# “RF energy harvesting” Can rf transmitter be a rf signal generator?

I'm trying to build an rf energy harvester. But having some problem.

The harvester should collect the 915MHz signal.

1. Can I use 915MHz RF transceiver + 915MHz amplifier as the rf signal source?
2. How do I determine the value of the capacitor in the picture? I had read some paper but they didn't tell how to choose the capacitors.

The purpose is to receive ambient 915MHz RF signal. In order to do experiments, I need a stable signal source so that I can change the distance and see how is the efficiency

• What is $V_{in}$ connected to? What voltage is being applied? What is the forward voltage of the diodes? (That's a big hint.) Put all the information in your question - not in the comments. You probably also mean MHz rather than mhz. Please fix. – Transistor Mar 23 at 11:00
• What do you expect to be the source of the energy you are "harvesting?" From the frequency given, it seems you are looking at an ISM band. Ask yourself how much power is allowed to be transmitted in those bands, then consider how much time you will need to gather a useful amount of energy. "Useful" being determined by what you are going to do with it. – JRE Mar 23 at 11:25
• Thanks. Vin is connected to the impedance matching circuit. The diode is hsms-285c(Avago) and the maximum forward voltage is 150-250mv. – penli Mar 23 at 11:32
• And what is on the other side of the impedance matching network? An antenna? What transmitted power source are you trying to receive, and from how far away? – JRE Mar 23 at 11:39
• Again, please put all the relevant information in the question and not sprinkled through the comments. Make it easy for people to help you. "Hz", not "hz". SI units named after a person have their symbols capitalised but are lowercase when spelled out. – Transistor Mar 23 at 11:43

## 2 Answers

1. Yes, you can use a transceiver + amplifier for 915MHz. Your usable range will be very low (1m at most) or charging time will be very long.
2. The last capacitor's value is determined by your energy requirements. You will have to calculate how much charge you need over what period of time.
3. The HSMS line from Broadcom is obsolete. Good luck finding alternatives.
4. For stable signal, you can use UHF TV stations, they transmit at much higher power levels than your ISM transceiver. The original RF harvester demo from TI (WiSP) used the same circuit with a high gain antenna aimed at a TV transmitter to power an MSP430 with a small LCD.
• Thank for your reply. I had found some online shop which providing hsms now. I mean the capacitor in the rectifying circuit. C1,C2,C3? – penli Mar 23 at 12:18
• By the way, do I need to consider the modulation of the rf signal from the transceiver? No matter it carries information or not? – penli Mar 23 at 12:29
• Alternatives are easy enough to find. I recently a bunch of diodes of similar performance that were not obsolete. Actually for use in a similar application. My hat band now blinks if you use your cellphone too close to my head. Entirely passive (and short range.) About 10cm. – JRE Mar 23 at 12:57
• You don't care about the modulation, this circuit turns it to DC anyway. As long as the duty cycle is high enough, you are collecting energy, one pico Joule at a time. As for the rectifier capacitors, the WiSP used 100nF 50V ceramic capacitors. Larger capacitors - longer charge times. – Lior Bilia Mar 23 at 13:04
• Thanks. And how about using dipole antenna for transmitter and harvest in this experiment though dipole antenna has a bad directional characteristic.. – penli Mar 24 at 8:12

If you are receiving a signal "from the air" then a resonant input circuit is essentially essential. Also, in real world use an aerial/antenna is required.

Using a signal source with enough voltage may allow use without a resonant circuit, but this will seldom represent reality.

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This paper appears to address your requirements.

Optimization of the Voltage Doubler Stages in an RF-DC Convertor Module for Energy Harvesting

This Avago application note

The Zero Bias Schottky Detector Diode investigates diode operation at zero bias. Should b read in conjunction with the following application note. Also Avago 1999.

This paper MAY revolutionise your results - or not :-)

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Possible alternative diodes:

The NSVR201MX available ex stock Digikey appears to have a fighting chance of doing the job. It's a single diode.

The NSVR351 is not in stock / 8 weeks lead time but is a dual series diode as is the original 285

Pricing and availability

• Thank you. I will read the paper. Is vector network analyzer essential for impedance matching? I don't have this equipment... – penli Mar 24 at 14:29