RadiMation Application Note 124

Emission: measuring intermittent signals

One of the first important things to determine when doing a Emission test is to know the emission pattern from the device. The standard EN55014-1(CISPR 14-1:2016 + COR1:2016) describes two general types (Paragraph 4 in CISPR 14-1:2016 + COR1:2016).

• Continuous interference

Continuous interference arises when the source continuously emits are a given range of frequency's

Sources include:

• Radio Frequency interference generated by wireless and radio frequency transmissions
• Television and radio frequency transmissions
• Industrial, scientific and medical equipment
• Digital processing circuitry such as micro controllers

• Pulse or transient interference

An electromagnetic pulse (EMP), sometimes called a transient disturbance, arises where the source emits a short-duration pulse of energy. The energy is usually broadband by nature, although it often excites a relatively narrow-band damped sine wave response in the victim.

Sources include:

Switching action of electrical circuitry, including inductive loads such as relays, solenoids, or electric motors

It is important to know this when you need to test a eut. It informs which method is best for the task as it also informs the settings needed to fit he cycle of the eut. It is also advised to look at this before deciding on test equipment as the measurement devices can be more or less suited for continuous or intermittent signals.

In this explanation we are only explaining the details of measuring a unstable/changing spectrum. For some devices it if obvious, for other devices it is less obvious if the have a stable or changing emission pattern.

The best way to determine if a device is continuous or not is to use a spectrum analyser. When you do a continues measurement of the spectrum you should be able to determine if it is a continuous or non-continuous signal.

Now that you have determined the type of emission you need to further investigate. For disturbed signals the test equipment needed is specified in paragraph 5.1, further guidance is included in Annex c (ref: Paragraph 4 in CISPR 14-1:2016 + COR1:2016)

It is important to know that it is preferable to use a analyser instead of a receiver. This is do to the nature of both devices. A fft based device will sweep the whole spectrum every time it does a measurement. A receiver will only measure a signal point at one time.

This means that the analyser will have a more complete picture of the different levels of all points which makes it more suited to work with changing signals. You can use a receiver to do this as well. This does not work as well as you have the chance that you are missing the signal because it is not being there when the receiver is measuring that point.

This can be resolved by increasing measure time (Observe a point longer and hold the highest value (Max/Hold) or increasing the amount of sweeps. Sometimes it is needed to increase measure time, sometimes the solution is to increase the amount of sweeps. because of the nature of the measuring device there is also a chance that you are constantly measuring when the signal is not there.

Radimation checks if the values measured in the final measurement are the same. If the measurement has changed then you get a warning that the detector has a different value the measured before. This is do to the effects mentioned above. Below I will discuss best practice methods to be able to work with discontinuous signals:

- Preferable use a analyser. This has advantages for discontinuous signals and is mostly used when the emission pattern is a changing pattern.

- Do a pre-test to determine cycle time of the eut (Time that a eut has used all it's process so that you have seen all emissions)

This is important to determine the measurement time. This must be at least equeal or longer then one eut cycle. For more information regarding the test conditions please refer to the the standard. Be aware that there are different test specifications for different devices. To check if you are using the right conditions to comply should be part of your preliminary research. These can be found in Annex A of CISPR 14-1:2016 + COR1:2016. Please read this so that you know that you know the test conditions for these devices.

A simple description of a test requirement for a changing load device is a drill. The requirement is that they are operated continuously and do not have load.

This is because you else would have to much change in the emission spectrum and therefore can not measure. If you can not comply to these requirements you will most of the times not be able to execute te test successfully.

Now that you know how you need to operate the device you are ready to determine the measurement and sweep time. There is not a default recipe to determin the settings needed. The settings are derived from you pre-limenary testing and then they are tuned to make sure that your settings are compatible with the EUT's cycle.

To determine the cycle of the eut it is the easiest to measure this. You do this by observing the eut. A way that is often used, is to measure the emission with a spectrum analyser.

You can do this manually from the analyser but this can also be done with the manual mode in Radimation.

- Make a manual continues measurement of the frequency area that you want to test. - Turn on max hold - Start a stop watch - Turn on the device and let it run a full cycle as you would in the test - You will now see that the analyser is filling up the peaks in the spectrum. - You will see the peaks rise every time a new highest value has been found. - When a device has done a full cycle all the processes should have been done, you can see this happen when you see that the graph is stable and not moving anymore. The last time a frequency's value jumps inidicates that you have observed a full cycle as no new activities are causing higher emission patterns and new frequency's are added. You have now seen all the emission this device can create - Stop your stopwatch

Now that you have observed a full cycle you can use this time as a parameter for your measurement time. If the signal is very irregular increase the amount of sweeps to average out the signal.