I am trying to apply phase lock loop to calculate phase shift due to doppler effect and finally calculate the doppler frequency($\Delta f$). I am assuming that the received wave is of the form, $r(t) =\cos(2\pi f0 + \phi) $ .The receiver in my setup is at rest and the source is moving at velocity of about 1m/s. But I am not observing the correct doppler shift (calculated using, $\Delta f = \frac{v}{c}f0 $). To calculate $\Delta f$ from $\phi$ I am using $\Delta f = \frac{1}{2\pi}\frac{d\phi}{dt}$ .I am testing using $f0=17kHz$ with sound. My low pass filter in PLL is a linear phase FIR filter with stopband and passband frequencies of 200Hz and 100Hz respectively. What am I doing wrong? I want to reduce the passband of the filter to 10Hz but octave is giving an error 'insufficient extremals'. Currently I am getting a maximum $\Delta f$ of about 6Hz while it should be around 50Hz according to the above equation.
EDIT: Just found out that amplifying/de-amplifying the signal before processing it is changing the $\phi$. Not sure why? Can anyone shed some light?