Measuring EMI with Homemade Magnetic Field Probe

Part I: Magnetic Field Probe

The theories behind the Magnetic Field Probe (Near Field Probe) is not very complex. Roy Ediss well stated the theories and the various designs of the Magnetic Field Probes in his article, Probing the Magnetic Field Probe. I implemented the design 2(a) which mentioned in the Roy Ediss's article. I used RG405 (Semi-rigid Coax Cable with Copper Outer Conductor) and a SMA connector [male body (inside threads) with male inner pin] in my implementation, it is shown in the Figure 1.

Magnetic Field ProbeFigure 1:Magnetic Field Probe

Part II: Preamplifier

The gain of the oscilloscope may not enough to measure weak EMI signals. Therefore I designed a preamplifier which can amplify the EMI signals by the gain of 10. The circuit board is shown in the Figure 4.

A 1.5 GHz Fully Differential Amplifier LMH6552 is used to amplify the EMI signal which is captured by the Magnetic Field Probe, and than the amplified signal will be sent to the transformer T1. T1 will convert the differential signal to the single-ended signal. Finally, the single-ended amplified signal will be sent to the oscilloscope Rigol DS2202 via BNC connector P5.

The preamplifier in the Figure 2 requires +-5V dual supplies. The dual supplies design is shown in the Figure 3, it uses the Inverting Charge Pump MAX1697 to generate the -5V from the +5V single supply.

 Preamplifier_Schematic
Figure 2: Schematic of the Preamplifier (Gain = 10)
negitive_voltage_Schematic
Figure 3: Schematic of the Voltage Regulator
DS2202_100Mhz_PreAmp_Board_S
Figure 4: Preamplifier Board

Part III: Testing

1. Testing the Magnetic Field Probe with RF explorer (Spectrum Analyzer)

In this experiment, the Magnetic Field Probe was directly connected to the RF explorer's left SMA connector wich has 15-2700 Mhz bandwidth. The ring of the probe was put right above the 100 Mhz Oscillator as Figure 5. The measured frequency spectrum is shown in the Firgre 6. In the frequency spectrum, we can clearly see the 100Mhz Fundamental Frequency and its Harmonics.

FRE_100Mhz_Experiment
.Figure 5: Measuring the EMI of a 100Mhz Oscillator
FRE_100Mhz
Figure 6: Frequency Spectrum showing the EMI of a 100Mhz Oscillator

2. Testing the Magnetic Field Probe with the oscilloscope Rigol DS2202

In the first part of this experiment, I connected the Magnetic Field Probe to the oscilloscope directly as Figure 7. The time domain and frequency domain results is shown in the Figure 8.

DS2202_100Mhz_Directly_PIC
Figure 7: Measuring the EMI of a 100Mhz Oscillator (Without Preamplifier)

DS2202_100Mhz_Directly_FFT
Figure 8: Time domain and Frequency domain results of the EMI (Without Preamplifier)

In the second part of this experiment, the EMI signal was amplified by the  Preamplifier Board before sending to the oscilloscope, the connection is shown in the Figure 9. The time domain and frequency domain results is shown in the Figure 10. The time domain and the frequency domain results in the Figure 10 are similar to the results in the Figure 8, but the amplitude of the signal in the Figure 10 is 4.53 times of the signal in the Figure 8.


DS2202_100Mhz_PreAmp_PIC
Figure 9: Measuring the EMI of a 100Mhz Oscillator (With Preamplifier)

DS2202_100Mhz_PreAmp_FFT
Figure 10: Time domain and Frequency domain results of the EMI (With Preamplifier)

Part IV: Datasheets

  1. LMH6552

Part V: References

[1] Roy Ediss. Probing the Magnetic Field Probe. Retrieved from http://www.compliance-club.com/archive/old_archive/030718.htm (Local Mirror)

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