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I've built a circuit that is going to supply 3.3v to a CEL Meshconnect Mini Zigbee module and now I want to test and verify. The only two things I know to look for is: -Make sure it's actually 3.3v -Find out how much ripple there is
Can any basic oscilloscope tell me how much ripple there is? How much ripple is too much for a Zigbee SoC? Are there any other factors(i.e. noise) that I'm not taking into account? Am I over thinking this?
It's more important to know how much current the module can draw. If your DC source isn't up to the task, the module will collapse the rail and all sorts of interesting behaviour can ensue (hiccupping, brown-out, etc.)
Industry practices for power supply ripple measurements must be done with a short probe (no long ground lead) or a direct-connect coax with proper termination, with 20 MHz bandwidth limiting turned on, and a combination of ceramic and electrolytic capacitors at the measurement point to attenuate CM noise and give you a 'true' indication of what the ripple is. In industry, it's common to use a 10 microfarad electrolytic capacitor in parallel with 100nF ceramic.
How much ripple your module can actually tolerate is hard to judge unless the manufacturer has specified it. Less is generally better when it comes to microcontroller-based gadgets, especially ones that do a lot of analog-to-digital conversion.
The ripple limit should usually come from the datasheet, manufacturer communication and/or your EMI requirements.
In order to determine how much noise you have on Vcc, do a popper measurement and read of the Vpp. Sounds simple, right?
The best paper I know of that describes how to do this measurement is this one: http://www.electrical-integrity.com/Quietpower_files/Quietpower-21.pdf
In short: Use a coax cable soldered directly to your board. Run the 50R coax into your oscilloscope set to 50R input impedance. Select AC-coupling. A bandwidth that is adequate (minimum 500 MHz). And infinite persistence.
If you make the measurement using a high impedance probe with a long "pig-tail" for ground - you may have extra noise not related to your Vcc noise picked up. When in doubt, always do the null-experiement: touch the probe tip to the ground point, so both tip and ground of the probe touches the same point on the board. If you don't get a flat line, something is being picked up by inductive coupling into the loop formed by probe and ground lead.
So do you have too much noise? Suppose the datasheet of this device calls for 3.3V +/-5% for the Vcc supply. That means you have +/-165mV as the limit. Let's assume you have a 2% accuracy of your DC regulator. And let's assume you have a 0-1% distribution drop in the connections between the regulator and the device (cables, connectors, traces, filters etc.). That leaves 2% to the AC-noise/ripple or +/-66mV (132mVpp).
If you are designing a supply for a daughter card module, you really should measure the voltage on the module using this principle. Note that you may very well find that the module was not well designed and there is nothing you can do on your board to fix it. That is one of the downsides of using modules.
CEL has an application note now: ZICM35xSPx Hardware Design Guidelines
It specifies +/- 0.3Vpp ripple.
