TDFS

Copy of Switched-Capacitor Filter Frequency Response using TDFS - on Fri, 03/20/2020 - 15:56

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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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Laser Temperature Regulator TDFS

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TDFS Loop Stability for Buck DC to DC Converter - Switching

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This design demonstrates the use of the TDFS (Time Domain Frequency Sweep) simulation method, to measure the open-loop frequency response of an operating closed-loop system containing switching elements.

The stability of the "Buck DC to DC Converter - Switching" design is assessed. This is a switching circuit, it does not use a state-average model for the modulator, so the standard AC Analysis method cannot be used. Rather, the frequency response is generated from time-domain simulation results. The TDFS approach can also be used for systems that contain sampling or digital control aspects.

This particular example is directly comparable to the design titled "TDFS Loop Stability for Buck DC to DC Converter - State Average". In that design, both the TDFS and AC Analysis methods are used to measure the open loop transfer function of an equivalent non-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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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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TDFS

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test TDFS

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Laser Temperature Regulator TDFS

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TDFS Input Impedance of Step-Down (Buck) DC to DC Converter - Switching

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This example demonstrates the use of the TDFS method (Time Domain Frequency Sweep) to measure the input impedance of a switching DC/DC power converter. The converter circuit is identical to the design shown in the companion example: “Step-Down (Buck) DC to DC Converter - Switching”.In this example, the TDFS impedance measurement model applies a 12V DC bias to the line input, in addition to a sinusoidal stimulus with 3V peak amplitude, and a frequency range from 3 kHz to 30 kHz. The input current is measured and the impedance vs. frequency is computed. The results show that the impedance magnitude (blue waveform) is approximately 5.0 Ohms at low frequency, This is approximately the reflected value of the 1 Ohm load resistor, multiplied by the effective DC/DC conversion ratio squared: 1 Ohm * (12.0/5.0)**2 = 5.76 OhmThe phase measurement (red waveform) shows -175 degrees at low frequency, indicating that this is effectively a "negative resistance". That is, the input current decreases when the line voltage increases. This behavior can be observed directly in the time domain waveforms, where the line voltage (brown waveform) and the input current (green waveform) are almost completely out of phase at 3 kHz. This negative impedance, or constant power load characteristic, can be destabilizing in power distribution systems. This will be demonstrated in a related design, and in Part 3 of the TDFS Blog Series, coming soon!

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TDFS Open-Loop Frequency Response for PMSM/SVM Motion Control System

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New TDFS

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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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