Author Topic: Why are Siglent channel logic module adaptors so expensive?  (Read 14323 times)

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Offline MyrvTopic starter

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Why are Siglent channel logic module adaptors so expensive?
« on: January 25, 2017, 05:29:48 pm »

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  :palm:).

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$369

For 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.



 

Online nctnico

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #1 on: January 25, 2017, 09:28:40 pm »
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.
« Last Edit: January 26, 2017, 01:07:25 am by nctnico »
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Offline MyrvTopic starter

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #2 on: January 26, 2017, 06:38:53 pm »
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.

I 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). 

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.

Ya, 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  :-\

 

Online nctnico

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #3 on: January 26, 2017, 06:52:49 pm »
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.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline David Hess

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #4 on: January 26, 2017, 07:14:40 pm »
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.
 
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Offline rf-loop

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #5 on: January 26, 2017, 09:41:56 pm »
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.

Perhaps 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 ?pF
SPL3016 (For SDS3000)  100kOhm 5pF
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Offline David Hess

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #6 on: January 26, 2017, 11:07:25 pm »
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.

Perhaps 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 ?pF
SPL3016 (For SDS3000)  100kOhm 5pF

The 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.
 

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #7 on: January 27, 2017, 02:01:48 pm »
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.
 
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Offline Fungus

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #8 on: January 27, 2017, 03:06:08 pm »
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).

That's a fairly typical price for those things. The usual excuse is that they sell you the cable and a 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.
 

Online nctnico

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #9 on: January 27, 2017, 03:52:08 pm »
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.
I'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.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline MyrvTopic starter

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #10 on: January 27, 2017, 03:53:10 pm »
I thought I would jump in here to try clarify.

Thanks for the clarification. Much appreciated.

It would be great if this kind of information was on the website.
 

Offline David Hess

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #11 on: January 27, 2017, 04:04:22 pm »
@Fungus: Others already pointed out the SPL2016 probe is a passive divider low-z probe which is more than adequate for it's purpose.

I 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   Passive
SPL2016   Coaxial   Passive
SPL3016   Coaxial   Active
« Last Edit: January 27, 2017, 04:10:09 pm by David Hess »
 

Offline Fungus

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #12 on: January 27, 2017, 04:24:22 pm »
@Fungus: Others already pointed out the SPL2016 probe is a passive divider low-z probe which is more than adequate for it's purpose.

I 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.htm

Also Hameg HMO: https://www.reichelt.com/Oscilloscopes-Spectrum-Analyser/HO-3508/3/index.html?ACTION=3&GROUPID=4044&ARTICLE=110074

If 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?
« Last Edit: January 27, 2017, 04:31:00 pm by Fungus »
 

Offline David Hess

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #13 on: January 27, 2017, 04:41:37 pm »
PS: If those cables include a software license then you can get a cheap replacement cable if you break yours, right?

You are a funny man.  If the cable breaks, then the software license breaks.
 

Online nctnico

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #14 on: January 27, 2017, 04:48:40 pm »
@Fungus: Others already pointed out the SPL2016 probe is a passive divider low-z probe which is more than adequate for it's purpose.

I 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.
AFAIK all Keysight MSO cables are passive. Tektronix also has lots of passive probes on their logic analysers.
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Offline David Hess

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #15 on: January 27, 2017, 05:24:34 pm »
AFAIK all Keysight MSO cables are passive. Tektronix also has lots of passive probes on their logic analysers.

The 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.
 

Online nctnico

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #16 on: January 27, 2017, 05:46:15 pm »
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.htm

This 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.
« Last Edit: January 27, 2017, 05:55:47 pm by nctnico »
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Offline MyrvTopic starter

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #17 on: January 27, 2017, 05:57:16 pm »
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.

Yes, 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.

 

Offline MyrvTopic starter

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #18 on: January 27, 2017, 06:08:39 pm »
PS: If those cables include a software license then you can get a cheap replacement cable if you break yours, right?

You are a funny man.  If the cable breaks, then the software license breaks.

This 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.


 

Offline rf-loop

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #19 on: January 27, 2017, 06:11:04 pm »


I do not remember ever seeing a passive logic analyser probe before but it is not something I have watched for.


I have many ancient Hewlett-Packard passive "real logic analyzer"
 ("real logic analyzer" = state and timing analyzing)  pods/probes.

Attached description
(HP 16550A manual)


« Last Edit: January 27, 2017, 06:18:12 pm by rf-loop »
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Offline Gyro

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #20 on: January 27, 2017, 07:25:55 pm »
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).
« Last Edit: January 27, 2017, 07:31:05 pm by Gyro »
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Offline David Hess

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #21 on: January 27, 2017, 09:41:52 pm »
What gives?  The probes on my 1999 Agilent E9340A logic anlyser passive probes are spec'd at 100k with 1.5pF parasitic tip capacitance.  :-//

That 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.
« Last Edit: January 27, 2017, 09:43:44 pm by David Hess »
 

Offline Gyro

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #22 on: January 27, 2017, 09:57:44 pm »
Sure David, but the E9340A does use 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
Best Regards, Chris
 

Offline David Hess

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #23 on: January 28, 2017, 12:11:02 am »
Sure David, but the E9340A does use 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

Based 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.
 

Offline Gyro

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Re: Why are Siglent channel logic module adaptors so expensive?
« Reply #24 on: January 28, 2017, 05:15:45 pm »
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.
Best Regards, Chris
 


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