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How to drive small inductive loads up to RF?

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horror-vacui:
Hi Everybody,

Could you help me giving some keywords, tips and hints for a measurement equipment to drive a wide range of slightly (=not a motor) inductive load with a sine wave for a wide range of frequencies?

I wish to investigate wireless power transfer with a small integrated RX coil. For energy harvesting a given voltage level is needed in the RX coil, which can be increased by increasing the frequency. I aim for a tiny RX coil, with even just sub-mW transferred power. A higher than typical frequency will help to increase the received voltage.

What affordable test equipment could drive a TX coil in a wide freq range from sub-MHz up to 100s of MHz? Waveform generators are not that fast, and I need only a sinewave (some AM modulation would be nice though, but it is not required). RF signal generators tends to be pricey, and provide more than I need, but in features, and in precision as well. I though of some SDR solutions with an additional power amplifier, but the wide range of non-resistive load values are usually disliked by RF power amplifiers. There could be either high current or high voltage levels at the TX coil depending on the frequency and the coil. Are you aware of a class of amplifiers which can deal with these requirements? I would be happy with two instruments for different frequency ranges as well. In inductively coupled wireless power transfer the current drive is more important to build up the magnetic field, but typical instruments have a limited current driving capability related to the often assumed 50 ohm load.

I am happy to build something by myself, but I would rather trade the time of designing and debugging for the cost of an instrument. Nevertheless the cost of the instrument should be also affordable: I am sure, I can not consider anything above $1000€ at the moment. Do you have any ideas or suggestions, which path to take? Any keywords I should search for?

Prying:
while im not sure what test equipment is avalible for this but cant this be done with a software defined radio IC/package?

TimFox:
Have you looked for used "RF Signal Generators" from eBay, etc?
They usually have an internal 50 ohm source resistor, and will tolerate almost any (passive) load after the output connector.

jonpaul:
Bonjour: Nikola TESLA (NOT the car!) investigated this at Colorado Springs in 1880s, with a 50 KW several megavolt tower at ~ 20 kHz resonance.

The physics of wireless power transfer and aircore transformer coupling is very well studied and documented.

The magnetic field falls off so rapidly that the efficiency is extremely low unless the coils are next to each other.

Just take any audio oscillator or function gen and wind a few coils.

Cost should be ~ $/EU 20..50 not 1000!


Bon Chance


Jon

David Hess:

--- Quote from: horror-vacui on June 25, 2022, 12:14:53 pm ---Could you help me giving some keywords, tips and hints for a measurement equipment to drive a wide range of slightly (=not a motor) inductive load with a sine wave for a wide range of frequencies?

I wish to investigate wireless power transfer with a small integrated RX coil. For energy harvesting a given voltage level is needed in the RX coil, which can be increased by increasing the frequency. I aim for a tiny RX coil, with even just sub-mW transferred power. A higher than typical frequency will help to increase the received voltage.
--- End quote ---

Wireless power transfer coils are resonated with a capacitor to achieve the needed circulating current and strong magnetic field for longer range.  They are driven close to their resonant frequency by a switching instead of linear stage.  A resonant switching regulator controller could be used and these are the types of circuit which should be studied.

For experimentation purposes at lower power, I might use a transconductance (current) output stage which will have a big advantage in speed for higher frequencies, but that is not how real inductive power transfer systems work.  In practice though a 50 ohm series terminated current feedback amplifier will be just as good and easier to implement.  There are some high power ones intended for DSL applications available.

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