Help needed - Siglent TPA10 / Tektronix TekProbe offset not working

[FlexibleMammoth]

Hi all,

I own some Tektronix active probes (ADA400A and P6247) that still use the TekProbe interface [1]. I have quite a hard time to get them running with my Siglent SDS5000X via the official Siglent TPA10 TekProbe adapter [3]. In particular, I am unable to do any offset adjustment, and I have a suspicion that my adapter is defective.

I realized I went into way too much detail, so here is the short version:

Could someone who owns EITHER a Tektronix Oscilloscope with a TekProbe interface OR a Siglent Oscilloscope with a TPA10 adapter please measure the voltage present at the offset pad of the TekProbe interface?
- Does the voltage change at all when you rotate the offset button?
- What is the voltage range when adjusting the channel position to min/null/max?


-------------Rambling and lengthy debugging below----------------

Info: Both probes support manipulating the DC offset. The ADA400A supports manual offset adjustment, while the P6247 offers DC Reject.
Info2: The mechanism by which the TPA 10 generates its offset is the following (provided by Siglent):

The mechanism of the scope controlling the offset over TPA10 is that the scope programs an offset DAC in the TPA10, and the TPA10 outputs the offset to the Tek's probe thru the TekProbe interface level II. If the offset is correctly added to the front end of the probe, then it can be captured and displayed on the scope.

What I tried:

ADA400A:
1) When DC offset is enabled on the probe:
    - The ADA400A allows manual adjustment on the probe itself.
    - The offset control of the scope does not work.

2) When DC offset is disabled on the probe:
    - The ADA400A no longer controls the offset, but neither does the scope.
    - The offset control of the scope does not work.

I think this expected due to the limited interface on this probe; there are only 4 wires going from the probe to the TekProbe connector (+/-15V, +5V, attenuation ID), so the offset voltage is ignored.

P6247:
1) When DC reject is enabled on the probe:
    - The P6247 rejects DC and shows correct signals.
    - The offset control of the scope does not work.


2) When DC reject is disabled on the probe:
    - The P6247 shows a constant DC offset of about 1.2V, which I assume means it is crashing into the V+ rail of the input transistor.
    - When applying the input calibrator of my scope to the probe input, i can see a faint square wave riding on top of the 1,2V DC offset.
    - The offset control of the scope does not work.

To find out what is going wrong, I measured the offset pad in the tekprobe adapter. It is always sitting at 2.5V, no matter how I adjust the offset. That seems strange to me, since for the 1103 Tekprobe power supply, Tek adjvertises a +-1V offset range [2]:

Outputs
Pin 1: n.c.
Pin 2: n.c.
Pin 3: +5 V.
Pin 4: +15 V.
Pin 5: Offset ±1 V.
Pin 6: –5 V.
Pin 7: –15 V.

The adapter works the same way (taken from [3]):

Specifications
Bandwidth 4 GHz
Power Supplies +15 V (±2%, 100 mA),
-15 V (±2%, 100 mA),
+5 V (±2%, 200 mA),
-5 V (±2%, 200 mA)
Offset Voltage -1.2 V ~ +1.2 V to the probe. Multiply it by the attenuation factor of the probe to calculate the system offset
range.

Now if the offset works the way I think it works, a fault where it is 2.5V all the time would surely cause the probe to crash into Vmax.

Could someone who owns a Tek Scope with a TekProbe interface please measure the offset pin? What is the voltage range? Does it change when you rotate the offset button?

If someone has a Siglent TPA10: Could you verify if your adapter behaves the same way? What is the voltage range? Does it change when you rotate the offset button?

Thanks a lot,
BR Andreas

[1] https://www.tek.com/en/support/faqs/what-tekprobe-interface
[2] https://www.tek.com/en/datasheet/probe-power-supply-1103
[3] https://siglentna.com/wp-content/uploads/2021/10/LPA10_TPA10_Datasheet_DS060APA_E01A.pdf

[FlexibleMammoth]

I edited the original post to get straight to the point - the lengthy introduction was obfuscating the problem.

If someone has a Tek scope with TekProbe interfaces or a Siglent TPA10, a simple voltmeter measurement would help a lot

[tautech]

From an email last year to Siglent EU from Siglent DSO product manager.....I think about this issue for you:

The mechanism of the scope controlling the offset over TPA10 is that the scope programs an offset DAC in the TPA10, and the TPA10 outputs the offset to the Tek's probe thru the TekProbe interface level II. If the offset is correctly added to the front end of the probe, then it can be captured and displayed on the scope.

•   If the TPA10 is connected to the scope without any probe, then the offset feedback loop is not created, so what the tester saw is on purpose.
•   As for the case that with the ADA400A probe, we guess its offset is not provided from the  TekProbe interface level II , but controlled by manual (we checked the probe user manual and found it has offset control button on probe, see figure below). If our guess is correct, then what the tester saw is on purpose.

However I have raised this again with some contacts and it may be a day or 2 before we get replies.
 

[TimFox]

I have a Tektronix power supply/adapter for TekProbe.  The "offset" pot does not function with some of the Tektronix TekProbe probes.

[alm]

Could someone who owns EITHER a Tektronix Oscilloscope with a TekProbe interface OR a Siglent Oscilloscope with a TPA10 adapter please measure the voltage present at the offset pad of the TekProbe interface?
- Does the voltage change at all when you rotate the offset button?
- What is the voltage range when adjusting the channel position to min/null/max?

I have a Tek TDS3052, which may not support the TekProbe offset feature. From what I remember reading in catalogues, the P6243 single-ended FET probe, which does not have the offset feature, is recommended for the TDS3000 series, while the P6245 probe, which does support offset feature, is recommended for the TDS600/700 etc. I measured pin 5 as indicated by the attached diagram without a probe connected, and it stayed at 0V regardless of the setting of the offset control. The offset control also works with simple passive probes. Note that this offset feature is separate from the vertical position control. The way the manual describes it, it can prevent the scope from clipping a signal that's out of range, so clearly the offset is applied in the analog input circuit. As far as I know, it controls the offset voltage fed into the vertical amplifier. It might be that when it detects an active probe with offset feature, it supplies a voltage on pin 5 instead, but it's quite hard to measure while a probe is attached, since the case covers the area around the connector.

I'm guessing this is different in the scopes that do support supplying an offset via the TekProbe2 interface, but I don't have access to such a scope.

I do have an ADA400A, and when the offset switch on the ADA400A is on, both the offset pots on the ADA400A and the offset control on the scope affect the vertical position of the signal. With the offset off on the ADA400A, only the offset control on the scope affects the vertical position.

[FlexibleMammoth]

From an email last year to Siglent EU from Siglent DSO product manager.....I think about this issue for you:

Yep that was me too. We were trying to figure out why offset was not working with the ADA400A. I feel that the TPA10 does not give us a „full“ TekProbe Interface - x1/x10 detection and offset for passive probes do not work. If ADA400A uses these mechanisms, it will not work correctly.

[FlexibleMammoth]

Note that this offset feature is separate from the vertical position control. The way the manual describes it, it can prevent the scope from clipping a signal that's out of range, so clearly the offset is applied in the analog input circuit. As far as I know, it controls the offset voltage fed into the vertical amplifier.

Very interesting. So there are two types of offset - your scope seems to only support traditional offset in the frontend, and my scope only supports offset on Pin5 (well at the moment it doesn’t).

I do have an ADA400A, and when the offset switch on the ADA400A is on, both the offset pots on the ADA400A and the offset control on the scope affect the vertical position of the signal. With the offset off on the ADA400A, only the offset control on the scope affects the vertical position.

I had my ADA open and found that only signal and 4 cables are going to the main chassis, those being +/-15V, +5V and what I assume is attenuation detection, since it is mentioned in the manual. So it seems that ADA is using traditional offset - which does not work with TPA10. Very interesting, thank you for the valuable input!

[alm]

The ADA400A does indeed switch the probe attenuation setting on the scope from x10, x1, x0.1 and x0.01 depending on the gain setting on the ADA400A.

The ADA400A also supports offset on the amplifier itself. I can't really think of a use of the offset control of the scope with the ADA400A since it is inferior: after the differential amplifier, so the offset can still saturate the differential amplifier. I guess one advantage is that you can see the magnitude of the offset on the scope, while on the ADA400A are just simple pots without any scale.

Of course the non-functional P6247 is another problem. Did you try to verify the problem is indeed with the offset signal, for example by covering pin 5 on the adapter with tape?

[tautech]

From an email last year to Siglent EU from Siglent DSO product manager.....I think about this issue for you:

Yep that was me too. We were trying to figure out why offset was not working with the ADA400A. I feel that the TPA10 does not give us a „full“ TekProbe Interface - x1/x10 detection and offset for passive probes do not work. If ADA400A uses these mechanisms, it will not work correctly.

Yet we don't fully understand how the 1V offset feedback pin works.
When the probe Offset control is ON does the scope sense the probes offset setting ? IDK.

Attached is a pic of the probes preamp.

[FlexibleMammoth]

The ADA400A does indeed switch the probe attenuation setting on the scope from x10, x1, x0.1 and x0.01 depending on the gain setting on the ADA400A.

Unfortunately, it doesn’t for me, similar to how passive probe attenuation is not recognised. This is actually a major annoyance, since every time I change the Probe gain, I have to remember to set the scope correctly.

The ADA400A also supports offset on the amplifier itself. I can't really think of a use of the offset control of the scope with the ADA400A since it is inferior: after the differential amplifier, so the offset can still saturate the differential amplifier. I guess one advantage is that you can see the magnitude of the offset on the scope, while on the ADA400A are just simple pots without any scale.

Of course the non-functional P6247 is another problem. Did you try to verify the problem is indeed with the offset signal, for example by covering pin 5 on the adapter with tape?

Yep, they could have put a nice vernier knob like on the old 7000 series to provide a mechanical readout… if I am annoyed enough I might actually make a project from that idea.

I thought about covering the pin, but have not tried it yet.

[FlexibleMammoth]

The p6247 is working with tape over pin5 as if the scope was set to 0v offset, so my probe is good.

[FlexibleMammoth]

Yet we don't fully understand how the 1V offset feedback pin works.
When the probe Offset control is ON does the scope sense the probes offset setting ? IDK.

Attached is a pic of the probes preamp.

Just tested the ADA400A with taped over pin5, no difference. Offset off = no one controls offset, offset on = probe only controls offset. So it appears that ADA does not care about pin5 and uses traditional offset?

My sds5000x does not actively respond to anything I do on the ADA.

[alm]

Yep, they could have put a nice vernier knob like on the old 7000 series to provide a mechanical readout… if I am annoyed enough I might actually make a project from that idea.

The ADA400A was a modernized version of the 7A22/5A22N/AM502/3A9. All of these had a simple concentric pot for course / fine without any scale. Clearly the customers for this plugin didn't mind this. The 7A22 (but not the other ones) did have a Vc out so you could hook up an external voltmeter to measure the offset voltage.

The 7A13 was a different product available at the same time that was marketed as a "differential comparator" and had the 0.1% accurate 4 digit vernier for very precise offsets. I don't believe Tektronix ever made a modern version of the 7A13 (except maybe in the 11000 series), but Preamble Labs, and later Lecroy, did with the DA1855.

The p6247 is working with tape over pin5 as if the scope was set to 0v offset, so my probe is good.

That's good to know! So the problem is that the Siglent adapter is incompatible with this probe, and probably with any probe that relies on the TekprobeII interface to set the offset.

Just tested the ADA400A with taped over pin5, no difference. Offset off = no one controls offset, offset on = probe only controls offset. So it appears that ADA does not care about pin5 and uses traditional offset?

I'd say the ADA400A is designed to be used with only the offset knobs on the ADA400A.

[FlexibleMammoth]

The ADA400A was a modernized version…

I do have a 7000 series mainframe, and was thinking about getting a 7a13, as the da1855 is unobtainium… the only „cheap“ one I saw went for €700 in combination with a scope.

That's good to know! So the problem is that the Siglent adapter is incompatible with this probe, and probably with any probe that relies on the TekprobeII interface to set the offset.

I’d say the Adapter is defective. 2.5V should not be at a pin specified for +/-1V by tek.
EDIT: Also the specs of the adapter list the offset as +/- 1.2V, which makes a 2.5V offset voltage even more unlikely...

I'd say the ADA400A is designed to be used with only the offset knobs on the ADA400A.

Very likely. I would still like to have working passive-style offset for other probes that do not support pin 5

[tautech]

The p6247 is working with tape over pin5 as if the scope was set to 0v offset, so my probe is good.

That's good to know! So the problem is that the Siglent adapter is incompatible with this probe, and probably with any probe that relies on the TekprobeII interface to set the offset.

From investigations it does seem TPA10 is a TekProbe2 device. however there is no mention of this in documentation or on websites. 
Should TPA10 also be TekProbe backwards compatible ?

Can anyone test TPA10 with an older Tek probe that only uses the TekProbe interface ?

[alm]

The p6247 is working with tape over pin5 as if the scope was set to 0v offset, so my probe is good.

That's good to know! So the problem is that the Siglent adapter is incompatible with this probe, and probably with any probe that relies on the TekprobeII interface to set the offset.

From investigations it does seem TPA10 is a TekProbe2 device. however there is no mention of this in documentation or on websites. 
Should TPA10 also be TekProbe backwards compatible ?

Can anyone test TPA10 with an older Tek probe that only uses the TekProbe interface ?

If you compare the TekProbe pinout from the Tek 1103 manual with the TekProbe II pinout (both attached to this post), they are identical except that TekProbe II adds clock and data for digital communication with the probe (where TekProbe and older probes used a resistor on the readout pin to signal attenuation). So a TekProbe II host should be backwards compatible with any TekProbe probe, save maybe the readout feature. To the point where Tektronix barely distinguishes the two. For example the TCA-1MEG TekConnect to BNC adapter is described as implementing the TEKPROBE interface, yet the pinout clearly shows the clock and data pins.

Does the Siglent adapter even support the digital interface? Or is it just implementing the TekProbe interface? Either way, a TekProbe interface should support any probe supported by the 1103, which I believe is pretty much all of them.

The ADA400A and P6247 are both compatible with TekProbe (I) scopes, like the TDS400/500/600 series and 1103, and are listed as currently available products compatible with the latest BNC scopes and BNC adapters. I'm not quite sure if they implement TekScope or TekScope II. They were introduced in 1998, when Tek says the current TekProbe (II) interface with clock and data was introduced. Yet the Tek 1998 and 2000 catalogs don't mention "TekProbe II" anywhere I can see (not even for the newly introduced scopes), and all of the then newly introduced probes list compatibility with the 1103. So I until I see evidence to the contrary, I assume that if anything the newer probes just add an optional digital interface for the scope to be able to get more information about the probe beyond what the readout resistor provided.

[tautech]

Thanks alm. 

It's not like Siglent has the relationship with Tek like they do with LeCroy so propriety TekProbe configuration and probe identification SW code they really need work out for themselves and it seems to need more work. 

Any further clues to get the TPA10 fully working would be much appreciated.

[alm]

I believe the original TekProbe was considered an open standard of sorts. The 1103 manual and the page I linked gives most of the specifications except the readout pin. The link I posted earlier gives empirical information about the values of the readout resistor, but I'm not aware of an official document specifying them. However, half an hour with a recentish Tek scope and a decade resistor should give you this information if you don't want to trust some random person posting on Usenet. There may be some subtle points, like noise and output impedance. I remember Steve on the TekScopes group mentioning that the poor CMRR of some FET probes became a problem when they introduce the TCP202, which would introduce ripple on the power rails. Still, implementing the 1103 specs should get them 95% there.

The TekProbe II standard is explicitly described by Tektronix as proprietary, so I can imagine reverse engineering that 100% may be more involved, although I'm guessing it's just an I2C EEPROM. There has been some effort reverse-engineering this EEPROM. So I could accept it for example not detecting the proper input impedance for an active probe. But the purely analog interface of the 1103 should be easy to implement, and this already specified the offset signal from -1 V to +1 V. So setting that to 2.5 V seems wrong no matter what.

I have no idea how to improve the TPA10 without Siglent's help other than taping off the offset contact or maybe generating your own offset signal from a pot.

[FlexibleMammoth]

Does the Siglent adapter even support the digital interface? Or is it just implementing the TekProbe interface? Either way, a TekProbe interface should support any probe supported by the 1103, which I believe is pretty much all of them.

I reported to Siglent some time ago that the automatic scale factor and termination (1MOhm/50Ohm) detection were not working. They told me that "The current [firmware] version is fully functional, but unfortunately automatic detection is not supported". It is not really clear if they refer to the scale factor, the input termination, or both.

Either way, there seems a lot missing here - let's assume my unit is defective and the offset on pin 5 normally works:

1) There is no analog offset control for passive probes or TekProbe1 probes.
That one is interesting - because this voltage is generated inside the scope and the signal path of the adapter is most likely a straight through. So that means when the TPA10 is connected, Siglent scopes disable the internal offset and only provide offset on pin 5. That sounds like a hack to avoid double offset problems with certain probes, but kills the offset completely for all probes that don't make use of pin 5.

2) There is no detection of the scale factor.
That one surprised me the most, as the metal ring around the BNC on the scope already does that.

3) There is no TekProbe 2 digital interface, so things like output impedance, probe names, probe termination, calibration factors etc. dont work.
Understandable, especially given their quite reasonable price for the adapter. But that would be amazing nontheless.

[tautech]

This issue is firmly in front of top ppls at Siglent in the hope they can get TPA10 working as it should however I have no idea what Tek active probes they have so to go sniffing the TekProbe I2C coms therefore any sniffings you can provide and post here for their analysis could be helpful to improve TPA10.
TIA

[FlexibleMammoth]

I made some calls to professors and lab engineers at our university. Unfortunately, across two campuses I found no one that still has a scope with a tekprobe interface - the only old scopes they had were before tekprobe.

For the moment, I do not have access to a native tekprobe device. I will report back once I find one, maybe they let me borrow it.

[FlexibleMammoth]

I have a Tektronix power supply/adapter for TekProbe.  The "offset" pot does not function with some of the Tektronix TekProbe probes.

Tim - which adapter do you have? A TPA-BNC , the 1103 standalone unit or something else?

[TimFox]

I have a Tektronix 1103 power supply/adapter and a Tektronix P6243 probe.
The 1103 does have an offset pot on each of the two channels, but I believe the P6243 does not use that analog voltage.

[FlexibleMammoth]

I have a Tektronix 1103 power supply/adapter and a Tektronix P6243 probe.
The 1103 does have an offset pot on each of the two channels, but I believe the P6243 does not use that analog voltage.

That is interesting - could it be that Siglent *did* implement the 1103 specs? Maybe a genuine 1103 psu only provides offset on pin 5, and the scope provides its internal offset as usual. The result is that you can adjust some probes at the 1103 and some at the scope.

If Siglent copied that Design, there is a problem: as the TPA10 shares an offset control with the scope, that knob will control  both the traditional frontend-style offset and pin5. Now if a probe supports both, then you end up with 2x the offset you select.

So maybe when the TPA10 is detected, they disable the frontend offset - not realising that there is a number of tekprobe probes that do not use the tekprobe-Style offset on pin 5 but rely on the scope.

In a real tekprobe scope there must be some way of addressing that problem… Or I am completely off the mark here.

(Of course there is a German word for the feeling of being wrong about something: „Ich bin auf dem Holzweg“ - literal translation „I am on the wood way“ :beer: )

[FlexibleMammoth]

While cleaning up my hard drive, I found that the Tek probe selection guide specifies what is TekProbe Level 1 and 2 (page 2).

My initial suspicioun was right - TekProbe Level 1 is just the metal ring around the BNC for detecting the attenuation factor.

So the 1103, and all active TekProbe probes, are TekProbe level 2.

Also:
- "Some probes require an external power supply (1103) when used with the TDS3000 series." (p.2)
- The P6243 offset is listed as N/A. (p.5)
- The ADA400A offset is listed as "±1 to ±40 V**" with the stars meaning "Based on Gain Setting" (p.9)
- The P6247 offset is listed as "depends on scope". (p.9)

I have a Tektronix 1103 power supply/adapter and a Tektronix P6243 probe.
The 1103 does have an offset pot on each of the two channels, but I believe the P6243 does not use that analog voltage.

That would explain this problem, too: P6243 does not support offset.

I have a Tek TDS3052, which may not support the TekProbe offset feature. From what I remember reading in catalogues, the P6243 single-ended FET probe, which does not have the offset feature, is recommended for the TDS3000 series, while the P6245 probe, which does support offset feature, is recommended for the TDS600/700 etc. I measured pin 5 as indicated by the attached diagram without a probe connected, and it stayed at 0V regardless of the setting of the offset control. The offset control also works with simple passive probes. Note that this offset feature is separate from the vertical position control. The way the manual describes it, it can prevent the scope from clipping a signal that's out of range, so clearly the offset is applied in the analog input circuit. As far as I know, it controls the offset voltage fed into the vertical amplifier. It might be that when it detects an active probe with offset feature, it supplies a voltage on pin 5 instead, but it's quite hard to measure while a probe is attached, since the case covers the area around the connector.

This is also consistent with the information in the probe selection guide.