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In 1990-ies hobby people used 50Hz-transformer power supplies and did not use switching power supplies for low-noise electronic applications, because switching power supplies were associated with introducing extra noise. Hi-Fi audio people avoided using digital electronics like panel meters in their instruments as well.
Low level of electromagnetic disturbance is important especially for receiving devices like radios, TV, mobile phones. For example, in hospitals and airplanes mobile phones have/had to be switched off. In our lab with sensitive photodiodes measuring nanowatts of light we experienced disturbance from mobile phones, WiFi and wireless PC keyboards.
It seems that meanwhile many problems can be identified and properly addressed by multilayer circuit boards, input filtering, shielding, SMD filter capacitors and inductors and lower CPU voltage.
One could expect that Wireless Power transmitter could potentially produce EMI. The receiving coil is in close proximity to the mobile phone receiver, and, although the frequencies are far apart, there could potentially be some harmonic pickup or non-linear mixing and rectifying effects.
Oscilloscope trace of TI Qi Wireless Power Evaluation Kit
By bringing oscilloscope probe close to the Qi transmitter one sees that switching frequency is 175 kHz. The frequency is not constant and changes automatically with load and coil mutual placement. In oscilloscope trace we see sharp switching spikes should lead to generation of many higher frequency harmonics.
Measurement with RF spectrum analyser
One of powerful devices to examine radio frequency spectrum is RF spectrum analyser.
We have available model Hameg HMS1000E that allows to measure spectra from 100 kHz to 1.6 GHz.
I did measurement on Texas Instruments Wireless Power Evaluation Kit by plugging in a piece of bare wire into the RF spectrum analyser input and putting another end of this wire between the wireless transmitter and receiver coils. Input resistance of RF spectrum analyzer is 50 Ohm so induced signals have to be pretty strong to be visible.
Below is measured RF spectrum span from 1 kHz to 5 MHz. One can see a comb of regular harmonics with frequency spacing of ca 300 kHz. Spacing is twice the switching frequency that was seen on the oscilloscope. This is double frequency, because on scope trace are two sharp spikes per oscillation period.
Next I took RF spectrum in the range 100kHz … 120 MHz. Probe between TX and RX coils. Left: Wireless power is ON. Right: Wireless Power is OFF. FM radio stations are visible between 88… 114 MHz. Amplitude scale is the same in both plots.
This comparison shows that wireless transmitter EMI level is quite low. Even when the pickup wire is placed between the coils the RF disturbance is comparable to FM radio station emission.
In summary, it looks like EMI from TI Qi Wireless Power kit is not a problem.
This is what one would expect. Wireles power is transmitted on just slightly higher frequencies than audio and nobody complains from EMI from audio speakers.
Other wireless charging appliances
Frequency of charging electric toothbrushes and torch are in 30…50 kHz range and shape is sine wave and is not producing much harmonics. <100kHz frequency is too small to be observed by RF spectrum analyzer.
From Philips toothbrush spectrum analyser showed a weak peak around 3.1 MHz.
From the pocket torch and from the Braun toothbrush no RF disturbance signal could be detected at all by the RF spectrum analyzer.
Next blog entry: 04 Wireless power board under varying load conditions