Stratix II: Coil circuit

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 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?

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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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!

I simply suggested to use less turns to achieve resonance with the present capacitive load.

Also the transmitter coil can be operated in a resonant circuit to achieve a higher output level.

You don't need EMC tests for a hobby circuit, but you are required to keep the respective regulations. If your device is disturbing e.g. administrative radio-telephony or public broadcast, you can get in trouble.

How about the power measurements?

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?

And,

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?

It's not clear to me, in which regard transmitted power is relevant for your application. Although the coil is a reactive load, part of the power feed is consumed by losses, a part is transmitted to the secondary and a small part is radiated. The general limits for radiated emissions correspond to a few µW RF, to give you an idea of the order of magnitude.

As you mentioned above, you connected the coild to a digital pin through a series resistor. I don't see, how the FPGA should filter the signal, it's a squarewave anyway. So only an external filter seems to be possible.

I previously mentioned, that a filter would be necessary if you utilize an ISM frequency. Otherwise, your circuit won't comply to EMC regulations anyway, at least if the coil has some extension related to the wavelength and the transmission path isn't shielded. There are additional rules, also some national regulations, that define, which frequencies and how much transmitted power are allowed for particular RF applications.

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It's not clear to me, in which regard transmitted power is relevant for your application. Although the coil is a reactive load, part of the power feed is consumed by losses, a part is transmitted to the secondary and a small part is radiated. The general limits for radiated emissions correspond to a few µW RF, to give you an idea of the order of magnitude.

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Thanx for the clarification.

I need to measure how much power I am transmitting to try and improve my communication distance. In addition, I will be asked how much power I am transmitting and I need to know how to answer! lol!

Could you tell me how to know the power I am transmitting? Its not simply multiply the current going into the coil by the voltage around it, on the resonance frequency, Is it?

Hi,

Please dont use 1.6MHz it is the high end of the Medium Wave broadcast band and will cause local interferance, you will be generateing a lot of harmonoics some of these will be in the shortwave region and will go world wide. If you do want to do a home radio link then I would suggest using some radio modules for the unlicenced bands such as 433MHz. A google search for ism 433 mhz gave lots of interesting links.