Designer’s Guide Community

Visjnoe Senior Member Offline Posts: 233	Phase noise bump Mar 22^nd, 2007, 2:47am Dear all, please find enclosed a phase noise profile of a VCO: I'm seeing a 'bump' in its profile at the oscillation frequency. What would be the physical explanation for this bump? Thanks in advance! Peter
Back to top	Typical_PLL_VCXO_Phasenoise.png IP Logged

mg777

Senior Member

Offline

Posts: 131

Re: Phase noise bump
Reply #1 - Mar 22^nd, 2007, 9:04am

The 'bump' is called a spur. I see a 32 MHz fund, a weak second harmonic, and a stronger third. The center freq is not specified.

Is the plot measured data or sim? If its measured then the 32 MHz could be a digital clock from an adjacent block, with duty cycle distortion. It's like how the old synthesized signal generators would exhibit spurs corresponding to the rpm of the equipment's cooling fan. If it's sim then your oscillator may have a weak second resonance riding on top of the fundamental mode - except that given the small signal level for this second mode we'd expect it to be highly sinusoidal.

M.G.Rajan
www.eecalc.com

Back to top

IP Logged

Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: Phase noise bump Reply #2 - Mar 22^nd, 2007, 9:43am You are seeing the harmonics of the VCO's fundamental frequency. Each harmonic is also contaminated with phase noise, that is why you are seeing them in the PNoise results. -Ken
Back to top	IP Logged

Visjnoe Senior Member Offline Posts: 233	Re: Phase noise bump Reply #3 - Mar 22^nd, 2007, 11:12am Dear, okay, thanks for the answer! So, if I get this right, the 'first bump' is actually the second harmonic? Correct? The plot is indeed obtained from simulation, not measurement and plots the phase noise relative to the fundamental carrier frequency (~32MHz). Kind Regards Peter
Back to top	IP Logged

Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: Phase noise bump Reply #4 - Mar 22^nd, 2007, 12:53pm You are doing a relative frequency sweep, so 0 Hz is the fundamental frequency. Thus, 32MHz would be the second harmonic. This would more obvious if you did an absolute frequency sweep from 0 to say 100MHz. They you would see the phase noise "tents" at 0, 32MHz, 64MHz, and 96MHz, which represent harmonics 0, 1, 2, and 3. -Ken
Back to top	IP Logged

jeffyan

Community Member

Offline

Posts: 47

Re: Phase noise bump
Reply #5 - Sep 19^th, 2007, 7:54pm

Ken Kundert wrote on Mar 22^nd, 2007, 12:53pm:

You are doing a relative frequency sweep, so 0 Hz is the fundamental frequency. Thus, 32MHz would be the second harmonic. This would more obvious if you did an absolute frequency sweep from 0 to say 100MHz. They you would see the phase noise "tents" at 0, 32MHz, 64MHz, and 96MHz, which represent harmonics 0, 1, 2, and 3.

-Ken

hi Ken,
if we do pss+pnoise analysis to PFD&CP, i think, we should set a absolute frequency sweep, right?
thanks.
jeff

Back to top

IP Logged

Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: Phase noise bump Reply #6 - Sep 20^th, 2007, 12:30am It's your choice. Generally if you want to see close in noise, particularly on a log scale, you use relative. If you want to see the noise over a wide bandwidth, absolute is preferred. -Ken
Back to top	IP Logged

jeffyan

Community Member

Offline

Posts: 47

Re: Phase noise bump
Reply #7 - Sep 20^th, 2007, 2:17am

Ken Kundert wrote on Sep 20^th, 2007, 12:30am:

It's your choice. Generally if you want to see close in noise, particularly on a log scale, you use relative. If you want to see the noise over a wide bandwidth, absolute is preferred.

-Ken

thanks Ken.
but the nosie plot is different near the dc and fref.
to PLL, the noise of cp is lowe-pass, and i think the noise near the fref can be filtered out greatly for narrow bandwidth PLL.So i think, using sweep type=absolute makes more sense in noise analysis of PLL. is it correct?

thanks.

Back to top

IP Logged

Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: Phase noise bump Reply #8 - Sep 20^th, 2007, 7:43am Oh yeah. Of course. Sorry, I wasn't thinking. -Ken
Back to top	IP Logged