Using my oscilloscope near my TV(a Panasonic TH-L42U20A) seems to introduce an unidentifiable source of noise. The scope probe is floating, neither probe or ground clip is connected to anything, however my scope shows a strange prominent 'ringing' waveform even though the input is floating.

The probe is about 30cm from the rear of the TV, and the waveform disappears when I bring the probe close to the floor, appearing again when I raise its height to about 15-20cm above the floor. Attaching the ground clip to ground attenuates the waveform slightly, but still has considerable ripple.

I suspect this might be some kind of EMI from an internal switching regulator, but it is unlike any switching noise I have seen before, and is considerably strong as well.

Oscilloscope waveform capture

(10mV p/div, 10us timescale)

  • \$\begingroup\$ What is the probe setting (1x, 10x)? \$\endgroup\$ Feb 10 at 2:27
  • \$\begingroup\$ The scope is set to x10, the sample rate can go up to 1GS/s but is auto set depending on timebase(I can change it but it is unnecessary). \$\endgroup\$
    – lemon
    Feb 10 at 4:33

1 Answer 1


These results are not unexpected at all. Nothing to do with the TV. Try it with charging bricks or other switching "brick" power supplies.

it is unlike any switching noise I have seen before

What you're seeing is most likely the impulse response of the scope probe assembly (probe, cable, scope's front-end).

That the problem with such "measurements": they can be qualitative in nature at best. You shouldn't be using a floating scope probe to measure such things. Use proper EMC near field probes.

I have had a go at probing the RCA outputs of my TV

The probe is about 30cm from the rear of the TV

These two statements are contradictory. Either the probe is properly connected (tip and ground ring) to the RCA outputs, or it isn't. Probes floating in the air are antennas. Every modern consumer "IT" device radiates quite a bit, just try tuning to anything on a short wave radio.

  • \$\begingroup\$ Sorry for confusing terminology, I was originally going to test the RCA outputs however the weird waveform drew my attention instead. \$\endgroup\$
    – lemon
    Feb 10 at 4:34

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