SBOA550 October 2022 OPA1671 , OPA2990 , SN74HCS04 , SN74HCS164 , SN74HCS30 , SN74LVC1G00 , SN74LVC1G123 , TLC04 , TLC14 , TS5A9411

8 Tracking Harmonic Filter

For applications requiring a tunable frequency output, a fixed harmonic filter may not be suitable. A switch-capacitor low-pass filter such as the TLC04 or TLC14 may be clocked coherently with the shift register to provide a tuneable corner frequency that accurately tracks the output of the generator. For instance, the TLC04 provides a 4th order Butterworth response having cutoff frequency in the range of 0.1 Hz–30 kHz. The TLC04 is clocked at 50 × the desired corner frequency, while the TLC14 is clocked at 100 × the desired corner frequency. For example, for a 1-kHz output, a 64-kHz clock may drive the TLC04 for a corner frequency of 64 kHz / 50 = 1.28 kHz. Figure 8-1 shows the same clock is divided by 4 prior to clocking an 8-stage Johnson counter to generate a 1-kHz sinusoid.

Figure 8-1 TLC04 Switched-Capacitor 4th Order Filter Used as a Tracking Filter to Remove Harmonics From the Output of the Davies Generator

This arrangement can be tuned to any frequency within the capability of the TLC04 simply by changing the input clock frequency. A simulation model of the TLC04 is not available, but a discrete switched capacitor filter simulation follows.

The primary drawback to switched capacitor type filters is the tendency to inject broadband noise into the output due the charge injection of the analog switches involved. Switched-capacitor filters may be constructed from discrete analog switches, but care must be taken regarding the charge-injection characteristics for the same concerns. For instance, the TS5A9411 has relatively low charge injection and is suitable for basic switched-capacitor applications.

Figure 8-2 shows a 1st order tracking harmonic filter based on the OPA1671 and TS5A9411. The OPA1671 is a low-voltage amplifier having low noise (7 nV/√Hz), wide gain-bandwidth (13 MHz), and high-output drive characteristics that can easily drive the switched capacitor load.

Switched-capacitor filter clocked at 64 × sinusoidal output frequency.

Figure 8-2 Tunable Sinusoid Generator With Variable 1st Order Harmonic Filter Constructed From a DPST Analog Switch

Figure 8-3 shows the waveform and THD performance.

The harmonic filter is constructed from a discrete DPST analog switch, THD ≈ 0.025%

Figure 8-3 Simulation Results of Tunable Sinusoid Generator With Variable Switched-Capacitor 1st Order Harmonic Filter

Figure 8-4 shows a zoomed-in view of the waveform, the artifacts of the switched-capacitor filter are apparent.

Figure 8-4 Zoom View of Waveform Reveals Artifacts of 1st Order Switched-Capacitor Harmonic Filter

The fuzzy-edges due to charge injection increases the system noise floor, and there is some slanting at the top of the curve. While these do not impact THD appreciably, it may cause other issues – nevertheless, this is a very simple solution for making an electronically-tunable harmonic filter.