Achieving higher RF pulse power with switched delay lines and a power combiner - Page 1
Forum section: electronics / general

author: daqq
author_rank: Super Contributor
timestamp: June 09, 2015, 09:14:55 pm
content: Hi guys,I've been reading into the realm of black magic. I was wondering whether it would be possible to do something like this:Assuming that we have a CW RF source and very fast RF switches: We run the source into a demux like structure - several RF switches, only one of them open at any given time. Switch 1 connects to a delay line (simply a long RF path) with a delay of 1 T (T being some very small time), switch 2 connects to a 2T delay line, switch 3 connects to a 3T delay line and so forth until switch N.All of the delay lines feed into a power combiner.Assuming we first switch switch N, then after a time T (minus the switching time) N-1, then N-2 until we get to one, then in every delay line we have an RF energy pulse running through them and they should reach the power combiner in phase (given proper timing and length) and be combined into a big pulse.Would such a thing work? I'm not asking if it's practical - the actual delay lines would be pretty bulky (a spool of microwave coax can't be good for losses, a few nanoseconds of waveguide can't be good for physical constraints), I'm pretty sure there are more elegant ways of getting such a small amplification.Though in theory you could feed the higher power pulse back into one of the delay lines and start all over, adding more power over time to it before finally releasing it... The timing requirements alone would be horrible probably...Could this work?Thanks,David

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2015, 09:48:25 pm
content: This is essentially the process used to generate impressive femtosecond laser pulses:http://en.wikipedia.org/wiki/Chirped_pulse_amplificationIn that case, a spacial optical function is used to generate a dispersed wave; this is amplified at relatively low power densities (using an optical medium with a wide bandwidth, so that it lases for all the frequencies in the chirp), then reassembled with the inverse mechanism.A one-dimensional electrical structure could be used for the transmission line case, such as an all-pass filter, or just some nasty transmission line (which is dispersive but still low-loss). Obviously, you need to be able to construct the inverse of that transmission line, as well.Doing it sequentially, with constant frequency but switched lines, is a discrete expression of the same thing. So, yes, it would work; you need low loss nanosecond switches, though. The easy part is turning on a switch that quickly; turning it off is harder (e.g., semiconductors loaded with minority carriers (PIN diode, avalanche transistor); gas discharge (T-R) tubes; etc., all which must wait the recombination or deionization time before becoming "open" again). Perhaps a series of 1/4 wave segments could be used to alternate the switch behavior, so only closing switches are required.MOSFETs would seem natural, but they must be low capacitance when off, which is a hard thing to achieve (the 'switches' in the RF amplifier are likely MOS to begin with, and they have enough trouble doing their job without having to transmit or reflect RF as well).Tim

author: KJDS
author_rank: Super Contributor
timestamp: June 09, 2015, 10:08:16 pm
content: The normal way of doing this is to put the delay lines into a ring with circulators.

author: ConKbot
author_rank: Super Contributor
timestamp: June 09, 2015, 10:10:35 pm
content: In theory? For the multiple delay lines converging in a combiner, yes, but youve pointed out some of the potential issues. Switching time will be the make or break thing here. A microsecond is ~200 meters in coax (vf=0.66) , a nanosecond is ~20cm. To even be practical, you'd have to get your switching time to the low nanosecond level, otherwise your coax loss will quickly swamp any power increase you get from your combiner.As far as adding to the signal in a loop, the operation of your power combiner will quickly spoil your day on that. Remember that you'll actually take a 3dB loss going from port A to sum, if port B isnt driven. (leaving you 6 db down from what you would get on sum if you were driving A and B equally and in phase) Eventually you'll end up with the 'loop' power being much higher than the added power, which will leave the second input effectively undriven. Plus end up beyond the isolation capablity of combiner. (~20-30 db ish depending on your combiner, generally) and you'll reach an equilibrium where the amount of power being added to the loop is equal to the cable loss, and the amount being coupled the wrong way though the combiner and back into the transmitter.First one is possible, probably not practical unless you want to learn a ton about PIN diodes, second one would probably only make a handful of db above the input burst before it reaches equilibrium.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2015, 10:28:14 pm
content: Quote from: ConKbot on June 09, 2015, 10:10:35 pmNote that adding signal to a fixed length loop demands that phase matching be correct; a wave is perfectly happy to circulate around any length of transmission line, but it will only be constructive for frequencies matched to the electrical length. Therefore, at a single frequency, this is indistinguishable from a resonant tank. Which is a much easier model to analyze with respect to matching. The matching issue is equivalent to the transmission-isolation of the combiner mentioned above.Of course, a simple resonant tank will not deliver a flat-topped pulse, but an exponential decay, when dumped into an output transmission line. The advantage you would have with a TL loop is, the pulse can be flat-topped (give or take echoes if the output match is imperfect), and can contain frequencies harmonic to the line length, if desired.Other methods are preferred for large pulses. For baseband, it's usually capacitive discharge (e.g., ESD, surge testing). (Explosive EMP is generally inductive discharge.) For very short pulses, a compression method is generally employed (saturable magnetic "compressors", shock lines), driven by a baseband pulse. For lower frequency RF, it's usually directly-switched, using amplifiers rated for the peak power, but skimping on heat dissipation or power input (using energy storage to compensate). For high frequency RF up into the UV range, it's usually a high-peak-power physics device, like a magnetron, gyrotron, maser or laser, excited by a baseband pulse and whatever suitable means (applied voltage, glow/arc discharge, flash tube, etc.). Anything higher is pretty much in the spooky-physics particle accelerator range (free electron laser, LINAC, synchrotron..). Or a good old fashioned nuke.Tim

author: daqq
author_rank: Super Contributor
timestamp: June 10, 2015, 02:03:27 pm
content: Thanks guys! So possible, but extremely impractical.