PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
Anyone in the group knows what is the difference between these 2
systems, since both are considered as frequency tracking system for
transciever? Why in some application we have to choose AFC?
THX a lot.
Adi
Reply by john●March 19, 20062006-03-19
shellte@hotmail.com wrote:
> PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
> Anyone in the group knows what is the difference between these 2
> systems, since both are considered as frequency tracking system for
> transciever? Why in some application we have to choose AFC?
> THX a lot.
>
> Adi
An AFC system does not use the carrier phase. It only deals with
frequency. A typical AFC will compute a frequency error at baseband and
apply a filtered version of it to a frequency adjustment point in the
receiver, thus bringing the tune frequency of the receiver into
alignment with the received signal. An AFC can be used to pull signals
into the frequency error range that the rest of the demodulator can
tolerate.
John
Reply by Randy Yates●March 19, 20062006-03-19
shellte@hotmail.com writes:
> PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
> Anyone in the group knows what is the difference between these 2
> systems, since both are considered as frequency tracking system for
> transciever? Why in some application we have to choose AFC?
> THX a lot.
In some applications, such as QPSK demodulation, we need phase lock
because the information is in the phase.
In other situations, e.g., FSK demodulation, we only need frequency lock.
In yet other situations, we don't need frequency "lock" but just
to get the frequency "close."
--
% Randy Yates % "How's life on earth?
%% Fuquay-Varina, NC % ... What is it worth?"
%%% 919-577-9882 % 'Mission (A World Record)',
%%%% <yates@ieee.org> % *A New World Record*, ELO
http://home.earthlink.net/~yatescr
Reply by Jerry Avins●March 19, 20062006-03-19
Randy Yates wrote:
> shellte@hotmail.com writes:
>
>
>>PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
>>Anyone in the group knows what is the difference between these 2
>>systems, since both are considered as frequency tracking system for
>>transciever? Why in some application we have to choose AFC?
>>THX a lot.
>
>
> In some applications, such as QPSK demodulation, we need phase lock
> because the information is in the phase.
>
> In other situations, e.g., FSK demodulation, we only need frequency lock.
>
> In yet other situations, we don't need frequency "lock" but just
> to get the frequency "close."
AFC doesn't imply frequency lock. Just as there can be residual phase
error in a PLL, so there can be a residual frequency error with AFC.
Zero error is achieved with an integrator in the loop. Since frequency
is the integral of phase, there is no long-term frequency error with a
phase phase lock.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Randy Yates●March 19, 20062006-03-19
Jerry Avins <jya@ieee.org> writes:
> Randy Yates wrote:
>> shellte@hotmail.com writes:
>>
>>>PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
>>>Anyone in the group knows what is the difference between these 2
>>>systems, since both are considered as frequency tracking system for
>>>transciever? Why in some application we have to choose AFC?
>>>THX a lot.
>> In some applications, such as QPSK demodulation, we need phase lock
>> because the information is in the phase. In other situations, e.g.,
>> FSK demodulation, we only need frequency lock. In yet other
>> situations, we don't need frequency "lock" but just
>> to get the frequency "close."
>
> AFC doesn't imply frequency lock. Just as there can be residual phase
> error in a PLL, so there can be a residual frequency error with
> AFC.
But that's what we call it - "phase lock" or "frequency lock,"
respectively.
I think you're trying to say that the frequency error in an AFC is not
zero. I agree. But I do maintain that an AFC achieves "frequency
lock." Lock does not mean zero error, but rather zero average
variation.
Explain to me if you disagree, Jerry.
--
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% <yates@ieee.org> % 'Big Wheels', *Out of the Blue*, ELO
http://home.earthlink.net/~yatescr
Reply by Randy Yates●March 20, 20062006-03-20
Jerry Avins <jya@ieee.org> writes:
> Randy Yates wrote:
>> shellte@hotmail.com writes:
>>
>>>PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
>>>Anyone in the group knows what is the difference between these 2
>>>systems, since both are considered as frequency tracking system for
>>>transciever? Why in some application we have to choose AFC?
>>>THX a lot.
>> In some applications, such as QPSK demodulation, we need phase lock
>> because the information is in the phase. In other situations, e.g.,
>> FSK demodulation, we only need frequency lock. In yet other
>> situations, we don't need frequency "lock" but just
>> to get the frequency "close."
>
> AFC doesn't imply frequency lock. Just as there can be residual phase
> error in a PLL, so there can be a residual frequency error with
> AFC. Zero error is achieved with an integrator in the loop. Since
> frequency is the integral of phase, there is no long-term frequency
> error with a phase phase lock.
I think I see what you're saying, now, Jerry.
There is a terminology problem. Allow me to define a new term: FLL
(frequency-locked loop). An FLL is just a PLL with an extra integrator
in the loop.
I think your point is that AFC is not necessarily one or the other -
it can be implemented with either a PLL or an FLL, depending on the
requirements.
It could also be something else in which there is no explicit phase
detector in the system. Instead, frequency error is somehow measured
and applied in a feedback control loop. This type of system existed in
the old Ericsson GSM phone architectures to bring the system within
100 Hz or so - enough for the equalizer and demodulator to do their
jobs.
Floyd Gardner ("Phaselock Techniques") states in his introductory chapter:
Perfect frequency control can be achieved by this method [PLL], whereas
conventional AFC techniques necessarily entail some frequency error.
--
% Randy Yates % "And all that I can do
%% Fuquay-Varina, NC % is say I'm sorry,
%%% 919-577-9882 % that's the way it goes..."
%%%% <yates@ieee.org> % Getting To The Point', *Balance of Power*, ELO
http://home.earthlink.net/~yatescr
Reply by Jerry Avins●March 20, 20062006-03-20
Randy Yates wrote:
> Jerry Avins <jya@ieee.org> writes:
>
>
>>Randy Yates wrote:
>>
>>>shellte@hotmail.com writes:
>>>
>>>
>>>>PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
>>>>Anyone in the group knows what is the difference between these 2
>>>>systems, since both are considered as frequency tracking system for
>>>>transciever? Why in some application we have to choose AFC?
>>>>THX a lot.
>>>
>>>In some applications, such as QPSK demodulation, we need phase lock
>>>because the information is in the phase. In other situations, e.g.,
>>>FSK demodulation, we only need frequency lock. In yet other
>>>situations, we don't need frequency "lock" but just
>>>to get the frequency "close."
>>
>>AFC doesn't imply frequency lock. Just as there can be residual phase
>>error in a PLL, so there can be a residual frequency error with
>>AFC.
>
>
> But that's what we call it - "phase lock" or "frequency lock,"
> respectively.
>
> I think you're trying to say that the frequency error in an AFC is not
> zero. I agree. But I do maintain that an AFC achieves "frequency
> lock." Lock does not mean zero error, but rather zero average
> variation.
>
> Explain to me if you disagree, Jerry.
AFC will pull a mistuned FM receiver toward center tuning, reducing the
tuning error to some fraction of what it would be open loop. That
fraction is fairly constant over a rather wide range. The error with
AFC, like the error without it, is in the same direction as the
mistuning and proportional to the amount of mistuning. What makes AFC
usually desirable is the constant of proportionality's being much less.
If that's what you meant, then we agree. In any case, my aim was to
clarify, not to correct.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by HelpmaBoab●March 20, 20062006-03-20
<shellte@hotmail.com> wrote in message
news:1142777745.042936.244100@t31g2000cwb.googlegroups.com...
> PLL is Phase Lock Loop, while AFC is Automatic Frequency Control.
> Anyone in the group knows what is the difference between these 2
> systems, since both are considered as frequency tracking system for
> transciever? Why in some application we have to choose AFC?
> THX a lot.
>
> Adi
>
One is normally high bandwidth (a PLL) and the other is predominantly low
bandwidth. The low bandwidth system does not need to achieve lock.
Tam
Reply by ●March 20, 20062006-03-20
As u mentioned, to demodulate the QPSK signal, what we need is only the
PLL but not the AFC, right?
Reply by ●March 20, 20062006-03-20
By the way, I asked some experts about this question, one said PLLs are
only useful for frequency tracking if there is some carrier power in
the spectrum of the received signal like AM, while AFC really refers to
carrrier recovery even though there is not carrier present.
The major difference between them is that AFC can achieve complete
carrier recovery, no matter which modulation technique is involved.
What do u think about that?