Here is a Fact, The GE PLL exciter is about 20db quieter than a
multiplied exciter, very likely for the reason stated below.
Think about it this way. To multiply a crystal frequency you have to
have harmonics.....(noise)..okay you multiply that and get even more
noise..well you amplify each time and get even better noise. By the
time you multiply the origonal frequency 12 times (GE) you have a
strong carrier at the required frequency with all kinds of noise, it
looks like FUZZIES, below it.
Merry Christmas
73
AC0Y
--- In Repeater-Builder@yahoogroups.com, Bob Dengler <[EMAIL PROTECTED]>
wrote:
> At 12/19/2004 10:29 PM, you wrote:
> >
> >When it comes to Phase noise of an oscillator, the higher the Q
of the
> >resonant circuit, the better the phase noise. An LC circuit
generally
> >will have a Q of around 100 where a crystal can have a Q of
10,000 to
> >500,000, thus a crystal oscillator generally yields superior
phase noise
> >performance over LC circuits, such as the VCO in the GE PLL
exciter. I
> >have seen instances where engineers have use
conventional "multiplier"
> >circuits (fundamentally similar to the old GE highband exciter,
without
> >the "issues") to achieve superior phase noise performance over a
PLL
> >circuit, because the phase noise of the VCO was the weakest link
in the
> >PLL circuit.
>
> Unless these multipliers without "issues" have very high Q tuned
circuits,
> I don't see where the improvement in phase noise would come from.
Noise is
> increased anytime a signal is multiplied by the factor 20*log(N),
where N
> is the multiplication factor. So for the highband VHF exciter
utilizing a
> crystal oscillator (x 12), the noise will be 21.6 dB higher
(referenced to
> the carrier) than the noise of the crystal oscillator itself. I'm
not
> ceratin of this but I don't think it's possible to reduce this
noise
> without resorting to
> very high-Q multiplier circuits or interstage filtering.
>
> Bob NO6B
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