[time-nuts] New topics (was Re: He isaTime-Nut Troublemaker....)

Didier Juges didier at cox.net
Thu Dec 25 14:29:37 UTC 2008


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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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

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