This video is part three of three part video series on air fuel alliance and
magnetic charging measurements using a scope based on magnetic resonance which
operates at 6.78 MHz. Now this particular video- part three- is focused
on making power and efficiency measurements PTU, PRU
sometimes called resonant coupling efficiency or RCE. Now frankly these
measurements are not included in the conformance test specification
determining deficiency in this type of system is usually done with the network
analyzer based on S parameters and impedance measurement and in fact,
Keysight has network analyzers with the built-in option for automatically determining
efficiency based on various parameters. But what I've learned from talking to
engineers that are designing these systems: once they get a system up and
running, they definitely want to use the scope as more as it reality check to
determine what is the power quality both their PTU and PRU, as well as
determine what the efficiency is. So they could then make tweaks and then make improvements.
Now the most important thing you need to do, when making either
power or efficiency measurements, is you must first be deskew your probes.
You are typically using or should be using a differential active voltage probe as
well as a current probe in order to measure power. Now there can be a delay
difference between the voltage probes and the current probe, and that must be deskewed.
Keysight offers a deskew board where you can automatically deskew your probes, but I have something better.
Simply a 10 ohm resistor. The scope actually has a built-in
waveform generator and we can produce a 6.78 MHz sinewave and calibrate it
based on that signal and this simple, inexpensive 10 ohm resistor actually does
a better job than the deskew board. Let's deskew our probes now.
The first thing I'm going to do is connect my deskew fixture, my 10 ohm resistor, to the output of the generator
I already have my generator set up for a sinewave at 6.78 MHz and the maximum output
swing which is in this case 10 volts peak-to-peak.Next, I'll connect my
current probe around the resistor and then connect the voltage probe across the resistor.
The scope, if it has a built-in power app that's an option on the scope, go into the analyze menu,
select power application, and then we have a list of measurements that can be
performed automatically. Now this was optimized for switch mode power supplies
but we can use it on magnetic resonance charging signals. I'll select power quality
go into the signals menu, here we show a diagram of a switch mode power supply,
you can ignore that. Assign my voltage probes to channel one,
current probes channel two, and over here is says deskew.
This is where I can do the automatic deskew. Here is shows a block diagram of the deskew, again all ignore that
since I'm using my 10 ohm resistor, then just press auto deskew.
It will automatically optimize vertical scaling of both the current waveform, the voltage waveform,
and then perfectly align the signals. In this case, we're measuring a skew of 5.48 ns.
Now we didn't perform that measurement with this deskew,
we would get significant error, and I'll show that later when we begin to make efficiency
measurements. I'll pull that deskew calibration back out and you'll see the difference
So let's get started making power quality measurements.
We're going to begin with a power quality measurement on the PTU, or the transmitting coils. Begin by connecting
your current probe to the PTU coils as well as the differential voltage probe across them.
Let's begin. I'll select what is called the analyze menu, and go into the
power application. These are all the automatic
power type measurements we can perform. This was optimized for switch mode power supplies,
but just a few of them we can use on magnetic charging.
We're going to select the power quality, go into the signals menu. Here's we can assign which
channel is the voltage. In this case, we can use the default channel one current
is channel two. The default number of cycles it performs a measurement across
is two in this case, which is not good for what we're doing.
Let's select about 20 cycles to perform the measurement cross, and then simply press auto setup.
This will optimize the scaling of both the voltage and current waveform and turn on the power waveform, which is
instantaneous voltage times current, and that is the purple waveform.
Then I'll press Apply and here we see automatically all our power measurements:
power factor, real power- this is the one you're probably most interested in-
parent power, reactive power, press factor of voltage, press factor for current,
phase angle- in this case is about 33 degrees, it turns on a DC RMS, you may not care about that one- and frequency.
Now we can add to it a countermeasurement that will give us a more
accurate measurement of the frequency. Let's do it on the current channel,
move that over to the side, so there we have five digit frequency measurement and we may
want to add a current measurement. RMS current measurement, select AC RMS in cycles, and there you can see we're measuring about 516, 517 mA.
Now if we want to perform the same measurement on the PRU, again we can
go back into that menu, and in this case I have my channel three and channel four
connected up to the PRU. Select three for voltage, four for current, auto setup.
Then apply. And there we have power on the PRU.
Probably the last measurement and maybe the most important one is the efficiency measurement.
So let's select efficiency, AC to AC, go into the signals menu, you find my input voltage is channel one, and for the current is channel two,
output voltage channel three, output current channel four,
duration 100 milliseconds. Again, this was optimized for a switch mode power supply.
I'm going to change that to about four microseconds. That's the time span that we will be making
a measurement across, then press auto setup. This auto setup takes a little longer, its got to optimize
four input channels, input voltage, input current, output voltage, output current, internal
two different power waveforms- one for the PTU, and one for the PRU.
Then we'll select apply. There you can see our input power, output power, and efficiency. Now you may
notice the input power is lower than what we were measuring when we did the
PTU power quality measurement. That's because again this measure was optimized
for switch mode power supplies it automatically turns on bandwidth limit
on all channels so we need to turn that off and we'll probably see it bump back up to
where we saw it before. So we're measuring about 70 percent efficiency.
Now, if you zoom in
you'll see I'm using the arbitrary waveform generator, and I've added some noise to the signal
to make it a little more realistic. If you want to make it more stable you can
turn on waveform averaging. Let's do that. Back it up to about 32, now we can get a more stable measurement.
Remember we deskewed the channel one and channel two voltage and current probes
and it was a deskew factor of 5.48 nanoseconds, I'm going
to remove that now, so remember, we're measuring- says about 69%-
I'm going to go into the channel two menu, which is where the deskew factor is, and I'm going to plug in
0 nanseconds. Notice it jumped up to 84%.
So if you don't deskew your probes, you're not going to get accurate measurements.
Thanks for watching this video on making power and efficiency measurements and a magnetic resonance system based on the
airfield line standard. We have two other videos, and you haven't watched them, I encourage you
to go watch those, part two especially on beacon timing current frequency
measurements. We also have three application notes that go right along
with these three videos. You can download them at the URL listed on your screen
and I thank you for watching this video