Tuning the antenna to be conjugately matched to input impedance of the die

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doenisz

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Hi guys,

I am currently designing a circular loop antenna to be matched to the input impedance of the rectifier, at 3.5GHz. I did HFSS simulations to verify the impedance of the antenna plus the bondwires, and I used Cadence Post-Layout simulations to find the input impedance of the chip itself.

Basically, at 3.5GHz, the input impedance of the chip is like 0.7-j270, and the antenna+bondwire is 1.5+j270. For context, I'm following the methodology described here: https://ieeexplore.ieee.org/document/7435352

However, due to the other secondary effects I cannot control, they may not be matched at 3.5GHz and my chip does NOT have a control loop to adjust the resonance.

I can change some of the antenna geometrical parameters by cutting some traces etc. but I'll probably need a drilling machine. However, my antenna will be very close to the die (to reduce bondwire, transmission line and loss effects) so drilling machine may cause my wirebonds to fail.

I can always find the resonance frequency and then order new PCBs as a trial and error but wirebonding new dies to the new PCBs would cause a lot of time wasted and money too.

So, I would like to ask, in case I find that the resonance occurs at a higher or lower frequency that 3.5GHz, what can I do to tune this resonance? I also thought of using a varactor to tune the input reactance of the antenna but I was wondering how it would change the radiation efficiency?

Thanks.
 

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What about the output impedance of the chip ? Is it very well controlled and fixed along a frequency band or it may show some/much deviation. You should ask this question first.
If there is no automatic mechanism, you will always loose or gain therefore you have to specify min. and typical values for a desired performance. Nothing is guaranteed in this world.

The good news is that the antenna impedance is very close to chip's input impedance so conjugate matching is almost there. A high valued coupling capacitor will terminate the matching.
Select a 0201 RF capacitor which does have a nominal value exceeding 3.5GHz.
 
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I don't know the output impedance because I just need to input impedance to know about the reactance, so that I can design an antenna to be conjugately matched to it, and then know the passive RLC boost on the input capacitance. What I mean by the input impedance is just the impedance seen looking into two RF inputs.

Right but all of them are simulated values. I found the antenna impedance from HFSS simulations (bondwires considered), and chip's input impedance from Cadence Post-Layout simulations. So, it's not certain if they will be matched once I get the chip packaged.

Not sure if I fully understand the capacitor idea. Could you elaborate on what it does? Should I add it in series to the antenna and then (potentially) tune it during the operation to satisfy matching?
Best.
 

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In that case, a strong matching is not necessary to my understanding.
Since the simulated impedances are close to reality, you can connect the IC directly to the antenna through a coupling capacitor. This capacitor will not serve to matching purpose but almost all IC have DC throughput that's why I said "connect a small capacitor"
The best way is to measure these impedances. If you're able to access to a Load Pull -even simple one- test bench, you can always match very well the IC to antenna. Theoretical values will you misguide.
 

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Thank you, I will keep these in mind.
I have one more question. For my antenna to be low-loss, I am using Rogers 4350B substrate instead of FR4. I initially designed the substrate thickness to be 10 mils, but I was wondering if this would cause mechanical problems like being brittle. Should I make it 62mils like standard FR4?
Never used Rogers 4350B so I don't know how rigid of a material it is.
 

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A lumped series capacitor will increase the resonant frequency of the loop, but at 3.5GHz will be hard to implement do to small dimensions of the loop.
You can design a printed capacitor to increase the resonant frequency, and/or a stub (or more) to decrease the resonant frequency.
 

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