At 2/9/2006 09:14 AM, you wrote:
> > Well, I'm going to go out on a limb & disagree with Jeff,
> > perhaps for the
> > first time ever.
>
>That's OK, we'll still let you hang around our tent and drink our beer at
>Dayton :-) Are you going again this year Bob?.
Not this year unfortunately; maybe '07.
> > My conclusion is that if
> > whatever you're
> > using to measure amplitude is sensitive enough & everything
> > is close to 50
> > ohms, tuning for max. trans. is good enough.
>
>My take on the situation is that I'd rather have everything as close to 50
>ohms as possible. If there is any variation in the system outside the
>duplexer/cavities that you can't control (such as Z changing due to antennas
>icing up), the transformation effects of the cavities should be minimal.
>Also, by keeping all of the passives at 50 ohms in and out, it eliminates
>much of the uncertainty when adding additional cavities (e.g. pass cavity
>ahead of a receiver) or when adding an isolator that was bench-tuned with 50
>ohm loads. I'll take consistent performance over optimum performance when
>we're talking a one or two tenths of a dB in insertion loss difference.
True; if something has to be swapped out it's easier to substitute 50 ohm
components instead of 67 - j16 ohm equipment.
>As far as using cavities as matching networks to eek more power out of a PA,
>without actually looking at the efficiency at different load Z's, there's no
>telling what the PA is truly happy with. Just because you can squeeze an
>extra dB out of the amplifier by providing it with an some odd load Z
>doesn't mean that's the ideal load Z to operate it at. If you have to burn
>up 50 more watts in heat to get an extra 10 watts out of the PA, that's bad.
>So, if you lack test equipment and have no choice but to use high-level
>signals for tuning the pass, you should still be tuning for minimum
>reflected power.
Alternately one could simply monitor the supply current while tuning to
make sure you're not drifting into a low efficiency region.
> > Yes, you can tune your RX cans to maximize power transfer into your
> > RX. But then what happens to your notches which you've just
> > moved as well?
>
>You can also tune Rx front ends with a network analyzer or SG/TG and RLB.
Yes but that sounds sort of risky in that the lowest noise figure of an LNA
is not necessarily achieved at optimum match. The waters get murkier with
LNAs because in addition to power transfer, you have the somewhat
independent variable of noise figure to contend with. Best to just tune
them on a 50 ohm system for best S/N period.
>You will also see that the "window" response of the front end of your
>typical two-way radio (Micor, M2, whatever) can be tuned to favor the side
>away from interference sources (e.g. your transmitter or other co-located
>transmitters) without any significant detriment to insertion loss at the
>desired Rx pass frequency.
My previous comments relating to LNAs may also apply to the JFET mixers in
G.E. radios, as I've had a few that did not completely optimize by tuning
for maximum RX limiter current per the manual; tuning for best quieting
yielded a dB or two better S/N.
Bob NO6B
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