RF magnetic field generation

[Malamala]

Hello! I want to generate an RF magnetic field at variable frequencies (from 1 to 20 MHz) using this amplifier: https://www.minicircuits.com/WebStore/dashboard.html?model=LZY-22+, by passing current through a loop of current (assume the inductive resistance is negligible). How should I proceed in practice? Can i directly connect the loop to the RF amplifier? Should I add a 50 Ohm in series? Thank you!

 

[DaveE]

Can i directly connect the loop to the RF amplifier?

It sounds like you can, it says it's protected from opens and shorts and is unconditionally stable. But it will work better with an impedance transformer to better match the very low load impedance to the 50ohm output. It's not clear how much current you'll get out of it when it's output is shorted, like most RF stuff it's only specified at 50ohms. Why don't you ask the guys that make it?

OTOH, if you get enough current with a 50ohm series resistor, then I'd do that, the amplifier will be happier and it's easy to understand.

Also, at 20MHz a 50ohm inductor is only 400nH (not really a short circuit), so a reactive matching network (Smith Charts etc.) would be the best because of the wide bandwidth. You'll need to understand your load impedance to do this right.

 

[Malamala]

It sounds like you can, it says it's protected from opens and shorts and is unconditionally stable. But it will work better with an impedance transformer to better match the very low load impedance to the 50ohm output. It's not clear how much current you'll get out of it when it's output is shorted, like most RF stuff it's only specified at 50ohms. Why don't you ask the guys that make it?

OTOH, if you get enough current with a 50ohm series resistor, then I'd do that, the amplifier will be happier and it's easy to understand.

Also, at 20MHz a 50ohm inductor is only 400nH (not really a short circuit), so a reactive matching network (Smith Charts etc.) would be the best because of the wide bandwidth. You'll need to understand your load impedance to do this right.

I actually checked it carefully and in my case $X_L = 10 \Omega$. If I connect the loop directly, and I want to compute the current through the loop, should I then divide the applied voltage by this 10 Ohm? or does the amplifier has an extra 50 ohms internally so i need to divide by 50 + 10 = 60?

 

[DaveE]

I actually checked it carefully and in my case $X_L = 10 \Omega$. If I connect the loop directly, and I want to compute the current through the loop, should I then divide the applied voltage by this 10 Ohm? or does the amplifier has an extra 50 ohms internally so i need to divide by 50 + 10 = 60?

It's hard for me to believe your load is constant 10 ohms from 100KHz to 20MHz, but whatever. Yes the amplifier will have a 50 ohm output impedance.

 

[berkeman]

It's hard for me to believe your load is constant 10 ohms from 100KHz to 20MHz

That does not make sense to me as well. @Malamala -- can you show us the calculation for this?

 

[DaveE]

I actually checked it carefully and in my case XL=10Ω.

Sounds like a 4:1 balun would get you pretty close to a match then.

PS: or is that 1:4? I'll never get that part right, LOL.

 

[Baluncore]

How should I proceed in practice?

Practice will be decided by your application's requirements.

You want a magnetic field, but have not specified the "spatial volume" or the "flatness" of that field within the excited volume. Will the test chamber be the volume of a matchbox, or a parking garage?

Why are you doing this? What you place in the excited volume, may change the field. Will the sample absorb RF energy?

Will the frequency be fixed for each experiment, or will the signal be wideband, or swept across the HF band? How will you prevent radiation of unwanted RF from the excited volume?

You could make a resonant cavity, a transmission line, a waveguide structure, or use Helmholtz coils. Some form of amplitude regulation will be needed to stabilise the magnetic field. I expect you will need some form of impedance matching tuner, maybe an automatic HF antenna tuner, to match the generator to the excited volume.