TDFS | PartQuest™ Explore

Switched-Capacitor Filter Frequency Response using TDFS

Mike Donnelly
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Mike Donnelly

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9 years 4 months

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Original member of the PartQuest Explore development team. Modeling and Applications Specialist

Description

This design shows the use of the TDFS (Time Domain Frequency Sweep) method to measure the frequency response of two low-pass filters. One is just a simple RC filter circuit with 1 kHz cut-off frequency. The other is an equivalent filter using a switched-capacitor implementation.

Both small-signal AC (frequency domain) and TDFS (time-domain) analyses are run, and both give the same results for the RC filter. However, because of the switching aspect of the switched-capacitor filter, the results of the AC analysis are meaningless for that circuit. However, the TDFS approach is valid for that filter also, as well as for any other circuit or system containing switching, sampled data or digital content.

For reference, note in the schematic that the R-C filter is connected to channel 1 of the TDFS instrument, and the switched capacitor filter is connected to channel 2, hence the v1 and v2 distinction in the name of the results for the magnitude (dbMag) and phase. Both are measured relative to the stimulus input vosc, the oscillator output.

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Stage testing of Switched-Capacitor Filter Frequency Response using TDFS

Mike Donnelly
Designer19

Mike Donnelly

Member for

9 years 4 months

1,337

designs

10

groups

Original member of the PartQuest Explore development team. Modeling and Applications Specialist

Description

This design shows the use of the TDFS (Time Domain Frequency Sweep) method to measure the frequency response of two low-pass filters. One is just a simple RC filter circuit with 1 kHz cut-off frequency. The other is an equivalent filter using a switched-capacitor implementation.

Both small-signal AC (frequency domain) and TDFS (time-domain) analyses are run, and both give the same results for the RC filter. However, because of the switching aspect of the switched-capacitor filter, the results of the AC analysis are meaningless for that circuit. However, the TDFS approach is valid for that filter also, as well as for any other circuit or system containing switching, sampled data or digital content.

For reference, note in the schematic that the R-C filter is connected to channel 1 of the TDFS instrument, and the switched capacitor filter is connected to channel 2, hence the v1 and v2 distinction in the name of the results for the magnitude (dbMag) and phase. Both are measured relative to the stimulus input vosc, the oscillator output.

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Copy of New TDFS - on Tue, 03/09/2021 - 13:36

bedrosian1134
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bedrosian1134

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Description

This design shows the use of the TDFS (Time Domain Frequency Sweep) method to measure the frequency response of two low-pass filters. One is just a simple RC filter circuit with 1 kHz cut-off frequency. The other is an equivalent filter using a switched-capacitor implementation.

Both small-signal AC (frequency domain) and TDFS (time-domain) analyses are run, and both give the same results for the RC filter. However, because of the switching aspect of the switched-capacitor filter, the results of the AC analysis are meaningless for that circuit. However, the TDFS approach is valid for that filter also, as well as for any other circuit or system containing switching, sampled data or digital content.

To see the actual frequency sweep results from the TDFS analysis, open the full waveform viewer and expand the folder "Switched-Capacitor Filter Frequency Response using TDFS -&gt; Swept Frequency Gain Measure". The waveform "v1_over_vosc_complex -&gt; dbMag" is the magnitude of the frequency response of the R-C filter, and "v2_over_vosc_complex -&gt; dbMag" is the magnitude of the frequency response of the switched capacitor filter, both expressed in dB. Note that the phase response waveforms are also available.

For reference, note in the schematic that the R-C filter is connected to the channel 1 input of the TDFS instrument, and the switched capacitor filter is connected to channel 2, hence the v1 and v2 distinction. Both are measured relative to the stimulus input vosc, the oscillator output.

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Copy of Switched-Capacitor Filter Frequency Response using TDFS - on Sun, 12/20/2020 - 02:17

vijay1607130725
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vijay1607130725

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Description

This design shows the use of the TDFS (Time Domain Frequency Sweep) method to measure the frequency response of two low-pass filters. One is just a simple RC filter circuit with 1 kHz cut-off frequency. The other is an equivalent filter using a switched-capacitor implementation.

Both small-signal AC (frequency domain) and TDFS (time-domain) analyses are run, and both give the same results for the RC filter. However, because of the switching aspect of the switched-capacitor filter, the results of the AC analysis are meaningless for that circuit. However, the TDFS approach is valid for that filter also, as well as for any other circuit or system containing switching, sampled data or digital content.

For reference, note in the schematic that the R-C filter is connected to channel 1 of the TDFS instrument, and the switched capacitor filter is connected to channel 2, hence the v1 and v2 distinction in the name of the results for the magnitude (dbMag) and phase. Both are measured relative to the stimulus input vosc, the oscillator output.

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SB Copy of Motion controller scratch - on Tue, 12/01/2020 - 18:16

sergeybekrenyov
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Copy of test TDFS - on Mon, 11/16/2020 - 13:09

Devon Johnson
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Devon Johnson

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Copy of Motion controller scratch - on Thu, 08/27/2020 - 20:43

michal.p.baranowski
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Copy of TDFS Loop Stability for Buck DC to DC Converter - State Average - on Tue, 08/25/2020 - 13:43

david.g.bedrosian+rohm_test_designs_production
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Description

This design uses both TDFS and standard AC Analysis methods to assess the stability of a closed-loop system. The design is the "Buck DC to DC Converter", using a state-average model for the modulator function.

The use of state-average models supports AC Analysis, but TDFS (Time Domain Frequency Sweep) is also used to show the equivalency of the methods. Note that the TDFS approach can be used for systems that do not support AC Analysis, such as switching circuits or systems that contain sampling or digital control aspects. This particular example is directly comparable to the design "TDFS Loop Stability for Buck DC to DC Converter - Switching". In that design, the TDFS method is used to measure the open loop transfer function of an operational switching circuit.

Note that the approach used to characterize the loop stability, by injecting a small sinusoidal stimulus signal in series with the loop and then measuring the complex ratio of the ground referenced return signal to the injected signal, is described in:

D. Venable, “Testing Power Sources for Stability”, Venable technical paper #1, Venable Industries

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Copy of Laser Temperature Regulator TDFS - on Tue, 08/25/2020 - 13:39

david.g.bedrosian+rohm_test_designs_production
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david.g.bedrosian+rohm_test_designs_production

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Copy of Switched-Capacitor Filter Frequency Response using TDFS - on Tue, 08/25/2020 - 13:36

david.g.bedrosian+rohm_test_designs_production
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david.g.bedrosian+rohm_test_designs_production

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Description

This design shows the use of the TDFS (Time Domain Frequency Sweep) method to measure the frequency response of two low-pass filters. One is just a simple RC filter circuit with 1 kHz cut-off frequency. The other is an equivalent filter using a switched-capacitor implementation.

Both small-signal AC (frequency domain) and TDFS (time-domain) analyses are run, and both give the same results for the RC filter. However, because of the switching aspect of the switched-capacitor filter, the results of the AC analysis are meaningless for that circuit. However, the TDFS approach is valid for that filter also, as well as for any other circuit or system containing switching, sampled data or digital content.

For reference, note in the schematic that the R-C filter is connected to channel 1 of the TDFS instrument, and the switched capacitor filter is connected to channel 2, hence the v1 and v2 distinction in the name of the results for the magnitude (dbMag) and phase. Both are measured relative to the stimulus input vosc, the oscillator output.

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Mike DonnellyDesigner19

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Mike Donnelly
Designer19

Mike Donnelly
Designer19

bedrosian1134
Designer198997

vijay1607130725
Designer237013

sergeybekrenyov
Designer236944

Devon Johnson
Designer250

michal.p.baranowski
Designer234398

david.g.bedrosian+rohm_test_designs_production
Designer234163

david.g.bedrosian+rohm_test_designs_production
Designer234163

david.g.bedrosian+rohm_test_designs_production
Designer234163