Capacitor Values for 900Mhz Rectenna (using dickson charge pump)

Im using a modified dickson charge pump for Rf applications. My goal is to make a 900Mhz rectenna but I am not sure what size my capacitor should be. Is there a formula or a way to simulate with ADS? I tried searching for both but couldnt find them on the net.

As a general principle you want the capacitors to charge to a DC level that does not change a lot during a cycle. Say 10 or 20 percent. Suppose you make the capacitors a very large Farad value, then they require an excessive number of cycles to charge to a suitable running voltage.

However too small a value causes the capacitors to charge and discharge quickly, reaching extremes of voltage. This reduces the amount of current they effectively carry in your circuit.

Look at schematics of antenna circuits for your operating frequency (900 MHz). A likely value to start with is a few pF.

Hey Brad, I read on literatures that 15pF is usable for 500+Mhz. Do I need to choose a lower value than that?(about 10pf or smaller) Additionally, may I know how u know that it is around a few pf and does lead ceramic capacitors work for 900Mhz?
(the last capacitor and resistor on the right are for loading so should be higher)

10 versus 15 won't make a lot of difference. To estimate a suitable value, consider how much current comes from your power source. This is the current that drives the charging/ discharging of the capacitors.

Example, if Amperes are miniscule, then you can use small capacitors. Large value capacitors are overkill in such circumstance.

Although there is a formula telling how much current causes how much voltage change on a capacitor, a simulation can tell you how your circuit will behave.

Sorry I have no experience with ADS.

I have not worked with such a high frequency. One wavelength is 13 inches, which should be all right with a small board of through-hole components.

They should work as long as you keep their wire lengths as short as possible.

I have reservations about the circuit functioning though, what is it supposed to do? If you are trying to harvest energy from cell phone transmissions I think you will be disappointed.

Brian.

oh, I know RF power are all quite low. Also brat, Could I do a simulation like finding s11 and s21 parameters of capacitor to determine it's operating frequency?

Naturally a lot depends on the degree of detail in the simulator's model of a capacitor.

This simulation resembles your voltage multiplier topology. It has generic components. Without a doubt it behaves differently from yours.

Of course it may be optimistic for us to expect the antenna to deliver 2V at 2mA.

Hey Brad, I tried simulating as well. The simulation results are strangely amazing. Even with 15pF stage capacitors, of 0.3V input, I can generate an output voltage of around 2.5V. Which is kinda high but i did not consider the parasitics in the capacitor, so I might try adding them in. However with that said, it is hard to determine the optimal capacitor value (how low it has to be) that work at that particular frequency as the voltages is not affected much with varied cap values in simulation.

Hey guys I manage to build the circuit but after testing it is off by 100 MHz. Still troubleshooting that part, I added 2 matching inductors and it is still capacitive so muz be my soldering skills. I have a question however, how do we determine the output capacitor and resistor values of at the output. Lets say my stage capacitor is only 1pF?

From the above post by Brad, I see that the output capacitor is around 100pF which is 10 times of the stage capacitors, not sure if I observed it right. Cant draw any conclusions about the output resistor though.

If you refer to the output capacitor then 1pF has slight smoothing effect. Any substantial load rapidly discharges it between cycles.