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× This discussion topic is moderated by MTT-S Technical Committees #6, #7, #23

current measurement for pulsed PA

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3 weeks 4 days ago #928 by Mohammad
Mohammad created the topic: current measurement for pulsed PA
Hello,
I have to measure a MMIC PA in pulse-mode and I am thinking what would be the best strategy to measure the drain current. For CW measurement I use a multimeter or reading from the power supply itself. But, now I need to measure it precisely, possibly as close to the drain as possible. I am not sure if multimeter/power supply reading will be adequate in pulse mode , that too if the pulse width is close to 1us. Could you please give some suggestion? Should I use a current sensing resistor with an oscilloscope or Is there any specialized instrument to handle such situation?

Thanks.

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3 weeks 3 days ago #929 by Farid
Farid replied the topic: current measurement for pulsed PA
There are two ways to measure the drain current in pulsed mode:
1: Hall Effect probe + oscilloscope
2: Differential voltage + resistance + oscilloscope probe

In your case, I think the Hall Effect probe is the preferred one.
The differential voltage probe requires an additional resistor in series which will cause a potential drop to be adjusted.

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2 weeks 5 days ago #930 by wheinrich
wheinrich replied the topic: current measurement for pulsed PA
Here the response from the expert on this in my department:
This scenario may be complicated because the MMIC probably has block capacitors at the supply feed so that the current will show low-pass characteristics. This may influence pulses in the µs range.
The following approaches appear to be possible:
1) Additional stabilization with a large C and measuring the average current. The peak current value can be estimated from the mean current value and the pulse width (duty cycle).
2) Use a current probe with a frequency range above 100 MHz. This should be sufficient but the probes are not small (nor cheap). See:
www.keysight.com/en/pd-2029388-pn-N2783B...t-probe?cc=DE&lc=ger
3) Implement a small current shunt resistor together with an instrumentation amplifier. See, e.g., the following paper:
Sebastian Gustafsson, Christian Fager, Koen Buisman, Mattias Thorsell, “Wideband RF Characterization Setup with High Dynamic Range Low Frequency Measurement Capabilities,” ARFTG 2016.
ieeexplore.ieee.org/document/7501956/

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2 weeks 5 days ago #931 by Mohammad
Mohammad replied the topic: current measurement for pulsed PA
Thank you very much for the response. I forgot to mention in my intial post that my pulse period is 200 us.
@Farid: I have never used a Hall Effect probe, do you know any reference material on this so that I can learn how to incorporate one into my setup?
@wheinrich: Sorry about my little knowledge, but this what I understand: If I pulse the gate bias ( and have a constant drain bias) the drain current should be a pulse (ideally). But, the feed inductance/capacitance plus the on-wafer DC bias probes inductance/capacitance will cause it to distort. So maybe I should be taking an average value of the pulse-on duration. Now the solution (1) is not clear to me, are you suggesting to put a large capacitor from drain to the ground and measure the average current from the supply and that divided by the duty cycle should give a good estimate of current peak? At this moment, I am thinking of (3) a small sense resistor+IA+oscilloscopr, but I have to see if the samll voltage drop across it will detoriate the performance as suggested by Farid. Also, what are disadvantages of simply using a bench-top multimeter with a very high sampling (50000 readings/second)?

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2 weeks 5 days ago #932 by Farid
Farid replied the topic: current measurement for pulsed PA
The reference is the following:
Tektronix TCP0030 current probe that has a current accuracy of 1 mA.
Good luck !

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2 weeks 5 days ago #933 by wheinrich
wheinrich replied the topic: current measurement for pulsed PA
Regarding (1): Yes, a large capacitor connected from the drain supply terminal to ground keeps the voltage constant during the pulse (and during the rest of the period). With this constant voltage and the assumption of ideal switching you can easily derive the peak current from the average.

Regarding (3) : You need to consider the required bandwidth for measuring the pulses. If the pulses are 1 µs long you need to sample in the range up to 10 MHz at least. I assume the multimeter uses a current shunt as well and introduces additional inductance due to the cables. Therefore I expect it to be too invasive and not applicable to measure the instantaneous current.
Is it a GaN MMIC with e.g. 28 V supply voltage or a lower voltage technology? In a 28 V system I expect that a voltage drop of 100 mV caused by a current shunt resistor is not critical, especially if you are doing pulsed CW measurements and no wideband modulated measurements for linearity evaluation.

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