Chuck, That was exactly my point. I have used phase combining of power amplifiers to increase power and I know it improves broadband noise performance (noise figure) even though that is usually not why we combine power amplifiers. My last phase-combined amplifier combines two 100W Ku band TWTs for a very compact airborne 200W satellite transmitter (much shorter than if I had been using a 200W TWT, which was a requirement in order to fit in a small diameter unit)
For power amplifiers, the improvement in noise figure (broad band, uncorrelated noise) is about 3dB, but I am not sure it applies when you combine two similar oscillators that may have a similar noise pattern. For instance, combining TWTs does not improve power supply ripple induced sidebands of course (when both tubes operate from the same supply). I would expect that combining low noise osacillators will improve the random noise by close to 3dB, but not all oscillator perturbations are broadband uncorrelated. Particularly, the supply induced, temperature and load pulling variations will not improve at all, limiting the benefits of the technique for low noise oscillators. The cost in complexity for combining oscillators is much more significant than for amplifiers. Amplifiers only need phase matching (electrical length). Oscillators need to be phase locked to begin with. Merry Christmas to all. Didier KO4BB -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Chuck Harris Sent: Wednesday, December 24, 2008 10:45 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] New topics (was Re: He isaTime-Nut Troublemaker....) Hi Didier, I presumed you at one time knew the difference between voltage ratio, and power ratio dB, but your question gave the impression that you might have needed a tiny nudge to refresh your memory. If both oscillators are in lock step, you obviously can combine them using an appropriately wound transformer to gain a 3dB increase in power. But it would seem that the noise, being random and independent, should add constructively as often as destructively. I don't know the answer to your question... I did once, but that was 30 years ago... but I get the feeling that the noise power is going to add up as some fractional power giving you a net improvement in signal to noise... probably close to 3dB, but not quite 3dB... I just don't remember anymore. -Chuck Harris Didier wrote: > Chuck, > > I am quite familiar with how to calculate a voltage or power ratio in > dB, but refering to the first issue, when you combine two oscillators, > does the noise improve by 3dB? > > Didier _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
