18.1.4 Step 4: Select Frequency Mode

In the case where the noise is at (or close to) a frequency that is harmonically related to the sampling frequency then the noise issue becomes severe, as illustrated above. In this case, the oversampling frequency must be adjusted in order to avoid the noise.

This is particularly important in applications where a frequency sweep test is required, such as EN61000-4-6.

Acquisition Module (PTC)

Available Frequencies (4 MHz PTC Clk)
Frequency Selection Frequency [kHz]
FREQ_SEL_0 66.67
FREQ_SEL_1 62.5
FREQ_SEL_2 58.82
FREQ_SEL_3 55.56
FREQ_SEL_4 52.63
FREQ_SEL_5 50
FREQ_SEL_6 47.62
FREQ_SEL_7 45.45
FREQ_SEL_8 43.48
FREQ_SEL_9 41.67
FREQ_SEL_10 40
FREQ_SEL_11 38.46
FREQ_SEL_12 37.04
FREQ_SEL_13 35.71
FREQ_SEL_14 34.48
FREQ_SEL_15 33.33
FREQ_SEL_SPREAD Variable frequencies
The acquisition module provides two strategies for frequency selection:
  1. A single acquisition frequency is selected, and oversampling takes place at this frequency only. FREQ_SEL_0 provides the fastest measurements and FREQ_SEL_15 the slowest. If no high-performance EMC standards are required, but the application equipment generates noise, which interferes with a particular acquisition frequency, the designer may simply change the frequency.
  2. A variable frequency is used during oversampling. FREQ_SEL_SPREAD varies the frequency during the acquisition oversampling. The delay is varied from 0 to 15 in a sawtooth manner on successive samples during oversampling to apply a wider spectrum of sampling frequency. Compared to single frequency acquisition, the frequency spread option reduces the sensitivity to noise at a particular ‘worst-case’ frequency, but increases the range of noise frequencies around that worst-case frequency which will show harmonic interference – albeit with reduced severity of the noise effects. In many applications, FREQ_SEL_SPREAD is sufficient to achieve the required noise tolerance.

Frequency Hopping Module

Module ID: 0x0006

The frequency hopping module utilizes three or more base frequencies and a median filter to avoid using measurements taken with harmonic interference. The frequencies should be selected to minimize the set of crossover harmonics within the problem frequency band.

Each of the selected frequencies is used for acquisition oversampling during successive measurement cycles.

Frequency Hopping with 3 Frequencies:

  • Cycle 1: All sensors measured with Frequency 0
  • Cycle 2: All sensors measured with Frequency 1
  • Cycle 3: All sensors measured with Frequency 2
  • Cycle 4: All sensors measured with Frequency 0
  • Cycle 5: All sensors measured with Frequency 1
If Frequency 0 is related to the noise frequency, then the measurements taken with F0 will show high variation. By using a median filter, the outlying measurements will be rejected.
Figure 18-1. Measurements Taken at Frequency 1 are Affected by Noise

Common Harmonics

No matter which frequencies are chosen, the possibility of noise at higher frequencies which are harmonics of more than one of the selected frequencies, exists.

Further up the spectrum, there are frequencies which are harmonics of all available frequencies, but those superset harmonics are at higher frequencies and so are blocked by the low pass filter.

In some applications, the potential for exposure to noise frequencies may be an unknown and variable quantity.

For example, a device utilizing a USB charger may not always be plugged into the charger that it was supplied with. Inexpensive replacement chargers are often found to generate high levels of common-mode noise, and at variable frequencies – often in the same band as the acquisition frequencies.

A similar situation occurs with applications tested to EN61000-4-6 for conducted immunity. The test equipment sweeps through injected noise from 150 kHz to 80 MHz, in steps of 1%. This gives an excellent chance of hitting an interference frequency, which is a common harmonic of the HOP frequencies.

In both cases, no static selection of frequencies can ensure harmonic avoidance by the median filter.

Frequency Hopping with Auto-tune

Module ID: 0x0004

Frequency Hopping with auto-tune provides the cyclic frequency hopping with median filter functionality, extended to quantify the variance of signals as measured by each individual frequency.

The module is configured with a stability limit. When signals measured at a particular oversampling frequency show a repeated variance exceeding this limit, the module switches this frequency to another, searching for a better performing option.