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 -> Swept Frequency Gain Measure". The waveform "v1_over_vosc_complex -> dbMag" is the magnitude of the frequency response of the R-C filter, and "v2_over_vosc_complex -> 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.

DGB's picture




Joined November 7, 2013
Design added Tuesday, January 12, 2016 | 1:18 pm PST