The objective of this activity is to create a receiving antenna using a coil connected to the input of an oscilloscope through a cable and observe electromagnetic signals produced by various electrical devices.
As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current –V is added as in CA-V or when configured to force current / measure voltage –I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage –H is added as CA-H. Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.
Types and sources of EMI
There are two main types of EMI, conducted and radiated. Conducted EMI is noise or interference that passes over wires and cables from a source to the receiver or “victim” circuit. A common conduction path is via power lines. AC power lines are a major source of EMI due to spikes and noise generated by a wide range of devices connected to the power lines. Power connections from switch mode power supplies to circuits as well brush type motors are good example sources. A huge amount of conducted EMI is passed through poor grounds.
Radiated EMI is the wireless transmission of signals from source to victim. Capacitive and inductive coupling from one circuit to another is one type of radiated EMI. This type of coupling is often called near-field interference. One example is crosstalk from one wire to another in a cable. All closed-loop circuits carrying current generate a magnetic field that can potentially induce a voltage into an adjacent circuit. The higher the frequency of the signal the more likely it is to radiate.
Any kind of noise is also a form of EMI. Noise can be generated either externally or internally. Manmade noise, coming from sources like fluorescent lights and auto ignitions, are particularly bad types of external noise. Spikes on AC power lines caused by switching loads such as motors, contactors, or relays turning off and on are typical examples.
ADALM1000 hardware module
2 12 inch / 30 cm long 3 wire female to female header jumper wires (should be included with ADALM1000)
From ADALP2000 parts kit:
Male to Male jumper wires
Male to male header pins (gender changers)
1 10 mH Inductor (103)
1 1 mH Inductor (102)
1 100 uH Inductor (101)
In this part of the activity we will be using magnetic coils to detect varying magnetic fields though inductive coupling. Connect 10 mH and 1 mH coils to the two scope input channels on the ADALM1000 as shown in figure 1.
Figure 1, Making and connecting the “antenna”.
The AWG channels should be configured in Hi-Z and Split I/O mode. Turn on the channel A and B scope traces. The signals seen will likely be rather small so the vertical range settings should be 10 mV/div or lower. You may wish to adjust the vertcal possition as needed to center the traces on the screen grid. The horizontial time/Div should be adjusted as needed to view the various waveforms you pick up with the coils.
The various electrical devices in the room (and elsewhere in the building) produce electromagnetic signals. Identify at least two different signals with whichever antenna works best. Hints: look for frequencies under 100 Hz and above 20 kHz. Some noise signals observed will be sinusoidal and some will be a series of pulses.
The following two scope screen shots are included here as examples of the kinds of signals you might pick up with the coils. The first one in figure 2 is with the two coils set next to or on top of an external powered USB hub. The second in figure 3 is with the two coils set on top of the keyboard of a lop-top computer. The amount of “activity” seen in the signal can be a different depending on if you are typing or using the touch pad device.
Figure 2, 10 mH and 1 mH coils next to powered USB hub.
Figure 3, 10 mH and 1 mH coils on lap-top keyboard
One potential source of EMI radiation is the M1k PC board itself. The various integrated circuits and passive components on the underside of the board will produce high frequency electric and magnetic fields. Use the 100 uH coil from the parts kit as the pick-up “antenna”.
Flip the M1k board over so the component side is facing up as shown in figure 4. Hold the pick-up coil vertically directly over various places on the PC board and observe the amplitude of the signals you pick up. As shown in the figure a couple of the good places to see large EMI signals is near the switching power supply regulator sections especially the inductors. Another place you might pick-up a strong signal is over the main crystal oscillator for the micro-controller.
Figure 4, Under side of M1k board facing up.
For example figure 5 shows the more than 100 mV p-p signal seen when holding the 100 uH coil directly over the master crystal oscillator Y1.
Figure 5, 100 uH coil held over Y1 Crystal Oscillator.
Why don’t you pick-up any strong signals in the analog input and output section of the board?
You can also try using the other higher inductance 1 mH and 10mH coils. Do they pick-up the high frequency EMI signals as well? Why or why not?
Up until now we have been using magnetic coils to detect varying magnetic fields. Now we will be using just an open (un-shielded) wire and the very high input resistance of the scope input to detect a changing or varying electric field through capacitive coupling.
Figure 6, Connect electric field pick-up wire to Scope input.
The AWG channels should be configured in Hi-Z and Split I/O mode. Turn on the channel A scope trace. The signals seen will likely be small so the vertical range settings should be around 50 mV/div. You may wish to adjust the vertical position as needed to center the trace on the screen grid. The 1 Meg resistor connected to the +5 supply rail should center the signal near 2.5 V because of the internal 1 Meg resistor connected to ground on the AIN pin. The horizontal time/Div should be adjusted as needed to view the various waveforms you pick up with the wire, 5 mS/Div is a good place to start to observe 60 Hz AC power line interference.
To detect the electric field surrounding 60 Hz power lines, hold the long header jumper wire near a power extension cord. Be sure to not touch any of the header wires to any bare conductors associated with AC Mains power!. To pick-up the biggest amplitude signal hold the header wire parallel and close to the extension cord. The longer the wires are parallel to each other the more the AC electric field will be coupled.
Shown in figure 7 is the waveform detected when the header wire is held parallel to an AC power extension cord. The frequency is 60 Hz as expected. The amplitude is about 300 mV p-p. We know that the voltage amplitude peak to peak of the 120 V AC power line is:
Or 336 Vp-p, so the coupling factor is about 1/1000.
Figure 7, Electric field around AC power cord.
a) Describe the “antenna” you are using.
b) Qualitatively describe (sketch or annotate a screen capture if you wish) two signals you observe. Give their characteristic frequencies.
c) Identify the sources of the two signals you observed. (Guess, but try to make a reasonable guess)
For Further Reading:
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