Teledyne DFP2 User Manual

Digital Filter Package 2 (DFP2)

Software

Instruction Manual

Digital Filter Package 2 Software Instruction Manual

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Digital Filter Package 2 and Teledyne LeCroy are registered trademarks of Teledyne LeCroy, Inc. Windows is a registered trademark of Microsoft Corporation. Other product or brand names are trademarks or requested trademarks of their respective holders. Information in this publication supersedes all earlier versions. Specifications are subject to change without notice.

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INTRODUCTION...................................................................................................3

The Need..........................................................................................................................................3

The Solution.....................................................................................................................................3

Enhanced Solutions.........................................................................................................................4

Kinds of Filters .................................................................................................................................5

Communications Channel Filters.....................................................................................................7

IIR Filters..........................................................................................................................................9

FILTER SETUP ...................................................................................................10

To Set Up a DFP Filter...................................................................................................................10

MULTIRATE FILTERS........................................................................................11

Description ..................................................................................................................................... 11

Example..................................................................................................................................11

CUSTOM FILTERS.............................................................................................13

Custom Filter Setup .......................................................................................................................13

Example 1: Creating an FIR Filter Coefficient File Using Mathcad........................................13

Writing Data to a Data File......................................................................................................15

Example 2: Creating an IIR Filter Coefficient File Using Mathcad .........................................17

Writing Data to a Data File......................................................................................................19

SPECIFICATIONS..............................................................................................19

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INTRODUCTION

The Need

In today's complex environment, data is frequently composed of a mixture of analog and digital components spread over a broad range of frequencies. In many applications, the relevant data is encoded or obscured. Capturing the right signals becomes a challenge. Engineers find it increasingly difficulty to examine only those parts of the data they are interested in. Tradition al (or even smart) oscilloscope triggering cannot always provide a satisfactory answer.

For example, servo motors from disk drives add a low frequency component to the high frequency data output. It is hard to achieve an accurate analysis of data unless the low component is removed.

Another common example is switched power supply units, which inject the switching frequency component into many system parts. Viewing digital signals mixed with this switching frequency component could be very difficult. Filtering is definitely required.

Yet another example is in ADSL residential connectivity, where data is transmitted over 256 narrow bands. Each band is only 4.7 kHz wide, and the gap between two adjacent bands is also

4.7 kHz. Examining such complex waveforms with regular DSOs is almost impossible; filtering out unwanted frequency components is necessary.

The Solution

At present, these needs are addressed in two ways. One way is building analog filters and placing them in front of the oscilloscope, providing an already filtered signal to the DSO. The disadvantages of this approach are many. Analog filters depend heavily on the accuracy and stability of analog components. Although in some cases analog filters are easily implemented, they are quite impractical for low (< 100 Hz) or high (> 100 MHz) frequency ranges. In comparison, digital filters can provide the desired results in those cases.

The second approach, practiced by many engineers, is using the DSO as a digitize r. The digitized data output is then transferred to a PC for processing. This solution freque ntly provides the required results, but it might be too slow or too limited in flexibility for some applications.

With Digital Filter Package 2 (DFP2), Teledyne LeCroy provides a solution that combines the best of both worlds. This package includes seven of the most useful finite impulse response filters (FIR), in addition (IIR) filter types (Butterworth, Chebyshev, Inverse Chebyshev, Bessel). You can easily set the Cutoff Frequency in addition to the Stop Band Attenuation and Pass Band Ripple for each filter.

It is even possible to use single filters or multiple filters cascaded for even more complex filtering. Once filtered, waveforms include mostly relevant frequency components, undesired parts being greatly attenuated.

If you want filters with special characteristics, the custom design feature allows you to design unique filters tailored to your specific needs. The required filter can be designed with a digital filter

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to a custom design feature. It also includes four infinite impulse response

gn or with a math package such as MATLAB or Mathcad. Filter coefficients can be directly

desi downloaded from the program into the scope, using the DSOFilter utility. It is also possible to specify the filter coefficients on an Excel spreadsheet and to use DSOFilter to download them from the spreadsheet to the scope.

DFP2's flexibility is shown by the following example:

1. A 25 kHz square with an unwanted 60 Hz sinusoidal component.

2. A high-pass filter set to attenuate signals lower than 1 kHz is applied to remove the unwanted 60 Hz component.

3. FFT of the unfiltered trace.

4. FFT of the filtered trace. Note the absence of the 60 Hz component.

wave combined

Enhanced Solutions

DFP2 can be coupled with other Teledyne LeCroy software products such as JTA2 or DDM2 to enhance the capabilities of these products and to provide improved solutions. For Jitter Measurement, for example, the DFP2 Band-pass Filter can be coupled with the JTA2 package to measure jitter over a narrow frequency range.

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Kinds of Filters

Low-pass Filter

High-pass Filter

Low-p

ass filters are useful for eliminating accumulated high-frequency noise and interference, and for canceling high-frequency background noise.

Sample applications are in datacom, telecommunications, and disk drive and optical recording analysis for accurate RF signal detection.

Band 1: Pass Band — DC to top of the transition region; signal passes unattenuated.

Band 2: Transition Region — edge frequency to edge frequency plus width; increasing attenuation.

Band 3: Stop Band — above end of transition region; signal is highly attenuated.

-pass filters are useful for eliminating DC and low-

High frequency components. Sample applications include Disk Drive and Optical Recording analysis (emulation of the SLICING function).

Band 1: Stop Band — DC to bottom of the transition region; highly attenuated.

Band 2: Transition Region — edge frequency minus width to edge frequency; decreasing attenuation.

Band 3: Pass Band — above edge frequency; signal passes unattenuated.

1. Filters are optimal FIR filters of less than 2001 taps, according to the Parks-MacLellan algorithm described in Digital Filter Design and Implementation by Parks and Burrus, John Wiley & Sons, Inc., 1987, and then adjusted by windowing the start and end 20% with a raised cosine for improved time domain characteristics and better ultimate rejection in the frequency domain, slightly increasing 1

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stop-band ripple height.

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