Is the RF on Coax, or in a waveguide? If it's on coax, why don't you insert some DC at the transmitter and pick it off at the switch? A coupling capacitor at each end and coils with a SRF below the frequency used by the transmitter to insert and remove the DC. That way you don't lose less than 1% of the RF.
-- Offworld checks no longer accepted!
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Did I say Phil?! So sorry, Michael. I used to know a Phil Terrell.
If it has an output connector it doesn't need to be modified. You build the DC inserter to connect to the transmitter, then connect the coax to the DC inserter. Just make sure to connect it so the power goes up the coax instead of the transmitter. :)
BTW, my name is not Phil.
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I used to know another Michael Terrell. He was two grades ahead of me in elementary school, and always starting fights. :)
Years later another moved to town, and he even had the same middle initial. Unfortunately, he was a con man who wrote lots of bad checks. The other one was in a bar fight two or three times a month. The two of them caused me lots of problems while trying to own & operate my electronics business.
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Hi Joerg,
How high is it possible to get the impedance of the input of the RF-to- DC converter, at 1-2 GHz, do you think? Putting it another way, when using either one or two of those HSMS-280X diodes, in LT-Spice, about the best I could do was around 300 Ohms at 1 GHz to 165 Ohms at 2 GHz.
That was with, for example, a large inductor (1-10 uH) to ground across the input, two diodes in series (with 33k across each), 2 pF to gnd, 10 uH in series, 1 Ohm in series with 3300 pF to gnd, and 150 Ohms load. And that just barely gives me 15V across the load, with about 1.53 Watts being dissipated in the load. Those impedances seem awfully low. Is there some trick to getting them much higher? Or maybe I should be asking what might be acceptable, in terms of impedance.
I also tried simulating with some directional couplers made with transformers and a few resistors, but couldn't do much better, except for maybe a little bit higher output voltage.
Of course, in either case, I was always able to trade a higher output voltage for a lower input impedance.
I guess I should look for some other diodes to try.
Also, I'm not so sure about the switch-mode power supplies. I'm afraid they will take too long to come up, and the pin diodes in the RF switch might fry. About the best I could do was around 200 us, with some tens of mV of ripple. Is there some simple way to power the RF switches directly (a "linear supply mode" for startup), until the switchers come up? It seems like there ought to be a way to have a diode circuit dump current into the caps at the switchers' outputs, until the switchers take over, or, probably better yet, just switch from the linear supply to the switcher when the switcher comes up high- enough.
I hope I'm missing something. Otherwise it looks like it might be time to think about just running some wires. But that'll still need some switch-mode supplies. And then I'll have to worry about people turning off the transmitter but leaving the RF switches on.
Thanks,
Tom
Send me the LTSpice *.asc file, the reply-to address in the header works by just clicking on it. If you placed the diode data in a lib file send that along. I won't get around to it until Monday though because I've got to do ministry work today.
Transformers are ghastly up there, it'll be microstrip or stripline. Directional couplers, ok, can be done but then the switcher would have to be a SEPIC because a 6dB coupler drops the voltage to half. 10dB down to a third and so on.
Yep, it's all in the diodes. The ones I showed were just a quick pick. I am sure there will be some with lower capacitance out there. But let's try the LTSpice file first. Just remember, simulation and the real circuit, two different things up at those frequencies.
I got one down to less than 100usec (had to) but even that may not be good enough in this case.
Many ways to do that. One is a pass FET from raw DC to output, with diode and resistor. A TL431 set slightly above target voltage (but within abs max limits of the RF switch) caps and regulated, and before all that fries up into a plume of black smoke the switcher comes on line and turns the pass FET off.
But: Take one of those RF switches and hang it onto a bench supply with
100ohm resistor in series for +5V and then +15V. Turn it on by plugging in the banana plug fast and hang a DSO in trigger mode "normal" onto it. If the electronics in the RF switch have largish bypass caps chances are the voltages won't come up fast enough anyhow. Because all those bypass caps need to be charged up. Then you'd be back to square one, having to supply a permanent power rail.
Switcher supplies are easy. Then you could even use ye olde and cheap MC34063 because you can guarantee that it won't see more than 30V. What's the problem with leaving the RF switches on? Battery drain? Maybe some protection mechanism could be put in place.
Action items that would make a lot of sense here: Ask the manufacturer or the RF switch what happens when RF power is applied while it does not have any supply voltages. Also, ask them how long a ramp-up for those supplies is considered safe while under RF power. Then, inquire whether any sort of sequencing is recommended (like 5V first, 15V after xxx usec delay).
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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I will be able to put the switch very near to the transmitter and run two separate coax lines out from there. The problem isn't actually how to get the DC out to the switch, it's how to get the DC at all. I was just hoping to be able to have a self-contained switch unit, with its own DC supply derived from the RF.
IMHO the problem in a "harvesting" scenario is mainly what happens the instant the RF comes on while the DC stuff still needs to settle. What does the switch do when it has do come on "hot" (under RF exposure) and what does the TX think when the load situation isn't defined for a split second?
Some things can be simulated (just send it over :-) ) but other things would require a chat with the manufacturers of the TX and of the switch module.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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