Re: RF noise on ground
|
Hello Dave
toggle quoted message
Show quoted text
I have been skipping over this thread, assuming that it concerned dealing with "ground bounce" on MSI logic chips on a buss. It¡¯s a synchronous logic thing from the days of 54/74 TTL logic. On closer reading it turns out that your noise issue might have more to do with kissing my GF while standing under high tension transmission lines. We were camped in the desert on a knoll along a DWP service road for the power line. When you stand under a transmission line your head is a tap on a voltage divider that exists between the HT line overhead and the ground you are standing on. What does this have to do with scopes you are asking? At home I lived a mile or so from KGIL 1260 and KMPC 1130 khz and 50 kw days. My scope probe tips (witches hats) obviously were a voltage divider tap between KMPC and my cement slab. Good for several hundred millivolts on my 465M. My mitigation technique was to cover the bottom of my bench top with aluminum foil connected to a ground post next to where the 465 sat. Jumper the ground to 465 ground and waala that tap moved a lot closer to ground. Audio circuits on perf-board looked much cleaner. Your luck may vary but freezer foil is pretty cheap. Eric WB6KCN -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Dave Peterson via groups.io Sent: Friday, April 9, 2021 9:34 PM To: [email protected] Subject: [TekScopes] RF noise on ground Could use some help here: I've always had persistent high frequency noise on my scopes. So far I've been focused on lower frequency signals with reasonable signal strength. Between BW Limit turned on, and HF Rej on the trigger coupling I've been able to live with it. But now I need to calibrate some vertical assemblies, and it's become untenable. In exasperation trying to find the source I've tied the scope input to ground, only to find an even stronger noise signal! See the pictures in: /g/TekScopes/album?id=262825 This is about a 100MHz signal. I also have to say I happen to live about 2000ft. away from Sutro Tower in San Francisco: . I've turned off all manner of potential sources, short of the household Wi-Fi. Can't do that with teenagers in the house! Could the wireless router really be the source of ground noise? I've also tried relocating - the pictures are taken from my living room. It's the same no matter where I go, or what equipment is on or near. This only comes from the scope inputs being tied to ground, or a floating probe, or on a DUT. It's very intermittent, variable, and dynamic when not grounded. Selecting GND on the input does suppress the noise, but clipping a 10x probe to the chassis yields the noise again. It's consistent across scopes, both 2236's and 465s. I have a pretty well grounded house. When we had our service upgraded the electrician drove a grounding post at the service entrance, and we have good ground straps throughout. I've also run a 4 gauge (0.2") ground line to my work bench and it doesn't much make any difference if I ground the scope to the bench or not. The supply ground and bench ground measure 0 ohms as well as my DMM can tell. Is this really RF energy coming from Sutro Tower? And why so specific a frequency? I see no other signals. The modulation is pretty obvious, but it's not dynamic. But who knows what all is coming out of that collection of antennae. And is there anything I can do to mitigate it? Thanks for whatever help the collective can provide. Dave |
Eric Schumacher