What is the frequency offset in phase noise ?

[m229]

frequncy offset

i dont know the exact meaning of frequency offset in phase noise.
for example when we tell phase noise @ 10 KHz frequency offset is -100 dBc/Hz what does it mean?
anyone knows this?

 

[lguancho]

Re: frequncy offset

Frequency offset refers to the frequency separation between the measurement point on the phase noise plot and the carrier.

 

[m229]

Re: frequncy offset

it means that @10 KHz offset we measure the phase noise at carrier frequency plus (+) or minuse (-) 10khz? is it truse?

 

[zorro]

Re: frequncy offset

Hi m229,

You have a signal, ideally a pure sinusoid (carrier).
Phase noise produces a quadrature component that cannot be removed by a limiter. This noisy component has a power spectral density (psd) that depends of frequency offset relative to the carrier. Integrating that psd over a given bandwidth gives a power.

In your example, if you measure the power in a bandwidth of 1Hz centered at a frequency 10 KHz apart the carrier, you obtain a power 100 dB below the carrier power. (The “c” in dBc stands for “carrier”.) If the bandwidth is 2 Hz, the power is 97 dB below the carrier power; if the bandwidth is 10 Hz, the power is 90 dB below the carrier power, and so on (if the bandwidth is narrow enough that the psd is constant in that band). At a different frequency offset the psd is in general different.

Is it clear?

Z

 

[liaowuliao]

Re: frequncy offset

Pls see the pic, sorry to draw it by mouse.

 

[m229]

Re: frequncy offset

Dear zorro
thanks for u'r reply.
signal that i use is video signal and it has 6, 7, 8 MHz bandwidth.
and i don't understand u'r explaination and if possible plz explain more about it.
because i want to know the exact meaning of freuency offset.
caculation for power and the relation between it and bandwidth (1 hz @10khz offset or 2 hz or 10 hz) for example u tell when 1 hz 100 db and when 2 hz 97 db and when 10 hz 90 db.
is this a standard?
sincerely u'rs
m229

 

[m229]

Re: frequncy offset

dear liaowuliao;
thaks very much for u'r graphical reply;
and when frequency offset is 100 KHz what happened?
best regards

 

[liaowuliao]

Re: frequncy offset

dear liaowuliao;
thaks very much for u'r graphical reply;
and when frequency offset is 100 KHz what happened?
best regards

When frequency offset is 100kHz, the PN would be smaller(less than -100dBc/Hz). You can view the PN value as P(offset)-P(Carrier),and the farther frequency offset is, the smaller P(offset).

 

[vagarwal]

Re: frequncy offset

Hi,

The 100 KHz (or whatever) frequency offset that you are referring to simply means, the phase noise at 100 KHz away from the center or the carrier frequency, (therfore the term 'dBc'), if you watch the spectrum.

Hope this answers the query. I am attaching a detailed note which may help you understand this.

Regards.

 

[m229]

Re: frequncy offset

Dear vagarwal;
thanks for u'r document; i will read it and if i have question then i ask u.
sincerely u'rs;