Stratix II: Coil circuit

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You can easily see, that it's impossible to achieve a several 100 MHz resonance above a certain coil inductance. But there's no need to have more than one turn with your coil, thus you may be still able to design a coupler with sufficient output level.

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Could you please elaborate more on this?

I understood what you said, still I do not understand the reason or the explanation of it.

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You should consider however, that you're effectively operating a RF transmitter with considerable output power (mW range)

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What is the best way to measure transmitted power in this case? Isnt the average power consumed by the coil is zero?

I know if I want to transmit more voltage, I have to transmit less current. But since I am dealing with FPGA, I am more interested to transmit higher voltage. How would I increase the transmitted power?

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and that your instrument won't pass EMC tests unless the coupler is completely shielded by a metallic enclosure. Even if you would use a free ISM (industry, science and medical) frequency, the clock signal has to be thoroughly filtered from harmonics.

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okay give me a moment here.

1. I have to make an EMC test?

2. How should I filter my transmitted signal? Just use a bandpass digital filter at the output of FPGA1 (the transmitter)? or programming this FPGA to do that job and output a filtered signal is a better option?

3. Will it really cause me, as a person, a problem to send a wireless signal with some harmonics outside the ISM band?

Thanx! Interesting discussion we have here!

Pin capacitances are listed in the device handbook, simply assume 5-6 pF for most pins.

You can easily see, that it's impossible to achieve a several 100 MHz resonance above a certain coil inductance. But there's no need to have more than one turn with your coil, thus you may be still able to design a coupler with sufficient output level.

You should consider however, that you're effectively operating a RF transmitter with considerable output power (mW range) and that your instrument won't pass EMC tests unless the coupler is completely shielded by a metallic enclosure. Even if you would use a free ISM (industry, science and medical) frequency, the clock signal has to be thoroughly filtered from harmonics.

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A coil with a capacitive load (oscilloscope probe or a FPGA input) is a resonant circuit, connecting a different load changes the resonance frequency. It's basically possible to design a wideband inductive coupling, but most likely an amplifier with suitable frequency characteristic is required.

You didn't tell about the waveform that's intended with your application.

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A great answer, and a confirmed one. I checked in the lab and what you said is absolutely true.

As a start I want to send a digital clock signal.

Now I have a problem. I am not sure how much capacitance the FPGA has, but if I only connect a coil of 100microH, I have Wo (the resonant frequency) at around 1.6MHz (which gives the maximum voltage to the FPGA input). I want to increase Wo alot, to a magnitude of 100s of MHz.

I know I have two options to play with on the secondary circuit side: Either to make my L (coil inductance) lower, or make my C (capacitance) lower. At this point I have these questions:

1. What is the capacitance at the input of the FPGA (Stratix II GX)? I can connect a capacitor in series to make the capacitance much lower in this case.

2. If I made my L lower, the communication distance between primary and secondary circuits should be reduced as a result. And since I already have the coils almost touching each other, I do not think this is an option. What do you recommend?

3. Is there another better way to operate the circuit on a much higher frequency?

A coil with a capacitive load (oscilloscope probe or a FPGA input) is a resonant circuit, connecting a different load changes the resonance frequency. It's basically possible to design a wideband inductive coupling, but most likely an amplifier with suitable frequency characteristic is required.

You didn't tell about the waveform that's intended with your application.