2019-08-29T16:35:46.000-0400

Simcenter SCADAS
Simcenter Testlab
Simcenter Testxpress

Direct YouTube link: https://youtu.be/aDS-Au5m0E8

**** Check out the free on-demand webinar Digital Signal Processing ****

When converting signals from their true analog form into digital form, frequency errors can be induced due to “aliasing”.

Aliasing occurs when a dynamic event is not sampled fast enough to truly capture what is happening. For example, in movies the wheels of a vehicle sometimes appear to being going backward, even though the vehicle is moving forward. This is because the frame rate of the movie camera was not fast enough.

This article has the following sections:

1. What is Aliasing

2. Preventing Aliasing with Filters

3. Simcenter SCADAS Anti-Aliasing Filter

4. Simcenter Testlab: Span versus Bandwidth

5. Conclusion**1. What is Aliasing?**

Aliasing is an effect that causes distortion in the spectrum of a sampled signal due to the sampling rate being too low to capture the frequency content properly. Aliasing causes high frequency data to appear at a lower frequency than it actually is (see *Figure 1* below): thus assuming a “false identity” frequency or “alias” frequency.

*Aliasing is caused when the digital sampling rate is not adequate to capture the fluctuations in an analog signal, and results in the wrong frequency being identified. *

Some essential terms to know when talking about aliasing:

**Sampling frequency**(Hz): The number of samples per second being acquired of an incoming frequency. The sampling frequency is two times the bandwidth.

**Bandwidth**(Hz): The frequency range over which measurements will be taken. Bandwidth is defined as half of the sampling frequency.

**Span**(Hz): The frequency range over which measurements will be taken and not be effected by the anti-aliasing low-pass filters (i.e.*the alias-free region of the bandwidth*). The span is 80% of the bandwidth.

**Nyquist rate**(Hz): Absolute maximum frequency (or bandwidth) content that can be digitized, and eventually reconstructed back, if sample rate is at least twice as Nyquist rate (Nyquist-Shannon theorem).

To properly sample all the desired frequency content of an incoming signal, one must sample at (or above) the *Nyquist rate*. In data acquisition, the sampling frequency is twice as high as the specified bandwidth. So, all frequency content below the specified bandwidth will be sampled at a rate sufficient to accurately capture the frequency content.

When the incoming signal contains frequency content above the specified bandwidth, the sampling frequency (2x bandwidth) will violate the Nyquist theorem for this higher frequency content.

In *Figure 2a*, a sine wave with inadequate sampling (violating the Nyquist theroem) is shown. The frequency is not identified properly.

In *Figure 2b*, a sine wave with adequate sampling shows the proper frequency.

When the Nyquist theorem is violated, spectral content above the bandwidth is mirrored about the bandwidth frequency. This means that frequency content X Hz above the bandwidth will then appear X Hz below the bandwidth. Watch the video at the top of this article to see mirroring in action.

Thus, higher frequency content appears to be at a lower frequency, or an “alias” frequency as shown in *Figure 3*.

Imagine trying to measure a signal that has a 125 Hertz sine wave and the measurement system identifies it as 75 Hertz!**2. Preventing Aliasing with Filters**

An anti-aliasing filter is a low-pass filter that removes spectral content that violates the Nyquist criteria (aka spectral content above the specified bandwidth). This makes it so a 125 Hertz sine wave does not show up as 75 Hertz.

The ideal anti-aliasing filter would be shaped like a “brick wall”, completely attenuating all signals beyond the specified bandwidth (*Figure 5*).

In the real world, it is impossible to have this “wall shaped” filter. Instead, a very sharp analog filter is used that has a -3dB roll off at the bandwidth and attenuates all frequencies 20% beyond the bandwidth to zero as shown in *Figure 6*.

This is why the “trustable”, alias-free region of the spectrum is from zero Hz to 80% of the bandwidth. This alias-free range is called the frequency span.

If the bandwidth was set at 1000Hz, the span would be 800Hz. *Figure 7* shows the span without alias effects and the bandwidth with alias effects.

The Simcenter SCADAS hardware has an anti-aliasing filter built into it. The video at the top of this article demonstrates how this anti-aliasing filter works.

**3. Simcenter SCADAS Anti-Aliasing Filter**

The anti-aliasing protection in a Simcenter SCADAS consists of at least two filters (one analog, one digital) as shown in *Figure 8 *below.

*Figure 8: The anti-aliasing filters of a Simcenter SCADAS input card have an analog filter and a digital filter. The analog filter operates at the full bandwidth of the card while the digital filter is applied while downsampling to the desired measurement bandwidth.*

When an analog signal enters a Simcenter SCADAS, an analog anti-aliasing filter is applied to the incoming signal with a cutoff at the full bandwidth of the module. The filter has an attenuation on the order of 150 dB per octave to prevent aliasing during the analog to digital conversion process.

After the signal is digitized, a second digital anti-aliasing filter is applied. This is applied to the digital signal during downsampling from the full bandwidth to the desired measurement bandwidth.

The anti-aliasing filters include both passive and active components. Operational amplifiers are used in the design of the filter and are part of the active components. Passive components include resistors, inductors, and capacitors. The passive components are used to protect the active components at high frequency and at high input levels.

The anti-aliasing filter on a Simcenter SCADAS can never be turned off. Depending on the input card, additional filters can be applied to prevent the Gibbs phenomenon. **4. Simcenter Testlab: Span versus Bandwidth**

In Simcenter Testlab it is possible to specify the span instead of the bandwidth. This way, you can be sure all data up to that frequency value will be alias free. See Video 2 below for instructions on how to set the default view in Simcenter Testlab as the span instead of bandwidth.

**Direct YouTube link:** https://youtu.be/bZnv4JOxs0U

This is done in the Simcenter Testlab main menu under "Tools -> Options -> General tab -> Frequency: Span/Bandwidth/Sampling Rate".**5. Conclusion**

- Aliasing is properly prevented by an anti-aliasing filter. Aliasing can cause spectral content to be mirrored about the bandwidth thus causing false representation of the frequency content. To prevent this an anti-aliasing filter is required.
- The useful measurement bandwidth (span) will always be smaller than the Nyquist rate. An anti-aliasing filter always needs roll off space between passband and stopband frequencies, so that signal gets minimally and maximally attenuated at passband and stopband respectively. The alias-free portion of the bandwidth is called the span. The span is the approximately 80% of the bandwidth (on many acquisition systems).Always set the bandwidth 20% higher than the highest frequency of interest to avoid aliasing.

Questions? Email charles.rice@siemens.com or contact Siemens Support Center.

**Related Digital Signal Processing Links: **

- Free On-Demand Webinar: Digital Signal Processing
- Index of Testing Knowledge Articles
- Digital Signal Processing: Sampling Rates, Bandwidth, Spectral Lines, and more...
- Gain, Range, Quantization
- Acquisition Filters and Overshoot
- Overloads
- Averaging Types: What's the difference?
- Overlap: What, Why and How to use it
- What is Fourier Transform?
- Time-Frequency Analysis: Wavelets
- Spectrum versus Autopower
- Autopower Function...Demystified!
- Power Spectral Density
- Shock Response Spectrum (SRS)
- Windows and Leakage
- Window Types
- Window correction factors
- Exponential Window Correction Factors
- RMS Calculations
- The Gibbs Phenomenon
- Introduction to Filters: FIR and IIR
- Digital Data Acquisition and Signal Processing Seminar

**SCADAS Data Acquisition Links:**

- Simcenter SCADAS
- Simcenter SCADAS Mobile and Recorder Hardware
- Simcenter SCADAS XS: Everything you need to know!
- Simcenter SCADAS Cable Guide
- How to use SCADAS T8 Thermocouple card?
- ICP versus IEPE versus Charge Accelerometers
- Simcenter Testlab Data Acquisition Tips
- Simcenter Testlab: Working with Charge Accelerometers
- Long Cable Lengths and ICP Transducers
- Single Ended versus Differential Inputs
- AC versus DC Coupling
- How to measure strain gauges with Simcenter Testlab?
- Simcenter Testlab: Measuring a String Pot