10067	2014-05-25 00:00:44	ashhar_farhan	scope trouble.
Message	Date	From	Subject
I have a weird problem with my rigol. I use it with a 50 terminated load the measure the carrier null of a simple diode modulator assembled on a separate small copper clad board that is fed from an external oscillaor board connnect via a short lenght of coax with bnc connectors.. If I short the probe to the ground, it still picks up the carrier of almost 100mv through the probe. I am quite puzzled. Simple electrical theory should put the hot-end of the probe at zero potential. If there is current in the scope coax's shield, should a balun help? I haven't dismantled the setup so I can probe and understand whatever is going on. Sent from BlackBerry® on Airtel
10068	2014-05-25 09:56:45	dixonglennb	Re: scope trouble.
Hi Farhan, Your problem is likely not with your Rigol and any scope will show the same thing. Try this: Use just one Rigol probe with the tip clipped to the ground clip. Now touch the grounded tip to your circuit ground. Did the signal come and go? If so, your circuit is probably injecting current into loop formed by the scope, the ac cord, the ac cord to the signal source and/or the ac cord to the power supply, and the circuit. Ferrites on the scope probes may break the loop, but I've found it easier to wrap the power supply wires around a 1" toroid a few times. If your signal source is a generator and not something on the board, do the same on that cord. Other things that may help: Single point ground for scope, power supply and signal source on the PCB. Make sure each connection (signal, DC, scope) has a low impedance direct ground return (hopefully coupled to the signal so a transmission line is formed) on the board Connecting scope, power supply and signal source chassis together with short wire (this usually reduces but does not eliminate the spurious signal). Your Rigol scope may have a terminal especially for this. Snap on ferrites on the AC power cords. Let us know if you make progress...could be something else, of course. 73, Glenn AC7ZN
10069	2014-05-25 10:02:08	dixonglennb	Re: scope trouble.
It appears you may be using a 50 Ohm coax as the scope probe instead of the high impedance probes. The same techniques apply. Ground the coax at the end farthest from the scope and touch it to the unit under test's ground. See if the signal comes and goes. Thanks, Glenn
10071	2014-05-26 06:12:38	kb1gmx	Re: scope trouble.
I see this on my 20mhz scope and other here... I have a Broadcast station on 1200khz about 4 miles away and the small loop formed by the probe and its ground lead is enough to capture a fair amount of RF. you can have the same thing is that is near the LO or other osc source. Allison
10072	2014-05-26 09:02:48	David	Re: scope trouble.
A balun might help if there is a ground loop but there is another possibility. How are you shorting the probe tip to ground? Are you using the ground lead on the probe? As kb1gmx@arrl.net points out, using the probe ground lead may be marginal. If it is a couple inches long, then shorting it to the probe tip actually makes a serviceable field probe useful in finding EMI. This is an even greater problem on high bandwidth oscilloscopes. I often end up removing the witches hat on the probe and shortening the ground connection or even better, making a coaxial connection from the probe to the circuit under test. If that is not enough, then I either use differential probe or design a 50 ohm test port into the circuit and use a coaxial cable to the oscilloscope input. On Sun, 25 May 2014 06:59:59 +0000, you wrote: >I have a weird problem with my rigol. I use it with a 50 terminated load the measure the carrier null of a simple diode modulator assembled on a separate small copper clad board that is fed from an external oscillaor board connnect via a short lenght of coax with bnc connectors.. If I short the probe to the ground, it still picks up the carrier of almost 100mv through the probe. >I am quite puzzled. Simple electrical theory should put the hot-end of the probe at zero potential. If there is current in the scope coax's shield, should a balun help? I haven't dismantled the setup so I can probe and understand whatever is going on. >Sent from BlackBerry® on Airtel