Anti-aliasing filter circuit for oscilloscopes

• US 4,621,217 A
• Filed: 09/21/1984
• Issued: 11/04/1986
• Est. Priority Date: 09/21/1984
• Status: Expired due to Fees

• Current Assignee: Tektronix Incorporated (Fortive Corp.)
• Original Assignee: Tektronix Incorporated (Fortive Corp.)
• Inventors: Saxe, Charles L., Jones, Roydn
• Primary Examiner(s): NOT, DEFINED
• Application Number:
  US06/652,700
• Time in Patent Office: 774 Days
• Field of Search: 315/1, 328/167, 328/151, 333/165, 333/166, 333/167, 307/521, 307/543, 324/121 R, 324/77 CS, 324/77 C, 324/77 B, 324/78 F
• US Class Current: 315/1
• CPC Class Codes: G01R 13/34 Circuits for representing a single waveform by sampling, e.g. for very high frequencies

Abstract

A digital storage oscilloscope is provided with an anti-aliasing filter circuit utilizing a first plurality of switchable analog filters that bandwidth limit frequencies greater than half the oscilloscope'"'"'s sampling frequency, and a digital transversal filter effective at lower sweep rates and sampling rates for also bandwidth limiting frequencies greater than half the sampling frequency. The digital transversal filter is controllable to provide a different band pass characteristic for different sweep and sample rates. Both the selection of filters and the control of the digital filter are responsive to the oscilloscope'"'"'s selection of sweep rate.

Claims

1. An anti-aliasing filter circuit for a digital oscilloscope having a controllable sweep-rate and including digitizing means for converting an analog signal to digital values for storage in said digital oscilloscope, said circuit comprising:

means for providing a signal input,

a plurality of low pass analog filters for receiving said signal input,

a digital filter for receiving said signal input, said digital filter having an electrically controllable pass band,

and means for controlling the sweep-rate of said oscilloscope simultaneously with selection of signal outputs of said analog filters at higher sweep-rates and selection of the signal output of said digital filter at slower sweep-rates for coupling to said digitizing means while controlling the pass band of said digital filter at slower sweep rates in order to pass selected frequencies less than half the sampling rate of said digitizing means.

2. The circuit according to claim 1 wherein said digital filter comprises a tapped analog delay device producing an output summed from the taps thereof and is controllable by means of selectively weighting the outputs of individual taps.

3. The circuit according to claim 2 wherein the weighting of a group of said taps describe a tapered sin x/x profile.

4. An anti-aliasing filter circuit for a digital oscilloscope having a sweep-rate control and including digitizing means for converting an analog signal to digital values for storage in said digital oscilloscope, said circuit comprising:

a signal input terminal,

a plurality of low pass filters having different bandwidths and means for coupling the input of each of said filters to said signal input terminal,

and means for selecting the ouput of one of said filters and for coupling the selected output to said digitizing means in accordance with the sweep rate control of said oscilloscope so that the pass band is reduced at lower sweep and sampling rates to selected frequencies less than half the sampling rate of said digitizing means.

5. An anti-aliasing filter circuit for a digital oscilloscope having a sweep-rate control and including digitizing means for converting an analog signal to digital values for storage in said digital oscilloscope, said circuit comprising:

a digital transversal filter including a tapped analog delay device providing an output summed at the taps thereof,

and means for coupling said digital transversal filter to receive a signal input,

wherein parameters of said digital transversal filter are selected in response to the setting of the sweep rate control of said oscilloscope to provide a reduced low pass characteristic substantially in proportion to lowering of the sweeep rate in order to pass selected frequencies less than half the sampling rate of said digitizing means.

6. The circuit according to claim 5 wherein said parameters of said digital transversal filter comprise weighting factors applied to at least ones of said taps to provide a selected pass band.

7. The circuit according to claim 6 wherein the weighting factors applied to a group of said taps describe a tapered sin x/x profile.

8. An anti-aliasing filter circuit for a digital oscilloscope having a sweep-rate control and including digitizing means for converting an analog signal to digital values for storage in said digital oscilloscope, said circuit comprising:

a signal input terminal,

a plurality of low pass analog filters having different bandwidths and means for coupling the input of each of said analog filters to said signal input terminal,

a low pass digital transversal filter and means for coupling the input of said digital transversal filter to said signal input terminal,

and means for selecting between the ouput of one of said analog filters and the output of said digital transversal filter and for coupling the selected output to said digitizing means in accordance with the setting of the sweep rate control of said oscilloscope including providing the output of one of said analog filters as a digitizing input at faster sweep rates wherein the selected output is sampled at a higher sampling rate, and providing the output of said digital transversal filter as a digitizing input at slower sweep rates wherein the selected output is sampled at a lower sampling rate,

said digital transversal filter being responsive to a particular selected slower sweep rate for setting the parameters of said digital transversal filter to provide a diminshed pass band as said sweep rate and sampling rate are reduced in order to prevent aliasing in the presentation of an input signal waveform by said oscilloscope.

9. The circuit according to claim 8 wherein said means for selectively coupling the input of said digital transversal filter to said signal input terminal comprises an analog filter having a lower pass band than any of said plurality of low pass analog filters, and means for sampling the output of the last mentioned analog filter at a predetermined rate to provide an input for said digial transversal filter corresponding to the shift rate of said digital transversal filter.

10. The circuit according to claim 8 wherein said digital transversal filter comprises a tapped analog delay device providing an output summed from the taps thereof and wherein the said parameters of said digital transversal filter comprise selected weighting factors applied to at least ones of said taps to provide a selected pass band.

11. The circuit according to claim 10 wherein the weighting factors applied to a group of said taps describe a tapered sin x/x profile.