Missing Pulses

Hi there, I have been checking waveforms on a working SPI clock line with both a full sized Digital Storage Scope and my HPS140, The waveform is one shot every time an input to the circuit is applied. On the DSO I have 8 positive going pulses(1uS in duration) with a spacing between them of 1uS. When I connect my HPS140 (trigger=NORMAL,slope= +)to the same test point I have only 7 positive pulses being displayed, the first pulse (that triggered the HPS140) is missing from the display. From measurements taken there is a delay on the HPS140 between triggering and the start of the display of aprox 1.75uS. My question is:- Do I have a faulty HPS140 or are they all like this ?

This scope is not equipped with ‘pre-triggering’.
Triggering is performed in software, so there are some instruction cycles between the trigger event and the displaying of the samples.

Mmm— that was not the answer I was hoping for, as it means that the HSP140 is not realy suitable for acurate measuring of events, even at fairly low frequencies. So trying to use it at MF and HF would be a no go area due to the inbuilt delay, I therfore question your advertising the HSP140 as suitable for professional and educational use.!

At the risk of offending you, and the wonderful people here at Velleman, I think you are asking quite a bit from a $150 (street price here in the US) pocket oscilloscope.

For an audio system pro working in the field the HPS140 has a lot to offer, however if my “professional” project were working on an SPI clock with 1us bursts I would have sought something more capable than what could be classed as a toy oscilloscope.

I agree that the lack of any pre-trigger capture ability and the inherent post-trigger delay should be made clear in the specifications, but for the price and diminutive size it is a very capable instrument–and cute as a bug’s ear to boot…

Hi there cliffyk, No offence taken, it’s just that if the specifications had made it clear that the triggering was software controlled then the HPS140 would not have been selected for the project I am working on. In all other respects the HPS140 is very handy to have around.

I understand, I have had that same thing happen more than once.

Over the years I have come to find that the cost of appropriate test equipment is sort of like 'scope bandwidth–figure out how much you would like to spend, then multiply it by 5…

I like the idea of *5 but it only works if you have a pocket big enough.

I am thinking of buying a HPS140 and wish to use it to measure single “square” pulses, but after reading this post I am not sure if it is the right tool.
If I only record the last ~90% of a pulse, can I make a reasonable estimate of the missing front? I do not need absolute 100% accuracy and am on a limited budget.
or, can I make a longer recording including the pulse and then “zoom in” on it?


Can you provide more info regarding the signal you wish to look at.
This will allow us to determine if the scope is suited for the job.

I am working on a hobby robot type project.
At the moment I am wanting to track a say 5ms, pulse from a pic, through transistors and to measure the current (across a resistor) of the high power pulse from capacitors to a solenoid. At a later stage I will be wanting to measure pulses from sensors and differentiate between pulses around 500usec long to an accuracy (well resolution) of say 5usec.
One day I may buy a fully functional scope, but I am hoping this will get me started, and be a handy tool in the future.

This should not be a problem.

Use an external buffer

Talk about missing pulses I’m glad I found this post. I was out trying to troubleshoot a hall effect flow sensor running at 7 hz I thought the sensor was bad but it turns out its how the scope triggers!

Hi jord4231, The missing pulses I had problems with were at a much higher frequency than the sensor
you refer to, but if you want all of the pulse for measurement then you will need a scope that has memory
before triggering, this little one has no facilities in that way.
Regards Stevey

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Thanks for the reply Stevey, its strange how it only misses them on some time bases. All the same I have plenty of scopes around but it might be time to upgrade my portable one :slight_smile: