Damn you, ground lead!!! - Page 1
Forum section: electronics / beginners

author: AlfBaz
author_rank: Super Contributor
timestamp: February 02, 2015, 10:43:50 am
content: It seems I keep coming up with ways to get bitten by the same thing!So I'm scoping a simple RC circuit configured as a low pass filter fed by my function gen.1k resistor and a 0.1u cap. I decide to wind up the sine frequency all the way to 20kHz, namely to see my measurement limits than anything to do with the LPF, and I see an incredible amount of noise on the output sine. Whats with all the noiseI zoom in on the noise and can just make out what appears to be a cyclic nature to what otherwise looks to be random jagged signal. I whip out the cursors and line them up with 2 nearby peaks and they're smack on 100Mhz. I fix the difference between the cursors and begin to scroll along the signal. The cursors line up with all the peaks and troughs.Where is this 100Mhz coming from, I think to my self as I tap my fingers on the bench and stare blankly at my function gen. I then realise I'm staring at the number 100MHz on the label of my wavetek arb.Damn it, its the dac's output I'm seeing. But how? The RC filter is killing the 20kHz signal nicely, why is this 100Meg getting through?I then look sullenly at the 6 inches of ground lead on the probe... You again!!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2015, 12:28:25 pm
content: Is the beast really that poorly filtered?It's probably not even the signal, as such, but common mode crap. My Wavetek 193 isn't exactly RF-grade anyway (at heart, it's no more than an RC timer, so the phase noise sucks), but the emissions from the output connector(s) with a cable attached (but not plugged into anything) are quite easily sensed by radio equipment.Fine equipment, but maybe not fine enough...On the plus side, 100MHz is easily filtered with ferrite beads. Give that a shot.Tim

author: AlfBaz
author_rank: Super Contributor
timestamp: February 02, 2015, 01:52:39 pm
content: Quote from: T3sl4co1l on February 02, 2015, 12:28:25 pmAccording to the manual it has 3 to choose from plus automatic mode (default) which is where I leave it. The choices are 20 and 40MHz elliptic, a 10Mhz Bessel and noneQuoteIt may well be, my understanding of common mode interference is very basic to say the least. However, having suffered previously at the hands of long ground leads I decided to take one of these scope probe to BNC adaptorsand turn it into one of these...The signal cleaned up nicely. I put it down to the inductance of the ground lead. Does shortening it reduce susceptibility to common mode noise?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2015, 04:11:24 pm
content: It's not so hard to imagine. Just think of the loop between the cable (as a single lone wire, ignore the fact that there's more than one wire inside there) and any kind of ground return (which can be space itself, but is often some other powered or grounded equipment coming back through the power line or ground). Thus, anywhere you have CM current along a cable, and a poor shield on that signal (like the ground clip, rather than the shield being 100% wrapped around the signal line), you get some voltage drop that appears on one conductor but not the other, and therefore, "ground loop" -- the introduction of a ground-current signal into a differential mode input (the scope probe).What does it come from? Inside the generator, maybe their output buffer circuit sucks, and drives voltage into the ground line. Typical construction is a short piece of coax running from the front panel connector to a header on the board (or soldered in), and both ends of that piece of coax are unshielded, so, boom, common mode interference there (even when reading the FG output direct via coax to the scope).It doesn't help that FG outputs are usually "floating", which usually means, they allow some (SELV level) DC between safety ground and signal ground, but not usually AC (usually being bypassed with a capacitor between grounds). But that bypass is usually done poorly -- a single capacitor at a single point, so there's plenty of opportunity for noise pickup and transmission along the way.Tim

author: timb
author_rank: Super Contributor
timestamp: February 04, 2015, 05:28:15 pm
content: Go read Jim Williams infamous App Note 47 from Linear Tech. Read the section on scope grounding and probes near the beginning and in the appendix. Seriously, go read it right now.Sent from my Smartphone

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 04, 2015, 05:34:52 pm
content: 6 inches? damn that long. mine is only 3 inches and can already dig in wonderful stuffs.