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2nd order PLL simulation

Discussion in 'Microcontrollers, Programming and IoT' started by NickS, Apr 6, 2010.

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  1. NickS

    NickS

    367
    0
    Apr 6, 2010
    Hello all I am new to your forum but it looks to be a great resource.

    I am trying to implement a 2nd order PLL for my OQPSK receiver simulation(in Matlab)
    But I think there is a flaw in my documentation though I am not sure.

    the Block diagram:
    Code:
    eta[n] --->(Phase Detector)--e[n]-->(filter 1)---c[n]--->(filter 2)---->phi[n]
                         |                                               |
                          ------------------( e^j )<---------------------      
    Code:
    filter 1 = KL*( 1 + alpha * z^-1 ) / ( 1 -z^-1 )
    filter 2 = mu * z^-1 / ( 1 -z^-1 )
    Which seems to make sense but I don't think the example code follows this block diagram
    EXAMPLE CODE:

    Code:
    Ts = 1/fs2;
    mu = Ts;
    BL = 10; %loop bandwidth
    zeta = .707; %damping factor
    wn = 2*BL/(zeta+1/(4*zeta)); %natural frequency
    KL = wn^2*Ts+2*zeta*wn; %filter constant
    alpha = -2*zeta/(2*zeta+wn*Ts); %filter constant
    c1 = 0; e1 = 0; %these denote c[n-1] and e[n-1] respectively
    phi = zeros(size(eta)); %empty vector the same size as input eta
    eta = -eta;
    for n = 2:length(eta)
       e = eta(n-1)*conj( exp(j*phi(n-1)) ); %phase detector
       c = KL*mu*(e + alpha * e1) + c1; %filter 1 ?
       phi(n) = c+phi(n-1); %filter 2 ?
       c1 = c;
       e1 = e;
    end
    
    So I don't think the variables c or phi are properly calculated to the block diagram. Can anyone fresh on Z transforms please confirm or reject my suspicions.

    Thanks
    -Nick-
     
  2. NickS

    NickS

    367
    0
    Apr 6, 2010
    Well I don't believe that I have asked a terribly difficult question to an experianced SWR engineer so perhaps it would help my cause to say that I am not doing this for homework.

    I graduated 2 years ago with my BSEE and my focus has been in hardware design. I recently have been required to simulate a simple OQPSK system since I know matlab reasonably well. I did find this example in a textbook by I can't make the loop lock. If I replace(almost all of it) and do a more dirrect phase comparison to find the error and simply replace the filters with a single scaled correction I can make the thing lock for a phase error but I need the second order property to be able to compensate for frequency as well.

    Any advice would be appreciated.
    -Nick-
     
  3. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    Demanding an answer is usually not a good way to get volunteers to help you.

    ---55p
     
  4. NickS

    NickS

    367
    0
    Apr 6, 2010
    Sorry if my tone seemed demanding. I noticed that many post was being viewed but not replied to and thought I knew the reason for being passed over. I figured I was being shunned because I was asking a question that(admittedly) looks like it could be homework.

    Having read the sticky in the locked homework section just thought I had to better clarify my intent. But it was not my intent to demand.

    My apologies to all that I have offended.

    I am just glad now that someone besides me has said something in this thread.
     
  5. Ian

    Ian Administrator

    1,487
    435
    Aug 23, 2006
    It could just be that although people have viewed it, there hasn't been anyone able to provide a detailed enough answer (or don't have time to reply at the moment). I've not used matlab for many years, so I'm not able to help in this case.

    Hopefully someone will spot this that is familiar with what you are asking :). Welcome to the site :D
     
  6. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    Yeah, sorry. :) I have no idea. I figured no post was more constructive than an "I don't know" post.
     
  7. NickS

    NickS

    367
    0
    Apr 6, 2010
    Thanks. I have been looking for this sort of forum since my university based engineering forum went down last year.

    I am talking to a software radio guru tonight so I will post what I learned tomorrow.

    Nick
     
  8. NickS

    NickS

    367
    0
    Apr 6, 2010
    I ended up using a common variant on the standard PLL called a Costas loop(still second order) It works great

    The block diagram is attached.
    The block diagram is defined thusly

    theta(n+1) = theta(n) + alpha*e(n) + f(n)
    f(n+1) = f(n) + beta*e(n)

    theta = phase correction (NCO register)
    alpha = filter gain 1
    beta = filter gain 2
    f = frequency offset (loop filter register)
    e = phase error

    The filter coefficients are chosen based on your needs but a good starting place is
    alpha = .01
    for critical damping beta = (alpha^2)/4
    for over damping beta < (alpha^2)/4
    for under damping beta > (alpha^2)/4

    Since I used OQPSK I had to cheat slightly on my phase estimation. Since my I and corresponding Q bits are offset by Tb/2 I can not just sample one point determine I and Q and map it to the constellation for phase estimate. So I use I(n) with Q(n+Tb/2) for a single phase estimate. But when applying the correction factor theta it has to be to the time aligned sequence.

    All that said here is the way I implemented it and hopefully this post will save somebody out there some time like it would have me.

    Code:
    IQpll = sampIQrx(startPoint:endPoint); %BB signal  2samp/sym [I(1),Q(1),I(2),Q(2),I(3)...]
    bits = length(IQpll)/2; % number of complex symbols
    alpha = .03; 
    beta = alpha^2/4; %critical damped
    
    e = zeros(1,bits); %initialize error vector
    theta = zeros(1,bits); %initialize phase correction vector
    dfc = zeros(1,bits); %initialize frequency offset vector
    
    theta(1) = PhaseEst; %starting point (converges faster for non-zero first estimate)
    dfc(1) = FreqEst*2*pi/28.8e3; %starting point (converges faster for non-zero first estimate)
    
    for n = 2:length(theta)
        Ipll = real(IQpll(2*(n-1)-1)*exp(1i*theta(n-1))); %apply correction
        Qpll = imag(IQpll(2*(n-1))*exp(1i*theta(n-1)));%apply correction
        angSh = mod( -angle(Ipll + (Qpll*1i)),pi/2); % shift symbol to 1st quadrant
        e(n) =(angSh -(pi/4)); %rotate by pi/4 and the value is your phase error
        
        dfc(n) = dfc(n-1) + beta * e(n-1); %loop filter calculation
        theta(n) = theta(n-1) + (alpha*e(n-1)) + dfc(n-1); %phase calculation 
        if abs(theta(n))>pi
            theta(n) = theta(n)-2*pi*sign(theta(n));% handles phase wrapping
        end
    
        II(n-1) = Ipll; %used I sample
        QQ(n-1) = Qpll; %used Q sample
    
    end
    
    Note that in the presence of no frequency offset dfc converges to 0 and theta converges to the phase offset. In the presence of frequency offset dfc converges to the frequency offset and theta never converges but does maintain a constant slope when locked.

    I find it useful to plot e, theta, and dfc after the loop is done running.

    -Nick-
     

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