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Factor affecting the phase noise in oscillator - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Post subject: Factor affecting the phase noise in oscillator
Posted: Tue Nov 21, 2006 3:59 am
Oct 03, 2006 3:03 am
I am interested
to know what are the factor which can effect the phase noise of the
oscillator other than the Q of the Resonator.
Post subject: Posted: Wed Nov 22, 2006 7:29 am
Joined: Mon Jun 27, 2005 2:02 pm
The other factors which affect phase
1. The NF of the active device.
2. The offset frequency
from the center frequency (At which the phase noise is being measured)
3. The average power through the resonator circuit and active device.
4. The Flicker corner frequency of the active device.
These factors are included in the Lesson's formula
which defines the phase noise of an osscilator.
Please look at
this link which gives a good overview:
Post subject: Posted: Wed Nov 22, 2006
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
The only thing I
would add to IR's very comprehensive reply would be the presence of
any dissimilar metal contact that can give rise to intermodulation (at
very low level, but significant). The spectral noise lines resulting
from all the processes mentioned by IR then mix with each other and
thus spread component sidebands even further out from the carrier. Thus
the slope of dBW/Hz is therefore reduced.
Post subject: Posted: Wed Nov 22, 2006 9:27 am
Joined: Mon Jun 27, 2005 2:02 pm
Another issue which is more physical rather than electrical
is of course PCB layout. You need to follow good PCB layout practices
in order to achieve good phase noise performance.
to that, you should apply adequate filtering to the oscillator's supply
rails. There is even a special circuit known as "Super Filter" which
is made of an NPN transistor including bypass capacitors of several
values at both the NPN's base and collector. This circuit reduces the
voltage from the supply to the level required by the oscillator. The
emitter of the NPN is connected to the supply rail of the active device
in the oscillator circuit. The only disadvantage is that you need to
provide a higher voltage to the NPN's collector due to the Vce voltage
drop. The NPN's DC operating point is set with voltage divider in the
I hope that you understood from the description how the
circuit looks like and operates.