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Resonant circuit matching
Jan 29^th, 2014, 2:55pm

Hi,

I'm designing a device that uses RF energy from a transmitter to power itself. To achieve best working distance a resonant circuit is used. So the power circuit of the device consists of an antenna, a resonant circuit and a rectifier (voltage doubler). And the resonant circuit is a simple LC circuit.

To obtain maximum voltage on the rectifier input, the inductance value of LC circuit should be as much as possible. But at the same time, when this value is maximal, the output impedance value of the resonant circuit becomes very high. Hence the output impedance value is differ from 50 Ohm, and a transmission line on its output changes the resonance frequency of this circuit.

How to match the resonant circuit to 50 Ohm transmission line? Or what is the proper way to solve this problem? I think that this construction should be trivial and widely used. But I couldn't find any useful information about this.

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aaron_do

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Re: Resonant circuit matching
Reply #1 - Jan 29^th, 2014, 4:53pm

Hi,

are you saying that there's a transmission line in between your LC impedance transformation and your rectifier? If so, I think you need to remove it. i.e. move your LC as close as possible to the rectifier. Otherwise, you may be able to design the transmission line to be exactly half a wavelength. If your transmission line length is fixed, then you need to design the LC impedance transformer so that the combination of the LC + the transmission line gives maximum output voltage. The smith chart is very useful here.

regards,
Aaron

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natnoraa New Member Offline Posts: 6	Re: Resonant circuit matching Reply #2 - Jan 29^th, 2014, 5:18pm Hi alfred.black, I agree with Aaron over here. However may i ask what's the purpose of your transmission line? in between the o/p of your resonant circuit and your i/p of the rectifier? as an impedance transformer or? you may need to redesign using the smith chart. please correct me if i understand this wrongly. natnoraa
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RFICDUDE

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Re: Resonant circuit matching
Reply #3 - Jan 29^th, 2014, 8:02pm

The proper way to view this problem is to consider what is it that maximizes the efficiency of the rectifier in converting the received power into a useful DC supply.

An optimized crystal radio is an example of a circuit that tries to maximize the audio output of a RF detector that is only powered from the received signal. The problem requires that the antenna is power matched to the diode to maximize the sensitivity and then the diode is also power matched to the DC load to maximizes the power transfer.

50 ohms is not important for the problem. The diode impedance at zero bias should be very high to maximize detector sensitivity. Usually a Schottky or germanium diode is used.

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