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Just a note: This is the second time I've ever used a VNA.

I am trying to measure S11 of the 1.88GHz RF Input of this Powercast RF Energy Harvesting board. Link to the datasheet. Powercast Board Datasheet Powercast Board Schematic

The basic block diagram for this device is RF Input> Rectifier> Capacitor >Boost converter>Load

The capacitor is charged by the rectified output of the rectifier chips and charges to a set "turn on" voltage. When this "turn on" voltage is reached, the boost converter turns on and uses the energy stored in the capacitor to power a load. The load in this case was a simple LED and series resistor.

Now, for the VNA. I have calibrated the VNA port, I have set up a 1GHz sweep with 401 points with 1.88GHz being the center frequency. I set the output power to -5dbm and turned off the "Power Slope" feature. When I turn the VNA trigger to continuous, I see that S11 steadily gets better and better (i.e. lower magnitude at 1.88GHz) and then S11 will get worse (i.e. higher magnitude at 1.88GHz).

The catch is that S11 gets worse right after the boost converter turns on and powers the LED. It repeats this cycle of S11 getting better and then LED turns on and S11 gets worse again. I'm not sure why the capacitor building charge would decrease reflection back to the input.

I've linked a video that I took of the phenomenon happening, you'll see that S11 gets better and then it jumps back up. At the moment it jumped back up the LED turned on, I just wasn't able to record both at once.

YouTube video of the phenomenon.

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  • \$\begingroup\$ The data sheet gives very little information as to the characteristics of the RF input. In fact, it only lists the frequency bands of operation and the permissible range of input power with no mention of input impedance. There are also no schematics. Without this information it is impossible to determine why S11 changes. However it is clear that turning on the boost converter affects the input stage. \$\endgroup\$
    – Barry
    Commented Mar 12, 2021 at 2:36
  • \$\begingroup\$ Forgot to attach the schematic. Its attached now. However, the chip responsible for the rectification(PCC110) has its datasheet behind an NDA thus making it fairly difficult to figure out what's going on. Thanks for you reply, I'm wanting to make sure there wasn't anything simple I'm overlooking. \$\endgroup\$
    – EECE
    Commented Mar 12, 2021 at 3:00

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You are measuring various values, because the effective value is changing as the power harvesting conditions change.

The RF arrives at a number of diodes. These diodes turn on and off differently depending on the bias voltage that they're feeding, and the current that's flowing through them. Very hand-wavingly, the impedance varies with current. Low current, high impedance. High current, low impedance.

The changing S11 is perhaps a sign of an efficient rectifier. It would be easy to make the S11 change less with power conditions by adding an attenuator pad at the input!

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  • \$\begingroup\$ @Niel_UK Im just not sure what conditions are changing. If I were increasing the power to the rectifier it would make sense why S11 would decrease because the rectifier gets more efficient as more power is input(for a certain range) but I'm only supplying a single power amount(-5dbm). \$\endgroup\$
    – EECE
    Commented Mar 12, 2021 at 21:52
  • \$\begingroup\$ @EECE The catch is that S11 gets worse right after the boost converter turns on and powers the LED. So the recitifer diodes are driving a different DC load, a different current, a different voltage, depending on whether the boost converter is drawing a current or not. \$\endgroup\$
    – Neil_UK
    Commented Mar 13, 2021 at 4:58
  • \$\begingroup\$ @Niel_UK Your right but S11 is changing the entire time the capacitor is charging like in the youtube video. So before the boost converter turns on the rectifier is only charging a 2200uF cap. \$\endgroup\$
    – EECE
    Commented Mar 13, 2021 at 5:12
  • \$\begingroup\$ @EECE when the rectifier is 'only' charging a 2200 uF cap, is the voltage into the cap staying constant, or varying? Is the charging current into the cap staying constant, or varying? So the recitifer diodes are driving a different DC load, a different current, a different voltage, depending on whether the charge state of the storage capacitor. I don't watch linked uTube vids, and I'm not about to start now. \$\endgroup\$
    – Neil_UK
    Commented Mar 13, 2021 at 5:18
  • \$\begingroup\$ I wasn't using 'only' to suggest it couldn't be the cause, was just stating that the boost converter wasn't involved most of the time. You seem agitated for some reason so I'll leave it at that. Thanks for your input. \$\endgroup\$
    – EECE
    Commented Mar 13, 2021 at 5:33

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