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Impact of Decoupling Capacitors and Trace Length on Radiated Emissions in a CMOS Inverter Circuit

In [1], we discussed the impact of decoupling capacitors and a PCB trace length on the signal integrity in a CMOS inverter circuit. In this article, we evaluate the impact of the capacitors and trace length on radiated emissions. It is shown that the radiated emissions from the PCB with short traces are lower than those with long traces. It is also shown that the decoupling capacitors have little impact in the monopole antenna range, a significant positive impact in the bicon antenna range, and a negative impact in the log-periodic antenna range.

CMOS Inverter Circuit

Figure 1 shows the block diagram of the inverter circuit and the PCB.

Figure 1: Block diagram of the inverter circuit and the PCB

In this study, trace length is varied between 3,000 mils (short trace) and 20,000 mils (long trace). Additionally, the PCB is tested in two configurations: without the decoupling capacitors and with decoupling capacitors by each inverter (0.1 µF and 1 µF).

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Radiated Emissions Measurement Setup

The measurements were performed in a semi-anechoic chamber, in accordance with CISPR 25 Edition 5 automotive standard. A monopole antenna was used in the frequency range of 150 kHz – 30 MHz with a bandwidth of 9 kHz and vertical polarization. A biconical antenna was used in the range of 30 MHz – 300 MHz with a bandwidth of 120 kHz and both horizontal and vertical polarization. A log-periodic antenna was used in the range of 300 MHz – 1GHz with a bandwidth of 120 kHz and both horizontal and vertical polarization. All measurements were taken with the average, peak, and quasi-peak detectors.

Measurement setup

Impact of the Trace Length on Radiated Emissions

In this section, we evaluate the radiated emission results from the PCB with short traces (3,000 mils) and long traces (20,000 mils) without decoupling capacitors by the inverters.

Case 3A: Short trace vs. long trace – Monopole antenna

Radiated emission results are shown in Figure 2.

Figure 2: Monopole antenna: a) short trace b) long trace

Observations

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Both traces showed similar failures at 1 MHz. Long trace failed average detector at 6 MHz. At 27 MHz, the short trace failed average detector while the long trace failed quasi-peak and average detector. The average detector failure for the long trace was about 7.5 dB higher than that for the short trace. Overall, the short trace outperformed the long trace.

Case 3B: Short trace vs. long trace – Bicon antenna

Radiated emission results for both traces are shown in Figure 3.

Figure 3: Bicon antenna a) short trace b) long trace

Observations

Both traces showed multiple failures. The short trace failed quasi-peak detector in the frequency range 72 – 173 MHz by the margin 4.26 – 24.09 dB. It failed average detector in the frequency range 42 – 300 MHz by the margin 0.67 – 38.12 dB.

The long trace failed quasi-peak detector in the frequency range 33 – 174 MHz by the margin 1.31 – 26.32 dB. It failed average detector at every frequency in the range 33 – 300 MHz by the margin 0.48 – 38.24 dB. Overall, the long trace showed more failures over a wider frequency range.

Case 3C: Short trace vs. long trace – Log-periodic antenna

Radiated emission results for both traces are shown in Figure 4.

Figure 4: Log-periodic antenna: a) short trace b) long trace

Observations

Both traces showed quasi-peak detector failures in the similar frequency range of 380 – 511 MHz, with the long trace exceeding the limits by a higher margin. The short trace showed average detector failures in the frequency range of 317 – 521 MHz, while the long trace showed failures in the frequency range of 308 – 844 MHz. Overall, the long trace showed more failures over a wider frequency range.

Impact of the Decoupling Capacitors on Radiated Emissions – Short Trace

In this section, we evaluate the radiated emission results from the PCB with short traces, without the decoupling capacitors, and with the decoupling capacitors by each inverter (0.1 µF and 1 µF).

Case 4A: Short trace with and without decoupling capacitors – Monopole antenna

Radiated emission results for both traces are shown in Figure 5.

Figure 5: Monopole antenna, short trace: a) without capacitors b) with capacitors

Observations

The decoupling capacitors had a negligible impact at the frequencies where the failures occurred.

Case 4B: Short trace with and without decoupling capacitors – Bicon antenna

Radiated emission results for both traces are shown in Figure 6.

Figure 6: Bicon antenna, short trace: a) without capacitors b) with capacitors

Observations

Quasi-peak detector: Capacitors eliminated failures in the frequency range 72 – 150 MHz. In the frequency range 150 – 173 MHz, many failures were eliminated, and the remaining ones decreased by 3 – 7 dB.

Average detector: Capacitors eliminated failures in the frequency range 42 – 100 MHz. In the frequency range 150 – 300 MHz, multiple failures were eliminated, and the remaining ones decreased by 2 – 10 dB.

Overall, the capacitors had a significant positive impact on radiated emissions.

Case 4C: Short trace with and without decoupling capacitors – Log-periodic antenna

Radiated emission results for both traces are shown in Figure 7.

Figure 7: Log-periodic antenna, short trace: a) without capacitors b) with capacitors

Observations

Quasi-peak detector: Capacitors did not eliminate or reduce the failures in the frequency range 380 – 511MHz. They increased the failures by 0.3 – 4 dB. Additionally, the capacitors created a new failure at 843 MHz.

Average detector: Capacitors did not eliminate or reduce the failures in the frequency range 317 – 521MHz. They increased the failures by 0.8 – 4.5 dB. Additionally, the capacitors created new failures at 843 MHz.

Overall, the capacitors had a negative impact on radiated emissions.

Impact of the Decoupling Capacitors on Radiated Emissions – Long Trace

In this section, we evaluate the radiated emission results from the PCB with long traces, without the decoupling capacitors, and with decoupling capacitors by each inverter (0.1 µF and 1 µF).

Case 5A: Long trace with and without decoupling capacitors – Monopole antenna

Radiated emission results for both traces are shown in Figure 8.

Figure 8: Monopole antenna, long trace: a) without capacitors b) with capacitors

Observations

The decoupling capacitors had minimal impact at 1 MHz. However, they eliminated failures at 6 MHz and 27 MHz.

Case 5B: Long trace with and without decoupling capacitors – Bicon antenna

Radiated emission results for both traces are shown in Figure 9.

Figure 9: Bicon antenna, long trace: a) without capacitors b) with capacitors

Observations

Quasi-peak detector: Capacitors eliminated failures in the frequency range 33 – 100 MHz. In the frequency range 100 – 174 MHz, several failures were eliminated, and the remaining ones decreased by 6 – 14.5 dB.

Average detector: Capacitors eliminated several failures in the frequency range 33 – 65 MHz. The remaining failures over the entire frequency region were either eliminated or decreased by 1.5 – 23.4 dB.

Overall, the capacitors had a significant positive impact on radiated emissions.

Case 5C: Long trace with and without decoupling capacitors – Log-periodic antenna

Radiated emission results for both traces are shown in Figure 10.

Figure 10: Log-periodic antenna, long trace: a) without capacitors b) with capacitors

Observations

Quasi-peak detector: Capacitors increased the failures in the frequency range 380 – 511 MHz by 2 – 10.6 dB. They introduced a new failure at 844 MHz.

Average detector: Capacitors increased the failures in the frequency range 319 – 511 MHz by 2.1 – 10.7 dB. They introduced a new failure at 844 MHz.

Overall, the capacitors had a negative impact on radiated emissions.

Future Work

The next article will present the conducted emissions results for the configurations discussed in this article.

References

  1. Bogdan Adamczyk and Mathew Yerian-French, “Impact of a Decoupling Capacitor and Trace Length on Signal Integrity in a CMOS Inverter Circuit,” In Compliance Magazine, January 2024.

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