The "need" for low-noise RF sources was prompted by the
electronics industry going hot and heavy on cellular telephony
which uses partly phase demodulation and clock recovery
circuits in digital electronics. Because of those particular
markets, "low noise" has become a Big Buzzword.

Whether you have one pole or twelve or whatever, you will
NOT need a specific "low noise oscillator!" The very ordinary
sweep oscillators of ten, twenty, or thirty years ago are quite
fine.

Len,

I'm sorry, but I disagree. Concern with phase noise
in *all* kinds of communications systems really
got hot in the 1970s. Adjacent channel rejection
is limited by phase noise performance. It is presently
the limiting factor in HF receiver performance.
Ham transmitter phase noise can easily be heard
during CW DX contests as a keyed increase in noise
floor.

Measurement of a crystal filter with steep sides
could be compromised by PM to AM conversion
on the slopes.

And in response to earlier posts, DDS synths *can*
give very good phase noise performance, but their
usefulness is usually limited by quantization spurs
unless a cleanup PLL is added.

73, John - K6QQ
retired RF circuit design engineer