BPSK modulation of fast pulses

Why would edges be smoothed? PIN diodes when properly used act as almost ideal RF switches. There are also integrated RF muxes for 2.45GHz apps,

Yike! At 10 volts? The diodes would probably break down.

You could possibly build a coaxial inverter (I posted one recently) and switch the junctions with some gaasfets or pins.

There's probably some balun-like thing that would invert or not invert depending on whether a pin is grounded. The grounding device could be a beefy phemt maybe. Hmmm, that's almost equivalent to the previous suggestion.

Phemts do seem to behave roughly like jfets, as far as analog switch biasing goes.

Interesting, but not a simple problem.

coax in =======================--------x---------+ |

---------------------------+---x---------+---->out | ======================= | +---->

| | | | | | x x gnd | | | | | | gnd gnd

where the x's are switches of some sort and the coax is longer than the pulse width. Add a ferrite maybe.

John

How about these?

Nice part, but the carrier lifetime is too long for a 20 Mhz data rate.

John

Yeah, true, even for these power levels it's a bit highish. The BAP63 would be around 300nsec, the BAP65 is under 200nsec, same price range:

If you really need no degradation at all of the edge speed, maybe build two of these things, and put the antenna of one of them upside down. You can then put the positive pulses through the first transmitter and the negative pulses through the other... Actually you could put both antennas inside a waveguide or similar, protruding through opposite faces, that way the pulses could be made to appear to radiate from the same location regardless of polarity.

Chris

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this entire discussion about BPSK of impulses has me scratching my head...

it seems for the case of impulses, it is not realy PHASE modulation but rather POLARITY modulation...

and even with conventional carrier based signals using doubled balanced mixes as modulators, that is also really polarity modulation and not really 180 deg phase modulation..

Purists are always correcting people when they say 240 Volts 2 phase AC power when they really mean 2 polarity.... Strictly speaking 180 deg phase shift is NOT the same thing as polarity inversion but it is close enough in many cases.

So for the special case of BPSK created with a DB mixer,,....I'm starting to think that the P should actually stand for POLARITY and not PHASE. For QPSK etc, the P is obviously phase.

And I guess if you made a BPSK modulator with a delay or complex IQ multiplier instead of a DB mixer, you could not tell the difference between the two signals at the receiver anyway...

Its probably 6 of one and 1/2 a dozen of the other but interesting to think about over a beer.

Mark

The idea of using two different antennas may be worth a try for testing purposes. It should work nicely in the broadside direction of the resulting "array". However, I did not get your idea on how I could make the antennas seem to radiate from the same direction.

Pere

You are right, BPSK is just a synonym form "sign inversion keying", sometimes also called PRK (phase reversal keying)!

Pere

Thanks for your input! For the pulses not to be degraded I need (almost) constant gain _and_ (almost) linear phase up to 10GHz. From what I have read so far, designs based on switches with PIN diodes are usually not characterized taking phase into account. Multiplexers behaving well up to 10 GHz are also not so common...

Pere

Also, multiplexers may not like the pulse amplitude!

Pere

Thanks for the info, John. I also have the feeling that some king of balun arrangement would be able to provide the inversion. But, to be honest, I have not been able to understand them in a rigorous way, i.e. to find a model that fully explains the role and differences of all those "gnd" symbols.

Pere

Thanks, Joerg. But all these have very limited isolation even at 3 GHz. Perhaps I am overlooking something but I think they would not work!

Pere

They're just the ground plane. That circuit should work pretty well with four pin diodes as the x gadgets, if you can find some pins with suitably fast carrier lifetimes to allow 20 MHz operation.

You'll need some dc blocks and biasing drivers, of course.

There's probably a more clever way to do it, possibly with a single switch to ground.

John

Well, you can't get around the capacitances. Only politicians can try to make those "go away" with some fancy creative budgeting :-)

You'll need T-structures and such to yield good isolation. OTOH PIN-diodes are very cheap, so that would not be a big deal.

Probably best to throw it onto SPICE and take a look, faster than crunching numbers on the old HP calculator. I've done lots of diode switches where phase was very critical but it was baseband stuff, under

Yep, 10V is a stretch. You could visit the web sites of the better RF vendors such as Sirenza and Hittite but my feeling is that you'll end up with diodes here. Or maybe a 2nd SRD generator.

I guess I meant to just combine the signals before making them radiate from one hole (e.g. horn antenna). Without knowing more about the application I don't know how much it matters anyway.

Chris

I do not know of anything that will properly preserve those fast edges. I suspect using two SRD pulse formers connected in opposite polarity through a broadband combiner (oops may be tractable). Hmmm how about feeding the SRD pulse former into the center tap of a wideband transformer, with a pair of oppositely driven gaasfets connecting one end or the other to ground, then the secondary has polarity inversions. Well maybe, those fast edges make things tough.

gnd---------fet_sw------------ ) ) ------- ) ( ) (

--------[pulse gen]---[SRD]----------------- ( ) ( ) ( ) ------- ) gnd--------fet_sw--------------