Choosing Oscilloscope for Measuring Current

[Madhav Aggarwal]

Hello,

We are developing a wearable device with NRF52 as a Beacon with CR2032 coin cell battery. We need to measure current profile for the beacon and calculate corresponding battery life. For this we also referred to  Measuring current chapter in the nRF52 DK documentation. Now, in order to measure current profile as described in documentation, we are looking forward to purchase a digital oscilloscope with following configuration:

Hantek 6022BE: Bandwidth: 20MHz; Sample Rate: 48MSa/s; Vertical Resolution: 8bit

Can you please confirm whether it would serve our purpose as we are tight on budget to purchase expensive one. Or, maybe you can suggest some cheap alternative to it.

Thank you,

Madhav

[Madhav Aggarwal]

Yes, I have checked Nordic Power Profiler Kit, and it looks quite promising, but I am specifically looking for an inexpensive oscilloscope. I am in a fix now whether to go for 48MSa/s scope which is only $84 on Amazon and portable too, or shell out more money to get Rigol.

It would be great if you can please clarify me on trade off between the two. 

[haakonsh]

For an oscilloscope it's quite slow, but it's more than enough to capture the current consumption waveforms of low powered devices. I think the size of the internal memory buffer is more important with regards to current consumption measurements. 

At 48Msps and 1Ms of memory you've got 1 second/48 = 20.8ms capture window. You will often want to trade sample rate for longer capture window, so a higher sample rate is not always better. 

What is the maximum sample rate that you can stream via USB with this device? 

[Madhav Aggarwal]

Thank you for providing a deeper insight. It can operate at 48MSa/s rate via USB 2.0. Is 20.8ms capture window sufficient for our purpose? 

[AmbystomaLabs]

Since we are talking about buffer size, the Rigol has a 12Mpt buffer vs. the Hantek has only 1Mpt.  So, for the same sweep settings the Rigol will have a 250mSec capture window vs. the Hantek's 20.8mSec,  Yep, over 10 times bigger.

Based on web reviews the Hantek seems to be popular with automobile mechanics.  If you are determined to have a USB device,  there are very good USB o-scopes out there, much better than the Hantek.  I would choose a dedicated device though, with its own monitor.

[AmbystomaLabs]

If your absolute limit on price is under $100 you should thumb around on Ebay first.  There are often a lot of reasonable quality devices like the Hantek that may have better specs.

I tend to think of all my tools as investments. Either I am saving time, money, fostering my career or all of the above.  I assume you will want to do more with this scope than measure BLE power consumption.

The problem you will run into with a 48MSa/sec scope is that even things at Pclk speeds (16MHz) will show up as a sine wave on it simply because the nyquist frequency is only 24MHz. Trying to get a scope to lock onto a sine wave is fraught with difficulty unless the amplitude is really steady.

However, on a reasonably fast scope such as the Rigol, a 16MHz signal shows up as a nice square wave since its nyquist region comfortably holds 31 harmonics of the signal.  Suddenly measuring the phase difference between two 16MHz signals is easy.  And frankly 16MHz clocks are pretty slow these days.  Likely you will soon want to work with faster signals and processors.