Test and Measurement Forum

Hi!

I got some problems with the Trigger of the PS3205.

I used different signalgenerators, and different PS3205 and I got always the same prob...
I use a direct connection from the signal generator to the scope. I use Channel A or B Trigger Mode. There is no external trigger connected to the scope.
The scope switches between "Falling" - or "Rising" Slope. PS3205 can't trigger the signal from the signal generators. it's just a simple Sine Signal, nothing special. Is this problem known?
Could you help me out, please?

Cheers Carsten

There is an attachment eclosed, where you can see that the trigger doesn't know whether it's a rising or falling slope. Here I used Channel B with the same configuration.

Hi and thank you for your post.

I cannot understand your problem, you say a sine wave yet all I can see is a triangular wave. Could you clarify your problem.

Kind regards
Ziko

Hi Ziko!

Thanks for your answer! Sure I will give it a try...
It's just a simple problem, but hard to explain...

First of all, I tried several functions. It doesn't matter if I use Triangular or Sine or any other.

My problem:
#1 I try to measure a simple signal (i.e. Rect, Sine or Triangular Wave). Doesn't matter what kind of wave it is!

#2 Upper part of the picture
You can see in the upper part of the picture a Triangular Wave. It is triggered with the "Rising" Slope. Ok? So I got the upper wave and that's ok for me. But...

#3 Lower part of the picture
In the lower part of the picture you can see the same Triangular Wave with the same configuration of the scope. It is triggered with the "Rising" Slope, too.
The displayed wave is now different from the upper once, but it should be the same! Do you understand?

The displayed wave changes at random between the upper and lower wave during my measurement. So I can say that Trigger Option "Rising" or "Falling" is out of order.

In addition:
If I use another analog or digital scope the upper wave (upper pic) is triggered with "Rising" slope and the lower wave (lower pic) is triggered with "Falling" slope. But PS3205 changes at random, doesn't matter if I choose "Rising" or "Falling".

Cheers Carsten

Is the 'scope triggering on *single* samples that go the "right" way? On my ADC200, I have a similar problem whereby the hardware triggers on single samples that, due to noise, are of of the correct type (ie, rising or falling). This happens even if a *single* sample is "wrong". Try zooming in at the trigger point and see if this is the case.

Note that in my case, only one out of, say, half-a-dozen samples triggers incorrectly - *most* are correct.

I've asked Picotech to kludge their way around this in the software by merely testing the direction of slope over more than one sample and ignoring those that are "wrong". So far, though, I've had no luck with this request...

Regards,

Mark Davis.

Hi Mark,

I asked for support, but they can't help me out!

I got from the support something like this:
Zoom in at the trigger point, there could be "a lot of noise". That could be the problem.

I know thats the problem!
I only connect the function generator to the picoscope without an DUT and get these trigger probs.
Like you said "most" of them are correct, but I need more than that, because I use picosope for an automatic measurement system.

Due to my pressure of time, I programmed a "trigger correction" for myself to hide the faults. I did it the way you were talking about. I take two samples and test the trigger direction. If it's ok, it will be displayed on screen, if not it will be hidden. It's not satisfying for me but it works for the moment!

Probably the will fix the problem sometime in their drivers...

Thanks for your post!

Cheers Carsten

Hi yes, this is correct due to the noise on the signal and the hysteresis level it does trigger however not as you think. If you zoom in you will see it triggering.

Kind regards

> it does trigger however not as you think

So what, *exactly*, is the hardware looking for, ziko? As I understand it, *any* noise - just a single "bad" sample is enough - will cause false triggering. In other words, the signal must be monotonically rising or falling at the trigger level. Is that right?

> Probably the will fix the problem
> sometime in their drivers...

As I said, it strikes me that a relatively simple kludge in PicoScope would bodge a "fix" - at least, "for now". I dunno about you, "cpinkle", but I find this problem a right pain in the bottom. (Luckily, I don't use my ADC200 *professionally*; I merely "play" with it.)

Regards,

Mark Davis.

but I find this problem a right pain in the bottom.

Me, too!!! A scope mustn't be confused by just "one bad sample" something like that. IMHO, it's a relativly simple but heavy problem of the driver. But I just fixed it for myself, because the support didn't solve my problem in time and I had to meet a deadline!

@Ziko:
Are Mark Davis and I the only one who recognizes these probs? I can't believe that... It's a basic feature of a scope and should work properly!?

Mark and Carsten are correct in that noise is causing the scope to appear as though its triggering on the wrong edge. We have identified this problem before and we have addressed it using something called hysteresis in our more recent products.

A good description of how hysteresis works can be found in section 7.7.2.1 of the PicoScope 6 help file which can be found here http://www.picotech.com/document/pdf/psw6-en.pdf

I'll briefly summarise the hysteresis implementation in our scope series below

The ADC200, ADC212 and ADC216s do not have any hysteresis at all.

The PS3204, PS3205 and PS3206 have hysteresis implemented in hardware where the exact value is dependant on the range but is typically around 2% of the input range.

The PS3223, PS3224, PS3423, PS3424 and PS2202 have a hysteresis implemented in software where the exact value is typically 5% of the input range.

The PS2103 and PS2104 have a hysteresis implemented in software where the exact value typically 2% of the input range.

The PS5203 and PS5205 have user configurable hysteresis (via advanced triggering) which is obviously ideal for any amount of noise on your signal.

User configurable hysteresis would be the ideal situation for Mark's trigger problems and is something that we are considering back porting to our PS3000 series; unfortunately there are no timescales on this.

cpinkle wrote:

but I find this problem a right pain in the bottom.

Me, too!!! A scope mustn't be confused by just "one bad sample" something like that.

This depends on what you want to do. In some scenarios I would want exactly that.
In any "real" scope this must be user-adjustable. So let's consider the 3000 series to be expensive beta test devices.

Hi,
The 3223, 3224, 3423 & 3424 all now have advanced triggering in PicoScope6 (6.0.12.2). This includes variable Hysteresis. The 320x series scopes do not at present but this might change in the future.
PeterF.