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Factor Affecting the Phase Noise in Oscillator - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: Factor affecting the phase noise in oscillator Posted: Tue Nov 21, 2006 3:59 am


Joined: Tue Oct 03, 2006 3:03 am
Posts: 11
Location: India
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

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello satish,

The other factors which affect phase noise are:

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.
5. Temperature of operation.

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: ... eNoise.pdf

Best regards,

- IR


Post subject: Posted: Wed Nov 22, 2006 8:50 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Satish
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

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
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.

In addition 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 base.

I hope that you understood from the description how the circuit looks like and operates.

Best regards,

- IR

Posted  11/12/2012

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