Eric Jacobsen
04-02-2008, 05:34 PM
On Wed, 2 Apr 2008 06:16:03 -0700 (PDT), [email protected] wrote:
><snipped.>
>> I'll second Vladimir's input on the noise, and also that phase noise
>> becomes very problematic at such low bit rates. So phase-modulated
>> signals may not be the best choice for such low rates.
>>
><snipped>
>
>Phase noise is problematic at low bit rates? How are the two
>related? Not enough pilot symbols available to estimate the phase?
>What else am I missing?
>
>col
As Vladimir mentioned, phase noise is relatable to the 1/f noise, or
if you want to look at it another way, the noise that is very close-in
to the reference oscillator. This is easily seen on a spectrum
analyzer and a very common way of quantifying phase noise is to just
measure the noise level close-in to the oscillator, typically within
1kHz or so.
If the loop bandwidth of the carrier phase PLL is wider than that it
has no problem tracking out the phase noise, since it's in-band wrt to
the PLL. Obviously, the carrier recovery PLL loop bandwidth has to
be much less than the symbol rate or it won't be able to lock onto the
symbols. So, as the symbol rate goes down the 1/f noise, or the
oscillator phase noise, becomes a bigger and bigger part of the PLL
bandwidth, and, therefore, harder to track out.
Symbol rates of a few bps are problematic for that reason. I'd think
it would require a different way of looking at the problem.
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
><snipped.>
>> I'll second Vladimir's input on the noise, and also that phase noise
>> becomes very problematic at such low bit rates. So phase-modulated
>> signals may not be the best choice for such low rates.
>>
><snipped>
>
>Phase noise is problematic at low bit rates? How are the two
>related? Not enough pilot symbols available to estimate the phase?
>What else am I missing?
>
>col
As Vladimir mentioned, phase noise is relatable to the 1/f noise, or
if you want to look at it another way, the noise that is very close-in
to the reference oscillator. This is easily seen on a spectrum
analyzer and a very common way of quantifying phase noise is to just
measure the noise level close-in to the oscillator, typically within
1kHz or so.
If the loop bandwidth of the carrier phase PLL is wider than that it
has no problem tracking out the phase noise, since it's in-band wrt to
the PLL. Obviously, the carrier recovery PLL loop bandwidth has to
be much less than the symbol rate or it won't be able to lock onto the
symbols. So, as the symbol rate goes down the 1/f noise, or the
oscillator phase noise, becomes a bigger and bigger part of the PLL
bandwidth, and, therefore, harder to track out.
Symbol rates of a few bps are problematic for that reason. I'd think
it would require a different way of looking at the problem.
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org