Zoom ExpectationsWhen using a 12-bit DSO, some people tend to get enthusiastic and expect miracles. Maybe we all should come back to earth and ask ourselves what we can realistically expect.
There’s the zoom, where the SDS1000X HD has been shown to exhibit some Sinc artefacts at extreme zoom settings. But shouldn’t we first of all get a feeling what a realistic zoom factor is?
Consider a signal that has to be viewed at a vertical gain of 1 V/div in order to fit on the screen:
SDS824X HD_PR_H50ns_Stop
Now we want to zoom in, e.g. 5x:
SDS824X HD_PR_H50ns_Stop_Zoom5
We already see a bit of noise creeping in. Apart from that, we should determine how much zoom is feasible, before we try to zoom in any further:
Just like the SDS800X HD, an SDS1000X HD will have 480 LSB (aka codes) per division. Consequently, 20x zoom is about the sensible limit, because then we get 24 LSB per division – and with this, there would still be a chance to see something meaningful. 20x zoom means 1 V / 20 = 50 mV/div:
SDS824X HD_PR_H50ns_Stop_Zoom20
Now the noise is stronger and it already gets hard to spot any details. Yet we can take it to the extreme and try 100x zoom:
SDS824X HD_PR_H50ns_Stop_Zoom100
We are now at 4.8 codes per division and what we see is just (mostly granular) noise – all this has nothing to do with the real signal anymore. Most obvious when viewing it in Dots mode.
SDS824X HD_PR_H50ns_Stop_Zoom100_Dots
We can take it one step further and zoom in horizontally as well (while still in Vectors mode), so that it becomes less obvious that we’re actually looking at pure noise:
SDS824X HD_PR_H2ns_Stop_Zoom100
As can be seen, the SDS800X HD doesn’t show any Sinc artifacts, yet the much more important insight should be that it is completely irrelevant what a DSO shows at such extreme zoom levels, where we see nothing but noise anyway.
Of course, we could also do it the correct way. Whenever we need to use some extreme zoom, then we also need a means to
a) Increase the vertical resolution
b) Reduce the noise
The tool of choice for repetitive signals is Averaging, because it increases the vertical resolution, suppresses any modulation (hence also noise) and doesn’t affect the bandwidth.
Even at the extreme 100x zoom setting we can get the following:
SDS824X HD_PR_H50ns_Stop_Zoom100_AVg1024
We leave the input channel at its original gain of 1 V/div, but set the final time base while still in Run mode and set up a math trace with averaging, which can be displayed at any vertical scale, i.e. also 10 mV/div for a 100x zoom. When the desired number of averages has been processed, we can stop the acquisition, yet this is not required, as the input channel is left unchanged anyway. In this example it is 1024 averages shown at 10 mV/div, hence a 100x zoom again.
Now compare this with the previous screenshots. This one now has much less curves and kinks than even the pointless capture at 2 ns/div before, proving that it’s just nonsense (=noise) what we get at such zoom levels without proper averaging.
Of course, 1024 averages are quite a lot. In theory, it would enhance the resolution by 10 bits, making for a total of 22 bits. The current platform doesn’t support sample data and digital signal processing results at a higher resolution than 16 bits, so this is what we get as soon as we use 16x or higher averaging. The ENOB on the other hand benefits a lot more from this, even though it’s limited to 16 bits as well. But it starts at just 8.4 bit according to the data sheet, and 1024x averaging will increase this by 5 bits for a total of ~13.4 bits.