Performing parametric measurements on digital modulation is often required to ensure reliable wireless transfer of data, such as secured financial transfers. One example gaining acceptance in today’s mobile technology industry is Near Field Communication (NFC). Most of today’s newer smart phones come standard with NFC technology. It won’t be long before our physical credit cards meet their demise and become a thing of the past — just like the VHS video tape. In the future — if not already — you’ll walk up to a payment terminal at your nearby supermarket, tap your smart phone on the payment terminal, and just like that the money’s gone!
Testing the analog quality of NFC digital modulation is possible with an oscilloscope. The key to performing automatic parametric measurements on digital modulation is to first strip the modulation away from the RF carrier. But let’s begin by taking a look at NFC communication between a payment terminal and a mobile phone as shown in Figure 1.
Figure 1. NFC communication captured on the scope.
The 13.56 MHz carrier and the first burst of digital modulation is generated by the payment terminal. The payment terminal periodically sends out “pings” or “polls” such as this to see if it can get a reply. The second burst of digital modulation with a much lower modulation index, is a reply from a mobile phone saying, “I’m here. Want to take my money?”, or something like that. Then there is additional handshaking that takes place after these first two bursts of communication before your money actually vanishes.
The NFC analog test specification calls for specific measurements to test the analog quality of both poller modulation (payment terminal in this example) and listener modulation (mobile phone in this example). Perhaps this is to ensure that a few extra zeros are not inadvertently added to your bill due poor signal quality. Let’s walk through making a few analog quality measurements on just the modulation generated by the payment terminal (poller).
The first task is to trigger on NFC communication. If you happen to own a Keysight InfiniiVision 3000T or 4000A oscilloscope, then you’re in luck. Keysight just recently introduced an NFC trigger option for these oscilloscopes. If you are using another model oscilloscope from Keysight or any other vendor, pulse-width trigger based on a specific time parameter along with trigger-holdoff might work for you.
The next task is to demodulate the modulated RF. For that we will first turn on the oscilloscope’s horizontal zoom timebase mode to zero-in on one set of pulses near the beginning of the first burst of modulation generated by the payment terminal. We will then use one the oscilloscope’s waveform math functions called “Envelope” to demodulate the captured RF waveform as shown in Figure 2.
Figure 2: Zooming in and using “Envelope” math function to demodulate the captured waveform.
The “Envelope” waveform math function is a frequency-domain operation based on a Hilburt transform. The purple trace in Figure 2 is the resultant envelope waveform, which closely tracks the positive extremes of the 13.56 MHz carrier. The reason that it appears noisy is because the Hilburt transform is based on both positive and negative extremes of the carrier. And in this case our RF carrier and modulation are not perfectly balanced. To remove the noise, we can apply a second math function (Low-pass filter or Smoothing filter) on top of the first math function (Envelope) as shown in Figure 3.
Figure 3: Filtering the Envelope waveform with a 5-MHz low-pass filter.
By applying the 5-MHz low-pass filter onto the somewhat noisy envelope waveform, we now have now successfully demodulated the RF waveform and are ready to perform some required parametric measurements to insure that poor signal quality doesn’t add zeros to our e-payments. One of the required measurements is the delta-time that modulation is below 5%. For that we can use the oscilloscope’s automatic negative pulse width measurement (-Width) based on custom measurement threshold settings as shown in Figure 4.
Figure 4: Using the scope’s automatic parametric measurements to measure the time below 5% modulation.
Although the oscilloscope’s automatic pulse width measurement is normally based on 50% measurement thresholds, most of today’s oscilloscopes will allow you specify user-defined measurement thresholds, such as 5%. In this measurement example we measured 2.17 µs. The NFC specification for this particular measurement is 1.8 µs to 2.3 µs.
Note that there are many other required measurements not covered in this article to ensure proper analog signal quality of NFC communication. Along with the ability to trigger on NFC communication, Keysight’s NFC option for InfiniiVision oscilloscopes also provides automated NFC test software for comprehensive testing of NFC-enabled devices. To learn more about testing NFC with an oscilloscope, see below.