This paper presents a simplified approach to the problem of the threshold extention capability of phase locked loop FM demodulator over the conventional FM discriminator.
For the unmodulated carrier signal each FM demodulator is analyzed to get the description of the output SNR in terms of the input CNR(or SNR) and the system parameters, namely, modulation index β, the loop natural frequency $ω_n$ and the test tone frequency $ω_m$.
Experimental results are then compared with the theoretical results. It is found that the PLL FM demodulator has a lower threshold point than the conventional discriminator by about 20dB for modulation index β = 75.
