Problems with EMI on my scope

[tkpmep]

I’m experiencing a noise pickup problem at home with my 100 MHz Instek 6103  oscilloscope, and I’d appreciate hearing thoughts on how to fix it. I don’t think this is a ground loop problem – the scope is the only device that is plugged into a  socket in the room in which it is being used, and  I have turned off all lights / fans / TV’s / appliances around the house. With no probe connected, the scope shows a clean flat trace even at 2 mV/div, regardless of  whether the input  is live or grounded. Unfortunately, that’s where  the good news ends.  If I connect a probe with grippers and short the two ends, the line thickens. Switching the timbase to its fastest speed (50 nsec / div) (leads still shorted), shows about 300 uV p-p of 100 Mhz noise, and switching in band limiting (20 MHz max) cleans up the noise, but I  still don’t see a perfectly straight line (vertical amp limitations?).

Placing a large square of aluminum foil under the test leads and wrapping one end firmly around the BNC connector ((leads still shorted) gives the same results, and separating the leads (foil still under them) induces ~2 mV pk-pk of 60 Hz noise, with an overlay of about 500 uV p-p of a 30 KHz triangle wave. Not sure where the 30 KHz is coming from, perhaps the foil is acting as an antenna for the scope’s switching power supply.  Photographs are attached. I’d be very appreciative for thoughts on how to fix noise pickup in general, as the project I'm working on (microphone preamp) has to work in unpredictable and possibly harsh conditions, and it's hard to polish the design if the basic test equipment has trouble giving me clean readings. Using a grounded conductive / dissipative mat to cover my workbench and attaching my wrist to it with a strap comes immediately to mind, but I think of these as anti-static devices, not anti-EMI devices. What's a good way to stop EMI from messing up all my measurements?
 
Thanks in advance for your assistance
 
Tom Philips

[wraper]

This thing don't look like probe but rather noise pickup antenna  .

[tesla500]

Those noise values seem quite typical with your probing setup. You'll want to use much shorter leads to reduce noise pickup, preferably a "proper" scope probe and not flying leads on the end of a coax. I found a large amount of 30kHz noise coming from fluorescent lamps with electronic ballasts, to the point where I could pick up about a volt of signal just by holding my probe up in the air. Changing to magnetic ballasts or to other lighting technologies may help. Anything below 150kHz is not covered by EMI standards, so they're free to radiate all they want.

The biggest thing you can do to reduce noise is to use proper "high-frequency" probing techniques, such as connecting your probes like this:

It's very hard for noise to get picked up this way.

You can also wrap your scope probe coax around a ferrite bead a few turns to reduce high frequency noise, this can be especially helpful for switching power supplies, while viewing small voltages on current sense resistors.

[tkpmep]

Thanks a mill for the quick response.  I've done this experiment with a real oscilloscope probe (Hameg) as well, and the results are very similar. I initially thought the 30kHz stuff might be from fluorescent lamps or the power supply for my Instek voltmeter, so I shut them off, as well as any other electrical device that might radiate EMI, with no change to what I saw on my scope. That said, your point is well taken, and I am going to get myself a good 1x/10x probe - a Pomona 6500, and will keep all connections short and try to cut down the buzz. Any thoughts on the grounded rubber mat? A waste of time and money?

Thanks again

[T3sl4co1l]

Noise sources by frequency (by no means exhaustive):
50-60Hz, harmonics: power line, electrostatic in nature
10-100kHz: lighting, some switching supplies, phase controllers (especially if repeating every 100-120Hz)
500-1700kHz: AM broadcast (do you live within a few miles of a commercial antenna?)
2-80MHz: shouldn't be much, but includes nearby shortwave or VHF / TV transmitters, or poorly behaved switching supplies (don't buy direct from China, amirite?); ESD transients may also pop up randomly around here (e.g. shifting in your chair)
88-108MHz: FM broadcast (do you live within a few miles of a commercial antenna?)
>110MHz: aviation (110-130), military (200-300??), UHF TV (various up to 700MHz?), etc.

You'll still be able to observe signals beyond 100MHz, the scope isn't a brick wall -- the amplitude won't be accurate, and the wave shape will be severely rounded (it's not picking up most or any of the harmonics).  That high frequency junk you were picking up looks like it might be a pair of nearby FM transmitters, triggered in such a way that it produces a stable display (showing the point time where the two different frequencies superimpose constructively).

Common mode chokes (one or more turns through a ferrite ring or clamp-on) are handy from time to time, but taking some time to understand the nature of the noise is important.  In your case, the high frequency noise is picked up when the "probe" acts as a loop antenna, and goes away in close proximity to the ground plane (working over aluminum foil isn't a bad idea, given that it's a bit of a short-circuit hazard!).  At least, if I understand your pictures correctly.

For your development application, you'll have best results with totally shielded connections -- don't just hang a BNC on twisted pair for example, that's a good antenna in the millivolts!  It has to be coaxial into circuit ground, shielded as hard as possible.  Even an "edge launch" configuration (like here) is noticeably more susceptible than a fully shielded enclosure.

As for your circuit's actual resistance to such influences: do take every opportunity to apply ESD and EMC protection and filtering to exposed connections!  Only use the bandwidth you require.  If your amplifier is covering more frequencies than you need, you will *see* more frequencies than you need!

Tim