kHz-GHz current multimeter - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: November 07, 2017, 02:05:19 am
content: Hi all,a simple question. I am looking for a lab multimeter that can measure high frequency (let's say from 1kHz to 1GHz) AC currents. I googled it but I only found that most instruments work up to few hundreds kHz or or few MHz.Do you know any high precisio instrument?Thank you

author: w2aew
author_rank: Super Contributor
timestamp: November 07, 2017, 03:10:12 am
content: Quote from: raff5184 on November 07, 2017, 02:05:19 amI'm afraid that you won't find a multimeter that can measure AC current beyond a few MHz maximum - doesn't exist. Mainly because signals at these high frequencies will be carried by transmission lines (generally), which can't be "broken" to insert a multimeter. Measuring currents at these frequencies is often done directly with wideband current probes and an oscilloscope - or derived from other RF measurements.

author: xani
author_rank: Frequent Contributor
timestamp: November 07, 2017, 07:12:43 am
content: 50 ohm load and a thermocouple

author: macboy
author_rank: Super Contributor
timestamp: November 07, 2017, 03:10:39 pm
content: Think of a multimeter as the "swiss army knife" of the electronics bench.Your swiss army knife has a blade, saw, tweezers, compass, screwdriver, scissors, etc. It is very handy. But all those functions can be performed better with single-purpose tools. The multimeter is the same, it is a multi-tool that does many things but does none of them exceptionally well.Measuring GHz range current is an exceptional task. A multimeter can't do it. A few companiesincluding Tekmake current probes that work into the GHz range. Tek's offering is described as "designed for permanent or semi-permanent in-circuit installation". This is due to the need for the conductor to be passed through the tiny little hole in the current probe head (it is not a clamp-on type like lower bandwidth ones). Of course these probes are designed to be used with a high bandwidth oscilloscope to view/measure the waveform of the current. That is just about the only way to get an RMS (or peak to peak, etc.) reading at GHz frequencies anyway... by digitizing and calculating the values. Other techniques including analog computation or thermal bridge techniques don't have the bandwidth.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2017, 07:15:18 pm
content: Combining a Tektronix CT-1 current probe and a Racal Dana 9301 or HP 3406A sampling voltmeter will yield a 25kHz to 1GHz RMS current meter.

author: raff5184
author_rank: Regular Contributor
timestamp: November 08, 2017, 08:39:05 pm
content: Thanks, do you think I can connect the CT1 directly to a Keysight 3024A Oscilloscope? At least up to the frequency scale of the DSO?

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2017, 08:47:09 pm
content: Quote from: raff5184 on November 08, 2017, 08:39:05 pmThe output of the CT1 is 50 ohms so sure. It was designed to connect to any 50 ohm input including an oscilloscope input via a 50 ohm coaxial cable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2017, 01:00:24 am
content: Dear lord, what are you doing at 1GHz that needs a current probe? You're just doing a half-assed power tap or directional coupler (without the direction) -- these items being preferable as the impedance is controlled, so you don't introduce reflections.Indeed, burden impedance is nowhere more significant than at high frequencies! You also have the problem of common mode impedance, which can be conveniently neglected at low frequencies, but is just as critical up there. This severely limits the extent to which you can even say you're measuring a current -- sure, you measured a signal across a resistance, but was that signal induced by electric or magnetic field?And absolutely, there are instruments which can deal with these challenges -- but it sounds very much like an X-Y problem: you have problem X, and suspect you need a solution Y which you are asking us about; but you should really be asking us about X in the first place!Musing:The most extreme current probe I know of is the bunching detector in a particle accelerator. This is a length of beamline (metal pipe what the particle beam zips around inside of) that has been sectioned, so that as a bunch of particles passes, the moving field of that packet of charge (in other words, a current!) induces a voltage across the ends of the sectioned pipe. The impedance is controlled by way of beamline characteristics (it's a circular waveguide) and the exact shape of the section (usually, it's made such that the pipe diameter makes a step increase, and a magnetic core is placed in the resulting annulus), and the signal is taken out by simply tying off coax cables across the section. The cables must be evenly spaced (equal angles around the section), so that there isn't a tangential reflection. Only a 1:1 ratio is possible (or N:1 for N cables in parallel, really), and the pulses are very short indeed (a 1mm long particle bunch, travelling at the speed of light, emits a cone of current only 3.3ps long -- because of the physics of acceleration, the particles are pushed tighter together into groups -- bunches -- that can be quite short, and therefore the sense pulse is quite short indeed).Tim