About PLL Loop BandWidth....

[super]

pll loop bandwidth

Dear all :
I have some questions about PLL Loop BandWidth.
1. If decrease Loop BW , In Band Phase Noise is increase , right ?
2. If decrease Loop BW , VCO phase noise can't sppress , right ?


Thanks.
8O

 

[toonafishy]

loop bandwidth pll rule of thumb

1. The phase noise at a single point within the loop bandwidth will tend to increase but the overall integrated phase noise within the loop bandwidth may or may not increase.

2. If you decrease the loop BW, there tends to be less supression of the VCO noise within the loop bandwidth.

 

[flatulent]

pll loop filter bandwidth

This is the classical feedback system. Inside the BW the output phase noise will be that of the reference. Outside it will be the VCO phase noise. The optimum BW for minimizing total phase noise is the frequency where the VCO and reference noise densities are equal.

This is because the VCO noise is high at low offsets but declines with increased offset. The reference noise is flat with frequency.

There are other things that may make this loop BW not the proper one. One parameter is switching speed between two freuencies in a synthesizer. The other is reference sideband levels.

 

[radiohead]

noise from dividers 20logn in pll

That is interesting Flatulent, I did not know that.

What reference are you talking about then: is it crystal oscillator? At what offset frequency typically does phase noise becomes flat?

I always thougt that this convergence level is a bit higher than about -174dBm/Hz. So depending on the output power we would get st about -174dBc at the flat level. Now if we would look for the crossing of this flat level with the VCO phase noise, this would result in a few MHz PLL loop bandwidth I suppose. Is this technique actually used in instrumentation eg?

Thanks for your help!

 

[flatulent]

what is loop bandwidth

The reference frequency is the input to the phase detector.

The reference noise gets multiplied by the divider number if the VCO output is divided down before being put to the phase detector. Included in the reference noise is the noise of the divider. Different technologies of dividers (TTL, ECL, etch.) have their own noise. This is why there is a noise specification on some dividers intended for use in synthesizers. So the total reference noise at the output is the actual reference multiplied by the divider ratio plus the added noise of the dividers.

The -174 dBm is the far away noise of the VCO. No circuit can be below this value unless it is cooled.

This is all described in Microwave Frequency Synthesizers by Ronald C. Stirling 1987.

 

[radiohead]

2009 steelers training camp

So I guess BW is optimal where the flat divider phase noise equals the VCO phase noise? I was a little confused by your first reply then.

I'm interested in the mathematical backgroup, although I can understand that this choice would minimize the "phase noise shoulders" that are often noticed.

I would like to get hands on this book. Did not find it on the ebook section though. Who can upload a digital copy?

testing latex ...
\[\sqrt 2\]

 

[khouly]

pll loop

if u analyze the pll as a system in s domian
it is so simple to see that the loop act as a high pass filter if u take the source before the VCO , which is the phase noise so as u increase the bandwidth , the phase noise of VCO will be suppresed

and if u take the phase noise of the crystal OSC is the source of noise , the loop will act as a low pass filter

there is one in UCB done his phd in wide band PLL , which he use a high bandwidth pll to supress the VCO phase noise and used a very clean xtal osc working around 80Mhz , and the loop bandwidth was 8 MHz

i think his name li lin ,

 

[super]

toddlers with shoulders easily out of joint

Thanks for yours reply.

If increase BW will improve close in phase noise (VCO be suppressed),
why we will reduce BW in application and measure?

Thanks.

 

[RFDave]

juicy boobs

Because if you increase loop bandwidth to improve close in spurious, you also allow more reference spurious through. There are also stability considerations, based on the linear approximation's used to derive the S-domain transfer functions generally used.
A rule of thumb is that the loop bandwidth should be less than 10% of the reference frequency (Primarly for Integer-N Loops).
For Fractional Loops, you may have a much higher reference frequency, but you may have sub-harmonic reference spurs generated, so you may not be able to go as wide as 10% of the Reference Frequency.

 

[Danielye]

loop bandwidth noise in plls

Within the BW, the phase noise can be estimated by 20lg(N) degradation of input reference. Beyond the BW, the phase noise depends on the VCO. In some cases, the narrower the BW, the better spur attenuation and phase noise, Especially the output freq. do no need to tune. Is this right?

 

[RFDave]

pll loop bandwidth effects

To get into some more detail, there are several things that the loop filter effects.
1) Phase Noise. This is the broadband thermal noise present at the output. For Phase Noise, inside the loop BW, the Noise is dominated by the reference source and 20LogN noise. Outside the loop filter bandwidth, Phase Noise is dominated by 2 things. First, the VCO output phase noise. Second, in a SDFractional loop, you have the rising noise from the SDM to filter out. There is an optimum loop filter bandwidth that can be derived to provide minimum total output phase noise, based solely on the loop transfer function. This is convered in "Architectures for RF Frequency Synthesizers" by Vaucher.
2) Spurious: In an integer-N Synth, the loop filter provides your reference spur attenuation, and so the loop filter BW needs to be narrow enough to provide spur suppression. Most radio's react worst to spectral tones than broadband noise at a higher level, so spur supression can be more important than low noise levels.
3) Lock time: Lock time is inversely proportional to loop filter BW, the wider the loop, the faster it locks. Lock time spec's can drive a loop filter BW wider.

To first order, this really doesn't depend on whether or not the output frequency needs to tune, since the loop filter is a baseband filter.

 

[Danielye]

pll bandwidth measure

I think this issue had been clarified by Mr. RFdave.

But I have another interesting question, How does comparison freq. affect phase noise and spur attenuation? Does the larger comparison freq. means better phase noise?

 

[zcr35]

loop bw to locktime

Mainstream newly designed PLL chips have linear phase noise performance of PFD. It's clear that in single loop, Fout=N*Fpfd.
In @nanlog Device's phase noise floor calculation, like ADF4107, phase noise in loop BW:
L(fm)=Inband phase noise floor(mainly depending on PFD performace)+20lgN=[-219+10lgFpfd(Hz)]+20lgN=-219+[10lgFpfd+10lgN]+10lgN=-219+10lgFout+10lgN
So, everything is clear now. Increasing Fpfd will reduce N as Fout is certain.
Another benifit from high PFD frequency will also give a wider loop BW, that will reduce lock time and make reference frequency spur farther.

In this case, the reference source's phase noise must be better enough, but it's easy to satisfy this as you choose a better Xtal.

 

[khouly]

hotmail

about the refernce spur , when the bandwidth of the loop is .1 of refernce i think the effect of spur is decrease

and to reduce spurs add parallel cap with filter this cap about 1/10 od the cap of the filter , this will make the ouput more clean
and if u wanna increase the cap , u must be aware of the phase margin of the loop or it will become unstable

 

[pavan200000]

walmart locations

Hi all,
can any one tell me how to measure the loop bandwidth of pll, I came to know that it can be done by FM input but i dint understant it.

Regards,
pavan

 

[biff44]

droit en haiti .

well, one way is to build yourself a low frequency analog phase modulator (a voltage tuned varactor diode one is fine), and place it in between the reference oscillator and the reference input to the PLL chip. Then hook an analog signal source to the phase modulator, set it to sine wave output. Then set the amplitude so that it is only changing the phase of the phase modulator by a "small angle". Now use a spectrum analyzer to look at the phase locked microwave oscillator's output. You should see the VCO carrier, and +/- from the carrier frequency, you should see two modulation side tones. Set your analog signal source amplitude so that the sidebands are, say, 30 dB down from the carrier. Now slowly sweep the analog signal sources frequency from 100 Hz to 10 MHz. At low frequencies, the two side tones stay at a constant amplitude at -30 dBc. At some frequency, the side tones start to get smaller. When they get to -33 dBc, read the analog signal generator's frequency. that frequency is what I call the "loop bandwidth". ie. the PLL can no longer track the phase input signal because it is varying too quickly, and you are 3 dB down.

Rich

 

[pavan200000]

carrier frequency loop bandwidth

Actually this was for presilicon testing (circuit simulations) I gues it will still work,
Thanks a lot

 

[biff44]

optimum pll loop filter bandwidth

Jeez, we should have known that!

https://www.youtube.com/watch?v=YcF_QlhIlww