AC to DC conversion without using diodes and no external supply

Use a transformer to increase the voltage to something usable before rectification.

the

s h

You've already asked this. The answer is simple: back diodes.

What frequency?

Linear Technology have an energy harvesting chip that might be what you need, LTC3108. This includes a syncronous rectifier foe efficiency.

Mark.

Not knowing your freq requirements... I would say using a precision rectifier that can put gain on your weak signal will do..

Looking at the "Improved Circuit" should give you some ideas.

You didn't say what was the max voltage coming in?

Did you miss the 'energy harvesting' point? There's chips designed for this area, one announced recently and pointed out upthread by another poster.

Grant.

I am looking at a frequency of 950MHz. the input voltage is 200mV (peak). Also for precision rectifier I have to use external supply for the op amp. I need to make all my electronics work with the harvested energy.

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thanks for the suggestion. Can you please give me more info on how to use back diodes for rectification? If you could point me to some links it would be great. I am trying to google on usage of back diodes for rectification.

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950MHz

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Hey mark, I have already looked at the LT3108 chip.yes it's used for energy harvesting, but it needs a dc input voltage. I plan to use that chip, but before that I have to rectify my AC voltage.

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Since this is a fixed frequency, just feed the antenna power (typically at 30-60 ohms) into a 950 MHz resonator and make an additional high impedance connection into the resonator.

A 1/4 wave resonator with a magnetic probe close to the grounded feeding the low impedance antenna power and a capacitive probe close to the high end of the resonator to extract the power might do it. Rectify the increased voltage in the conventional way e.g. with a simple RF probe (series capacitor, diode to ground).

It is not at all self evident that any active rectifier circuit would work at such high frequencies.

.

Where is the 200mV peak 950MHz signal coming from? What kind of source impedance is there?

Seems to me that it is essential to have answers to these before even beginning to consider how to go about solving your problem. The source impedance will have a direct impact on how much energy you can transfer over into whatever circuit you can try to come up with.

mkaras

How is this measured ? Open circuit or loaded transmission line ?

200 mV peak is 140 mV rms and if that was unloaded voltage, the voltage into a matched load would be 70 mV rms. Assuming 50 ohm impedance levels, this is -10 dBm or 0.1 mW power available from the source in matched condition.

If the original measurement was actually open circuit peak-to-peak, the available power would be 25 uW.

If both the power source transmitter antenna as well as the energy gathering antenna are simple omnidirectional Ground Plane antennas, at

950 MHz, the path loss of 1m distance (already in the far field for that frequency) is 32 dB and at 10 m 52 dB and at 100 m 72 dB.

To get -10 dBm at the energy gathering antenna, the master transmitter power would have to be +22 dBm (160 mW) at 1 m, +42 dBm (16 W) at 10 m and +62 dBm (1600 W) at 100 m in free space. We are quickly talking about power levels at which human exposure limits must be checked.

In order to transmit back any measurements to the master receiver, co-located at the main transmitter site and assuming -120 dBm master receiver sensitivity, the slave transmitter power would have to be -88 dBm @ 1 m, -68 dBm @ 10 m and -48 dBm @ 100 m.

At 100 m and assuming 100 % RF->DC->RF conversion efficiency, the transmitter power would have to be -120 dBm + 72 dB + 72 dB = +24 dBm (250 mW). Add to this the losses in the slave (perhaps 10-30 dB), so clearly such distances (100 m) are not practical at these frequencies and these kinds of antennas.

Not true, the application circuit actually produces an AC source from DC by using a resonant oscillator which is then synchronously rectified. The device can be used with AC, see page 14: "Any source whose peak voltage exceeds 2.5V AC or 5V DC can be connected to the C1 input through a current-limiting resistor where it will be rectified/peak detected." In fact it mentions vibrational sensors as a possible input source which is very likely to be AC.

So it seems by placing a 1:12.5 step-up transformer from your 200mV input you could produce a 2.5V AC signal suitable for use.

Mark.

[snip]

950MHz is a RFID "TAG" frequency.

What is the source impedance? How about a transmission line transformer? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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           Only as good as the person behind the wheel.

Apparently the internal oscillator is operated at 10-100 kHz, so it would make sense to optimize the synchronous rectifier for that frequency range too.

I very much doubt that the synchronous rectifier would be too useful at 950 MHz, which the OP had in mind.

At 950MHz?

Grant.

!!

When I worked with them they were still down around 130kHz, time flew. And they self power from the loop field, transmit during the breaks in loop excitation. Were used back then for cattle, dog and cat 'chipping'.

Easy to get power transferred 'cos it's a tuned circuit to known frequency, and fairly short range -- rx inside or close to loop antenna.

Grant.