I was watching Dave Jones video and he mentioned something I dont understand.I think he said that when the scope is in 1M ohm setting,then electromagnetic interference starts coupling with the probes,that they will act as antenas.
Is it true that oscilloscope is more susceptible to interference when set to 1 mega ohm than in 50 ohm mode?
There are all kinds of intereference,magnetic,capacitive,electromagnetic ( photons ),I am not sure what type he had in mind,he said something about led lights,I guess thats RF photons?
Why is there more interference at 1 M ohm setting? Free space have 377 ohm impedance,that should transfer more power to 50 ohm terminated scope.
Isnt coax cable supposed to have shielding as standard? Then it should be imune to interference,except the magnetic type unless it have mu metal shield
Right, impedance of free space is 377 ohms, and 1M is a lot higher; so what can we conclude? The interference is not electromagnetic, but (near field) electric.
Conversely, if we use a low impedance amplifier and a small loop antenna (a ~short circuit), we expect to observe magnetic interference.
(The same principle applies to low-frequency-equivalent transmission line properties: below Zo, equivalent inductance is the primary concern; above, equivalent capacitance is the primary concern.)
Indeed, these are always near field probes with very odd impedances (very far from 377 ohms). You can only make an antenna near 377 / 2 ohms (give or take geometric factors) when its size is comparable to a wavelength at the frequency of interest: in other words, it's not a near field probe anymore, but excellent for radiated (far) fields as well.
Tim
As you say, the source of EMI radiates a certain amount of power. A fraction of this power arrives at the scope probe, regardless of whether it's 50 ohms or 1 Mohm.
However, the scope displays voltage, not power. For this reason, the impact is much higher on the 1 Mohm input, as the same power results in much higher voltage than on the low impedance input.