about phase frequency detector in PLL
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about phase frequency detector in PLL

 
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Guest
Posted: Fri Dec 09, 2005 1:35 am    Post subject: about phase frequency detector in PLL Reply with quote
In the Phase frequency detector of PLL, we should get up and down
signal --narrow/broad depending on the phase / freq dfference... but I
am getting spikes at a constant interval in the output up and down
signals... what should I make of it??

any suggestions??
Thanks

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Tim Wescott
Guest
Posted: Fri Dec 09, 2005 1:35 am    Post subject: Re: about phase frequency detector in PLL Reply with quote
msdeshp@yahoo.com wrote:
(top posting fixed)
Quote:


Andrew Holme wrote:

msdeshp@yahoo.com wrote:

In the Phase frequency detector of PLL, we should get up and down
signal --narrow/broad depending on the phase / freq dfference... but I
am getting spikes at a constant interval in the output up and down
signals... what should I make of it??

any suggestions??
Thanks

What interval? At the reference / comparison frequency?

Are the up and down spikes of equal width, and coincident in time?

Are they very narrow spikes? That might be OK. Or are they more pulse than
spike??

What PFD are you using?

Is the loop locked?

Thanks for your reply.
This is a charge pump pfd...the refrence frequ is 212.5MHz and fout is
4GhZ, divison ratio is 20...
They are very narrow spikes, not pulses.. and are or equal width and
at regular intervals....
I can send you the jpeg file if that ok with you, pl let me know..

Meghana


You are most likely seeing exactly what you should.

Phase detectors aren't perfect, and neither are charge pump filters, so
there's always some offset or leakage that the phase detector has to
null out with little spikes from the "up" or "down" output. You just
have to design your PLL to perform well in the face of the noise.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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Guest
Posted: Fri Dec 09, 2005 1:35 am    Post subject: Re: about phase frequency detector in PLL Reply with quote
Thanks for your reply.
This is a charge pump pfd...the refrence frequ is 212.5MHz and fout is
4GhZ, divison ratio is 20...
They are very narrow spikes, not pulses.. and are or equal width and at
regular intervals....
I can send you the jpeg file if that ok with you, pl let me know..

Meghana


Andrew Holme wrote:
Quote:
msdeshp@yahoo.com wrote:
In the Phase frequency detector of PLL, we should get up and down
signal --narrow/broad depending on the phase / freq dfference... but I
am getting spikes at a constant interval in the output up and down
signals... what should I make of it??

any suggestions??
Thanks

What interval? At the reference / comparison frequency?

Are the up and down spikes of equal width, and coincident in time?

Are they very narrow spikes? That might be OK. Or are they more pulse than
spike??

What PFD are you using?
Is the loop locked?


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Andrew Holme
Guest
Posted: Fri Dec 09, 2005 1:35 am    Post subject: Re: about phase frequency detector in PLL Reply with quote
msdeshp@yahoo.com wrote:
Quote:
In the Phase frequency detector of PLL, we should get up and down
signal --narrow/broad depending on the phase / freq dfference... but I
am getting spikes at a constant interval in the output up and down
signals... what should I make of it??

any suggestions??
Thanks

What interval? At the reference / comparison frequency?

Are the up and down spikes of equal width, and coincident in time?

Are they very narrow spikes? That might be OK. Or are they more pulse than
spike??

What PFD are you using?

Is the loop locked?
Back to top
Mark
Guest
Posted: Fri Dec 09, 2005 1:35 am    Post subject: Re: about phase frequency detector in PLL Reply with quote
Narrow spikes are exactly what you should see when the loop is in
lock...

try to detune the VCO and you will see the pulses created to correct
the error.


Mark
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Jim Thompson
Guest
Posted: Fri Dec 09, 2005 1:35 am    Post subject: Re: about phase frequency detector in PLL Reply with quote
On 8 Dec 2005 12:17:14 -0800, msdeshp@yahoo.com wrote:

Quote:
In the Phase frequency detector of PLL, we should get up and down
signal --narrow/broad depending on the phase / freq dfference... but I
am getting spikes at a constant interval in the output up and down
signals... what should I make of it??

any suggestions??
Thanks

Suck thumb? Check the VCO spectrum?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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Guest
Posted: Fri Dec 09, 2005 5:35 pm    Post subject: Re: about phase frequency detector in PLL Reply with quote
Mark wrote:
Quote:
Narrow spikes are exactly what you should see when the loop is in
lock...

try to detune the VCO and you will see the pulses created to correct
the error.


Mark

Thanks for your reply Mark

Can you pl elaborate how I should detune the VCO?
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Chris Jones
Guest
Posted: Sat Dec 10, 2005 1:36 am    Post subject: Re: about phase frequency detector in PLL Reply with quote
msdeshp@yahoo.com wrote:

Quote:
In the Phase frequency detector of PLL, we should get up and down
signal --narrow/broad depending on the phase / freq dfference... but I
am getting spikes at a constant interval in the output up and down
signals... what should I make of it??

any suggestions??
Thanks

In practice any real charge pump can only produce pulses down to a certain
minimum width. Below that width, the peak current of the pulse is never
achieved, and the relationship between pulse width and integrated charge in
the pulse becomes non-linear. It is desirable always to operate in the
region where the charge pump is linear, particularly in Fractional-N
synthesisers. In order to achieve this, a certain minimum time is often
applied to both the up and down currents. Provided the up and down
currents are well matched, the only disadvantage of the scheme is slightly
increased noise, but this is usually small in comparason to the thermal
noise in the loop filter resistors etc.

Chris
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Mark
Guest
Posted: Sat Dec 10, 2005 1:37 am    Post subject: Re: about phase frequency detector in PLL Reply with quote
depends on what type of vco it is

an LC VCO can be perterbed by bringing your finger near the coil

xtals can be pertrubed a little by squeexing them or adding come
capacitnace to one lead

voltage or temperature changes can work too.

almost anything that tries to change the frequency of the vco will
require the PD to generate a correction...thats the point of a PLL
afterall..

Mark


Mark
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maxfoo
Guest
Posted: Sun Dec 11, 2005 5:35 pm    Post subject: Re: about phase frequency detector in PLL Reply with quote
On 9 Dec 2005 08:20:15 -0800, msdeshp@yahoo.com wrote:

Quote:

Mark wrote:
Narrow spikes are exactly what you should see when the loop is in
lock...

try to detune the VCO and you will see the pulses created to correct
the error.


Mark

Thanks for your reply Mark

Can you pl elaborate how I should detune the VCO?

You need to disconnect the VCO tune input from the loop filter and apply an
external voltage to the vco. Vary the voltage and monitor the PFD to see if the
phase switches. I've seen a few engineers connect the PFD polarity ass-backwards
try that too.
Which PFD are you using anyways?
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