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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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X10 Low Impedance Probe
Hi folks,
The other day, I was trying to troubleshoot a 100MHz amaplifier in a piece of equipment ( actually, a Cushman CE-3 Spectrum Monitor ). I was using a 150MHz tektronix scope with a 10X probe. I was getting different results with each measurement, it was driving me crazy. My friend Bob, the RF guru, said "Your trouble is that you're using the HI-Z input to the scope. For VHF, Hi-Z is crap. It just doesn't work. Especially if you have a pigtail for the ground." Guilty as charged... He then asked if the scope had a 50-ohm input ( which it does ). "Then just make up a probe with a 450-ohm series resistor, and some little ground clip. That's all you need. It will be stable, and it will work up to about a gig. The impedances in this RF stuff tend to be low anyway, so you don't really need Hi-Z." Makes sense. Any way to buy a probe like this off he shelf? - Jerry Kaidor ( ) |
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mike wrote: wrote: Hi folks, ...snippety.. Makes sense. Any way to buy a probe like this off he shelf? I'll bet Tektronix and HP/Agilent will sell you one, but you don't want to know what it costs. Problem with commercial probes is that they have a rod resistor with a probe point soldered into the end. Lots of stress, easy to break, expensive to fix. The resistor on the end of a coax is a pretty good solution. Pack it into a ball-point pen. When you break it, replace the resistor. 1/4W carbon composition works well. If you try to use a metal film, you may have to worry about the inductance. Don't know any way to tell without measuring it. If you want accurate/repeatable measurements at 100MHz. you need to build the measurement point into your circuit. ANYTHING with a ground clip will require an inch or more of wire just to let the probe reach the circuit. Ground inductance is your enemy. Think of it as a series resonant circuit. The tip capacitance and the ground + tip inductance resonate at some frequency and make a short at your circuit. Obviously, the accuracy decreases rapidly as you approach this frequency. Doesn't take much ground wire to mess things up. mike tp://www.geocities.com/SiliconValley/Monitor/4710/ A good hint is to downlod the manual from Tektronix website, called "ABC of probes". Gives a lot of basic info. The "DIY" approoach is nice and will work well. good Tektronix Probes in the low-Z Area are on2nd hand market like ebay. I am not shure about the correct probe number, AFAIK it is P3056 (10:1 low Z, made in the 1970+). I recommend you look in Tek Catalog for the description , under "obsolete probes" reference page. Probe is subminiature and works well, price was about 15 USD. Specified for up to 3 GHz use, which is more than adeaquate for my Tek 7104/485/2465B scopes ;-) hth, Andreas |
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