to me. Other manufacturers would list those 1.5pF loads as 8pF. Passive oscilloscope probes with the same design are not considered 1.5pF.

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2017, 05:15:45 pm
content: Ha, maybe so (although the likes of Tek don't seem to have seen fit to challenge them on it). I must admit, it wasn't until I re-read the probing solutions datasheet that I questioned the headline spec myself. I suppose you could argue that the 7.4pF component in series with 370R in the equivalent circuit is kinder on a signal than a straight parallel 8pF. It would be hard to test their model without knowing the cable characteristics anyway.

author: nctnico
author_rank: Super Contributor
timestamp: January 28, 2017, 05:41:33 pm
content: You also have to look at the goal of the probe. An MSO typically samples at 500Ms/s at most so realistically you'd be looking at signals in the less than 200MHz range. At 200MHz that 8pf would be equal to 100 Ohm. Add 370 Ohm to that and you are around 470 Ohm in total. The 1.5pf has an impedance of 530 Ohm so all in all you are looking at a load on the signal around 250 Ohm at 200MHz (which isn't bad).

author: Howardlong
author_rank: Super Contributor
timestamp: January 28, 2017, 06:08:52 pm
content: Quote from: Gyro on January 28, 2017, 05:15:45 pmI just took one of the Agilent LA probes apart, well as far as I could get. The resistance of the approx 8" probe to the pod is 90k or so. That 8" cable is coax for most of its length, there's a ground point about 1.5" from the tip in a tiny sealed micro pod thingy which I won't be able to get into without rendering it useless.The long woven ribbon cable from the pod to the LA has a distributed resistance of about 190 ohms on the signal lines. It is single ended despite being woven, every other conductor is grounded. There are plenty of texts online documenting why distributed resistance cables are used for scope probes, I assume it's for similar reasons here.There is nothing inside the pod other than a PCB to wire the woven ribbon cable to the flying probe connector. There are no components at all, neither active nor passive, just a PCB and a 10x2 0.1" pitch header style receptacle for the probes.If the design follows other passive probe designes like Tek ones, then the 8" probe with nano pod thingy will probably have some distributed resistance too.Making cables with distributed resistance is unlikely to be particularly cheap.I don't know if the Siglent's cable is active or passive. Rigol's MSO1000Z series are active and use comparators in the pods to buffer the single ended signal to a diferential pair running over normal ribbon cable to the scope.

author: David Hess
author_rank: Super Contributor
timestamp: January 28, 2017, 07:44:03 pm
content: Quote from: Gyro on January 28, 2017, 05:15:45 pmThe same procedure used to normalize oscilloscope input capacitance can be used to measure it and it applies to a x10 probe input if someone had a reason to do such. Put a parallel RC circuit in series with the probe tip, with a 50k or 100k resistor in this case depending on the probe, and a trimmer capacitor. Use a square wave signal source and adjust the trimmer until the properly terminated output is compensated. Now the capacitor can be measured to determine the probe capacitance and it will be somewhere around 8pF. The low value series dampening resistor will not even show up.Quote from: nctnico on January 28, 2017, 05:41:33 pmI agree; the application needs to be kept in mind. Logic circuits are almost ideal for passively probing because of their low impedance (1) and the ones which are high impedance like open drain/collector outputs are slower so either way, the probe input capacitance is moderated.I am just objecting to their misleading specification which makes it seem like an active probe. If I measured the input capacitance, it would be about 8pF and not 1.5pF. I wonder if Agilent released this unusually detailed datasheet in response to complaints.(1) My 300 MHz x10 oscilloscope probes work great when probing 25 ohm and lower impedance sources also!Quote from: Howardlong on January 28, 2017, 06:08:52 pmThe input network must be in the grabber probe instead of the pod. It is just a series circuit like you would find in a x10 oscilloscope probe so no ground connection is required.QuoteThe active probe designs do not need distributed resistance in the cable back to the logic analyzer. That would be preferable if you did not have a source for the special cable.

author: Howardlong
author_rank: Super Contributor
timestamp: January 28, 2017, 09:17:02 pm
content: I was trying to avoid going into a long description with lots of photos and links, but I can see I may well have to!I was trying to draw a comparison between the two methods, that of a passive based LA probe assembly and that of an active one.The Rigol MSO1000Z LA probe has a bunch of LMH7322 comparators in the pod, and all cables and probes are normal conductors.The Tek P6316 probe is passive and I did a teardown of it herehttps://www.eevblog.com/forum/testgear/mdo3000-hacking/msg765377/#msg765377which shows the pods have passive circuits as well as distributed resistance in both the main ribbon cable and each individual probe lead.The HP/Agilent/Keysight probe (54620-61601 but also has other numbers for electrically equivalent items) I took apart is the one that's been commonly used in their lower end MSOs for about 15 years, as well as some of their dedicated LAs. As well as what I've mentioned, I agree there's a lumped resistive/capacitive netwrok in each individual probes' "nano pod". This nano pod thingy sits about 1.5" from the probe point, and that 1.5" may well just be a bit of wire. The nano probe thing also has an optional ground connection as well as the lump network, and proceeds as coax for the next few inches to the pod proper. I would not be surprised if this has distributed resistance, but without destroying it I can't tell for sure. The mini grabbers are nothing more than that, the probe tips plug directly onto header pins and the like, or you can connect with the mini grabbers if desperate, frankly I usually find mini grabbers too flimsy and unreliable, they drop off too easily.Anyway, here's a teardown of the main pod:https://www.eevblog.com/forum/testgear/dsox2000-and-3000-series-licence-have-anyone-tried-to-hack-that-scope/msg367778/#msg367778"nano pods":Anyway, the point of my note is that these LA cables are not just reassuring expensive ribbon cables!Edit:typos, and added some pics.