Computing stuff tied to the physical world

Pushing the scope limits

In Hardware on Jan 17, 2012 at 00:01

After having become reasonably familiar with the new Hameg HMO2024 oscilloscope, I wanted to find out just what its limits are. Came up with this screenshot:


There’s quite a bit of information in here.

First of all, these scopes can do double-rate sampling when you don’t activate all the channels. Channels 1 and 2 share some hardware, and so do 3 and 4, so by enabling only one of each block, the scope can actually double its sampling rate to 2 GSa/s i.s.o. 1 GSa/s and also “stack” its memory depth to 2 MB per channel, i.s.o. 1 MB. Or to put it another way: a 4-channel oscilloscope can be used as a 2-channel unit with twice the specs.

I switched the acquisition to display in “sample-and-hold” mode instead of the normal Sin X interpolation mode, so the steps shown above represent the real sampling that’s taking place. As you can see, there are 4 steps per division, with 2 ns/div, so that’s indeed 2 billion samples per second.

The top was zoomed out as far as I could while still showing the fastest supported 2 ns divisions in the detail window, which ends up being 50 µs/div. Running the scope timebase any slower will cause it to reduce its sample rate so it can still capture the required 12 horizontal divisions full of sampling data. In this case we see 12 x 50 µs = 600 µs of data on the screen, while the detail view is zoomed in to 2 GSa/s (25,000x).

That’s 1.2 million samples of data – a hefty pile of bits flying around in this thing while it’s running!

But there’s actually more, because of the stacked memory. When I pan across the 50 µs/div display, I can see that data was collected from 575 µs before the trigger point to 525 µs after the trigger point. Which is 1100 µs of collected data, IOW that’s 2.2 MB of acquired sample data – again, more than the specs in the brochure!

The vertical signal acquisition hardware is also very impressive. This is one of the few scopes I know of which will go down all the way to 1 mV per division. That’s real sampling, not some sort of digitally enhanced sensitivity, as you can see from the fact that the data steps really are 1 pixel in the vertical direction.

It might not seem important to go down that low, but it’s quite useful actually when measuring voltage drop over a shunt resistor. Which is what I’ve been doing quite a bit to display current usage of JeeNodes while in ultra low-power sleep. The other benefit is that you can still go down to 10 mV/div with the standard 1:10 probes that come with the scope (I’ve got a few 1:1/1:10 switchable no-brand probes for when I really need the extra sensitivity).

The red line is created by using the “Quick Math” function, subtracting the channel 3 signal from channel 1 in this case. The nice thing is that the math function supports 20x more magnification than the input channels. So with some trickery, this scope can display down to 50 µV/div: subtract a spare channel set to “GND” from the input signal, and magnify the resulting math output 20x. It’s a coarse (digital) way of magnifying the display, but still.

The actual data shown above is what the scope displays with no probe connected, BTW. This is the residual noise in the scope’s input circuitry and it really is impressively low-noise – just a quarter millivolt peak-to-peak.

At the other end of the range, the scope will go up to 10 V/div, i.e. 80V full scale. That’s ≈ 6 orders of magnitude of usable range on each channel. Wow… Hameg (and R&S) did some pretty serious engineering to achieve this.

PS. I’m ready to ditch my DSO-2090 USB scope – it’s unlikely that I’ll use it with this HMO2024 now at hand.

Update – same for the Logic Analyzer – a LAP-16032U, if you’re interested in it, let me know.