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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