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Digital Filter Applications
Useful Applications Of The Digital Filter Package (DFP)
LAB
in PDF format
Filters are circuits or devices in which the output gain and phase vary as a function of the
frequency of the input. This frequency sensitivity makes them useful in removing undesirable elements of a signal or
compensating for some frequency dependent distortion within the
signal. LeCroy's Digital Filter Package (DFP) option offers a selection of several standard or a user defined, custom digital filter configuration. These can be
applied in the analysis and measurement of waveforms as
illustrated in the examples which follow.
Figure 1 - Using a band stop filter to remove a 5 MHz sinusoidal signal from a 2 MHz square wave.
Figure 2 - Using a high pass filter to eliminate 60 Hz pickup.
The first class of applications to be shown is the removal of
undesirable spectral components of a signal. Figure 1 contains an
example of a waveform which consists of a 2 MHz square wave combined with an unwanted 5 MHz sinusoidal component. The time domain view of this signal is shown in trace A and the
frequency spectrum is shown in trace B. By applying a band stop filter with band limits of 3 and 5 MHz the unwanted 5 MHz
component is attenuated and the 2 MHz square wave is evident at the filter output (Trace C). The spectrum of the filter output (Trace D) shows the reduction in the 5 MHz component.
Figure 2 shows how a high pass filter is used to eliminate 60 Hz pickup from a 25 kHz pulse width modulated signal. The high pass filter is set to attenuate signals lower than 1 kHz thereby removing the 60 Hz signal.
If the acquired signal has a shaped baseline, as shown in
figure 3, it is possible to use a low pass filter to separate the
baseline and then subtract it from the acquired waveform. In this
example a low pass filter (Trace B) is used to extract the baseline which is then subtracted from the acquired signal in trace C.
Figure 3 Removing baseline shaping by separating and
subtracting the low frequency content of an acquired waveform
The last of our spectral separation examples, figure 4, shows the use of a low pass filter in a detector simulation. Modulation from an amplitude modulated signal is extracted by peak
detection and filtering. The absolute value function performs full wave peak detection and the DFP provides the necessary low pass filtering.
Figure 4 Using peak detection and filtering to demodulate an AM signal.
The next set of applications uses filters help recover signals from noise and control channel
bandwidth. These types of situations arise in communications systems and echo ranging systems.
The acquired waveform in figure 5 (Channel 2) is a 12.5 MHz damped sine badly contaminated with noise. The judicious use of band pass filtering improves the signal to noise ratio significantly.
Note that the fast Fourier transform (FFT) displays are used to assess the effects of the filtering operation. Trace A shows the spectrum of the acquired signal and trace C shows the spectrum of the filtered signal. The band pass filter is used to reduce the acquired signals bandwidth to 16 MHz, thereby eliminating large noise components outside the
filters pass band. The recovered signal is shown in trace B. While averaging could produce even better results it would
require multiple acquisitions which are not always available.
Figure 5 - Using DFP to evaluate the effects of different filter types on an NADC signal
The final example, shown in figure 6, is the evaluation of a band limiting filter for a digital
communications signal. In this measurement the effects of filter selection for a North American Digital Cellular
(NADC) waveform are evaluated. Comparing a normally filtered signal (raised root cosine) against an unfiltered waveform with DFP filtering shows a near exact match. The user can vary the type of filter or adjust parameters to see the
effect of other types of filter configurations.
Figure 6 Use of a band pass filter to increase signal to noise ra-tio and recover a signal from broadband noise.
Channel 2 contains the NADC signal without filtering. Channel 3 is the same signal with the normal raised root cosine filter. The DFP raised rot cosine filter is applied using trace A. The overlapped traces B and C are used to compare the two versions of the signal.
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