Spectrum analyzer with anti aliasing?

[rvardimon]

I was wondering if someone can share a Labview VI for a spectrum analyzer including a digital anti-alias filter.

Let me explain why I'm asking this:

We are trying to replace our spectrum analyzer with a labview code spectrum analyzer connected based on a fast digitizer.

We wrote a code to sample from the digitizer and display the Power Spectrum Density (PSD), based on the "FFT and Power Spectrum Units.vi" example.

The problem occours when measuring white noise - the measured PSD is larger than the input signal due to aliasing from higher frequencies.

From different references, I found that this problem is solved in spectrum analyzers in 3 steps:

1. over sampling the data in a larger sampling rate

2. data filter with a low-pass filtering

3. data undersampling

Since the anti-aliasing procedure is done in all commercial available spectrum analyzers,

I wonder if someone knows of such code implemented in Labview.

Ran

[Dennis_Knutson]

An anti-aliasing filter is done with hardware before the acquisition. You cannot do it after with software.

[rvardimon]

Hi Dennis,

You are right: an anti-alias filter is analog. However, is can be followed by a digital filter in order to allow different spans for the power spectrum.

I'm attaching an explanation from this reference:The Fundamentals of Signal Analysis - Agilent Technologies:

"Fortunately, there is an alternative which is cheaper and when used in conjunction with a single analog anti-alias filter,
always provides aliasing protection. It is called digital filtering because it filters the input signal after we have sampled and digitized it.
In the analog case we had to use a new filter every time we changed the sample rate of the Analog to Digital Converter (ADC).
When using digital filtering, the ADC sample rate is left constant at the rate needed for the highest frequency span of the analyzer.
This means we need not change our anti-alias filter. To get the reduced sample rate and filtering we need for the narrower frequency spans, we follow the ADC with a digital filter.

This digital filter is known as a decimating filter. It not only filters the digital representation of the signal
to the desired frequency span, it also reduces the sample rate at its output to the rate needed for that frequency
span. Because this filter is digital, there are no manufacturing variations, aging or drift in the filter.
It is easy to design a single digital filter to work on many frequency spans so the need for multiple filters per channel is avoided.
All these factors taken together mean that digital filtering is much less expensive than analog anti-aliasing filtering."

[johnsold]

LabVIEW has several VIs for various kinds of filters, for resampling of data, and for decimating arrays - which is not exactly the same thing.

As long as your physical, analog filter meets the anti-alias requirements for your initial sampling, you can do a variety of processing in software.  If you want to resample at a lower rate, you will need to do a second anti-alias filter meeting the new Nyquist criterion.  This filter can be digital, provided that it operates on data sampled at the original, high rate.

Do you have other questions?

Lynn

[rvardimon]

OK, I will try that.

Thanks,

Ran