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Thread: Bandwidth of Analogue Multimeter

  1. #1 Bandwidth of Analogue Multimeter 
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    I need to make high frequency current measurements. I have a standard electromagnet based hand-held analogue multimeter. However, I am unsure if this type of meter has a bandwidth limit. The currents I am measuring will be oscillating at a frequency between 1 MHz and 10 MHz. I would appreciate any insight into the bandwidth of this type of meter.

    David


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  3. #2 Re: Bandwidth of Analogue Multimeter 
    Forum Ph.D. Leszek Luchowski's Avatar
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    I would try to find some manufacturer info on that. The measuring mechanism itself (coil, spring etc) would obviously never cope with this kind of frequency, but you would be using one of the AC settings, so the electronics will convert your current into DC before feeding it through the coil. So now it's up to the kind of electronics you have in your meter.

    Also, is it current or voltage you're measuring? And how big is it - microamps or amps? Millivolts or volts?

    If it's "macroscopic", from say 0.2A or 1V upwards, your meter might just use a rectifier bridge (along with resistors) to do the conversion. With luck, it might cope. Otherwise, we're talking about an input amplifier, and its bandwidth can be very different depending on the brand and model.

    To be really professional, you should use a meter with a probe (instead of just plain copper leads), or, better still, an oscilloscope.

    Good luck,
    and keep us posted - Leszek.


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  4. #3 Re: Bandwidth of Analogue Multimeter 
    Forum Sophomore Vaedrah's Avatar
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    I agree with Leszek - an oscilloscope (measuring voltage drop across a known resistor) would be best - however if this is intended for a 250 kV Tesla coil I doubt its insulation would stand up to the task!

    If the wave is reasonably sinusoidal then you could use a biased diode detector. IN4148's are quite OK to 10 MHz. If you use one to rectify the AC waveform, and another to act as a DC reference for temperature tracking, then your DC voltmeter could be used to measure the difference.

    Given that the bias might only be 10 uA for the diodes, a 9 V battery with 1 Meg resistors would be OK. Both this and the multimeter could be insulated from ground and handle the EHT.

    Biased diodes can detect down to about -50 dBm @ 50 Ohms, so sensitivity shouldn't be a problem. However the AC to DC detection will be affected by harmonics - sparks will probably create these!
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  5. #4  
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    Standard meters run on 50 to 60hz.

    You need to use an oscilloscope.
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  6. #5  
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  7. #6  
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    Not with a standard DVM you won't, at the very best a few kilohertz might be achieved but going into RF is a different ballgame, at RF your meter will have appreciable capacitance and inductance and therefore will affect the performance of the circuit, there are lots of ways of doing this, eg, input powre measurements with rf present and non-present (and knowing the stage efficiency will give an approximate idea, using a scope with a small resistance in series BUT you must also take phase angle into account, it can be a nightmare for the inexperienced, so please give some more detail of the circuit and conditions with which you are dealing with.
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  8. #7  
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    I have a B&K meter that will handle up to 40 Mhz as a frequency counter, it however won't accurately measure AC voltage that's not a pure sine at a low frequency. You need a true RMS meter to do that, and a good one. A scope would be best with a tip add on that does a divide by 10 or 100 or 1000. More or less it's a precision resistor on the end that allows for measurement of very high voltages. A good true RMS DVM might be ok if also used with the right voltage divider. I would use a scope, I would also make sure it has really good insulated probes, even with the divider on the end.

    Measuring current is actually easier then measuring voltage. You would just need to know the exact resistance of a junction in the circuit and measure the voltage drop across it. Scope would still be best.
    Pleased to meet you. Hope you guess my name
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  9. #8  
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    Treat it as a 4 wire black box, measure the supply current quiescent with no RF output then, with. IF you want to truly measure high frequency currents it's a real minefield and not easily done. You can measure it with a scope but you will be lucky to get within 20% of the real value. You also need to be aware that if there is a mismatch between the RF generator and the load all your measurements could be meaningless.
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