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A High-Performance Open Source Oscilloscope: development log & future ideas

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tom66:
One other thing:-

The present prototype when in 'Auto' memory depth (which is currently the only memory depth exposed to the user and otherwise behaves similarly to the Rigol DS1000Z 'Auto' function) uses all available RAM as a history buffer. With 256MB of RAM and at 50ns/div (~23k wfm/s, 610 pts), this gives approximately 17 seconds of history buffer that is recorded in real time.  In my mind, this is far more useful than any infinite or variable persistence feature, and as far as I can tell, only Siglent expose this in normal use - which led to Dave complaining about it as it was turned on by default.    As far as I can see, there is no reason not to enable this function by default, as it is just a case of walking through memory pointers.  If the user selects a larger memory size, then the instrument will have less record time, but should always have the amount of memory available that the user requests.

Most people know this function as segmented memory.  The only difference is it's a continuously active segmented memory function, which adapts to current settings to make the most use of the memory available.  It avoids that headache of pressing the 'STOP' button and missing the trigger by a few milliseconds.

This is one time the user might want to turn down the waveform rate as e.g. reducing the update rate to 1k wfm/s would increase the memory time to over 6 minutes.  Giving the user that trade off is valuable (this is pretty much always found on scopes with segmented memory).  Depending on the future platform choice,  I expect a later version of the scope to support at least 1GB of RAM which would give around 900 Mpts of usable waveform memory.  So at 23k wfm/s, instrument could record ~1 minute of waveform history and select any one of those timestamped frames or analyse any single given capture.

nctnico:

--- Quote from: tom66 on December 06, 2020, 09:29:59 pm ---One other thing:-

The present prototype when in 'Auto' memory depth (which is currently the only memory depth exposed to the user and otherwise behaves similarly to the Rigol DS1000Z 'Auto' function) uses all available RAM as a history buffer. With 256MB of RAM and at 50ns/div (~23k wfm/s, 610 pts), this gives approximately 17 seconds of history buffer that is recorded in real time.  In my mind, this is far more useful than any infinite or variable persistence feature, and as far as I can tell, only Siglent expose this in normal use - which led to Dave complaining about it as it was turned on by default.    As far as I can see, there is no reason not to enable this function by default, as it is just a case of walking through memory pointers.  If the user selects a larger memory size, then the instrument will have less record time, but should always have the amount of memory available that the user requests.

--- End quote ---
There are a few remarks to be made here:

1) Siglent and Lecroy scopes only capture enough data to fill the screen regardless the memory depth the user selects. This is wrong for a general purpose oscilloscope. It simply doesn't suit all use cases.

2) Having a history buffer running in the background is standard on Yokogawa and R&S oscilloscopes as well. The memory left after the user's memory depth selection (which can be set to auto meaning to use just enough memory to fill the screen) is used as a history buffer.

3) Segmented recording is close to history mode but the user selects a specific record length and number of records instead of the oscilloscope doing this automatically. The distinction is between the oscilloscope determining something automatically versus the user being very specific in order to tailor the oscilloscope configuration to a particular measurement. Having a history buffer with 100k segments while the user is only interested in 5 is counter productive.

4) Variable and infinite persistence are required on a DSO. I regulary use infinite persistence for tests which take hours to weeks. I just want to see the extends of where a signal goes (and it doesn't need crazy high update rates).

Another nice feature to have is detailed mask testing. Again it seems oscilloscope makers aim for high update speeds but in doing so they throw the baby out with the bathwater. To give an example: I have a product which outputs a low and high frequency signal during several seconds. A 10Mpts oscilloscope can sample this signal with enough detail however it turns out that mask testing seems to use peak-detect and decimates the data to a couple of hundred points. It would be nice to be able to compare traces with a length of 10Mpts (or more). It doesn't matter if it is slow; it will always be faster and more accurate compared to checking a signal visually.

2N3055:

--- Quote from: nctnico on December 06, 2020, 09:47:36 pm ---1) Siglent and Lecroy scopes only capture enough data to fill the screen regardless the memory depth the user selects. This is wrong for a general purpose oscilloscope. It simply doesn't suit all use cases.

--- End quote ---
Nico,
we keep geting back to this, and every time I read this definition of yours, I don't know if you have problem explaining it or have misunderstanding how it works (which I, honestly think you don't).

I think best way to explain this is to try call it that LeCroy is sample rate defined, sample buffer length is calculated in time (not samples) and it is same as displayed time base, with defined maximum.
That means it will keep sample rate and retrigger rate as high as possible at all times, until it reaches max memory allowed, and only then it will start dropping sample rate.

That is very good strategy for general purpose scope because it maximises retrigger rate, and captures only data needed for time span we are interested in. It is simple to think about from operators standpoint: I have 120ns of data. It was taken at 5GS/s so I know there is no aliasing on my 200 MHz signal...

It is not so good for FFT, where we want exact control over sample buffer size and sample rate...

nctnico:

--- Quote from: 2N3055 on December 06, 2020, 11:16:00 pm ---
--- Quote from: nctnico on December 06, 2020, 09:47:36 pm ---1) Siglent and Lecroy scopes only capture enough data to fill the screen regardless the memory depth the user selects. This is wrong for a general purpose oscilloscope. It simply doesn't suit all use cases.

--- End quote ---
Nico,
we keep geting back to this, and every time I read this definition of yours, I don't know if you have problem explaining it or have misunderstanding how it works (which I, honestly think you don't).

--- End quote ---
Let's keep it at me not being able to explain it.  8) I know perfectly how it works and why it is bad in which situation. It is based on my own hands-on experience; I have owned a Siglent oscilloscope in the past and also own a Lecroy oscilloscope (I don't think there is any DSO brand left from which I have not used/owned a DSO myself; yes including Picoscope).

tautech:

--- Quote from: 2N3055 on December 06, 2020, 11:16:00 pm ---
--- Quote from: nctnico on December 06, 2020, 09:47:36 pm ---1) Siglent and Lecroy scopes only capture enough data to fill the screen regardless the memory depth the user selects. This is wrong for a general purpose oscilloscope. It simply doesn't suit all use cases.

--- End quote ---
Nico,
we keep geting back to this, and every time I read this definition of yours, I don't know if you have problem explaining it or have misunderstanding how it works (which I, honestly think you don't).

I think best way to explain this is to try call it that LeCroy is sample rate defined, sample buffer length is calculated in time (not samples) and it is same as displayed time base, with defined maximum.
That means it will keep sample rate and retrigger rate as high as possible at all times, until it reaches max memory allowed, and only then it will start dropping sample rate.

That is very good strategy for general purpose scope because it maximises retrigger rate, and captures only data needed for time span we are interested in. It is simple to think about from operators standpoint: I have 120ns of data. It was taken at 5GS/s so I know there is no aliasing on my 200 MHz signal...

It is not so good for FFT, where we want exact control over sample buffer size and sample rate...

--- End quote ---
Maybe just maybe he will one day understand just why these different strategies are used but maybe not as wfps has never been of high concern for him....no guesses as to why.  ::)

3 choices, ASIC, ADC allowing for large captures and ADC with optimised wfps...pick your poison and understand its limitations.

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