Hameg HZO50 current probe weird high-frequency interference - Page 1
Forum section: electronics / testgear

author: Svuppe
author_rank: Regular Contributor
timestamp: February 23, 2014, 12:09:01 pm
content: Hi everyone.I have got a DC to 100 kHz current probe for my scope. A Hameg HZO50http://www.hameg.com/0.554.0.htmlThe specs looks nice on paper. In reality it also performs reasonably well, but it suffer from some very weird high frequency bursts at around 24 MHz, that interfere with the scope trigger if I want to look at low currents.This is what I am seeing without any wires going through the probe:Zooming in:And even closer:And now for the weird part. I can't remove these bursts in any way. I first thought they were generated inside the probe electronics itself, but if I turn the probe off, the bursts remain. Even if I remove the probe battery completely, the bursts are still there.I then took the probe with me to work, and tried to use a completely different scope...... The bursts are STILL there.I have tried to hook up the probe to my spectrum analyser, and this is what I get (still with the probe turned off by removing its battery):Terminating the cable in hi-Z or 50 Ohm makes no difference. Adding ferrite beads to the cable doesn't do anything either.Sometimes the bursts almost disappear when I touch the scope ground (one of the other BNC inputs) with my multimeter probe (and the other end just lies on the table without touching anything). However, touching the scope ground directly with my fingers makes no difference.Do you have any ideas as to where this interference is coming from, and what I can do about it?

author: madworm
author_rank: Frequent Contributor
timestamp: February 24, 2014, 02:42:42 pm
content: Some time ago there was a post about a Hameg HMO scope that had power-supply issues, it had strange bursts as well. Maybe this one is also affected.

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 24, 2014, 03:36:45 pm
content: Does your scope have a 20Mhz bandwidth limit switch? The 24Mhz noise is close to the cutoff, but it might reduce the noise enough to stabilize triggering. The probe only needs flat response to 100khz, so any kind of low-pass filter would help. It looks periodic, so waveform averaging probably wouldn't help much, but it might be worth a try as well. This noise looks a lot like switching power supply line noise. I had a lot of that in my shop until I finally sniffed out all the wall warts and other electronics in the vicinity and trashed a few of the poorly designed ones. A computer monitor turned out to be the most serious culprit in my case. Good luck.

author: Svuppe
author_rank: Regular Contributor
timestamp: February 24, 2014, 04:58:17 pm
content: Yes, my scope has 20 MHz BW limit, but it didn't help much.I've had a closer look at work today, and the frequency of the bursts are not the same there and at home. They are very similar in the timing, as in two bursts followed by a little delay, but different frequencies. And at work I managed to find the noise source. It was the backlight in the Tektronix scope I used there. If I placed a few fingers near the screen, and my other hand on the current probe, the bursts increased in amplitude by 4-5 times.At home I haven't found the offender yet, but I've tried to put a 200 kHz low-pass filter in the line, and that seems to have eliminated the problem

author: tszaboo
author_rank: Super Contributor
timestamp: February 24, 2014, 06:01:46 pm
content: Quote from: Svuppe on February 24, 2014, 04:58:17 pmYes, scope screens can be really noisy. Also, lighting in the room, close-by PCs, screens, laptops.