A graphical method is proposed for analysing hybrid feedback systems with PFM mechanisms. The system comprises a pulse frequency modulator (PFM) in the feedforward path and a linear constant plant in the feedback. The describing function approach is employed applied not to the original system, but to an equivalent system. The approach takes into account two types of pulse, impulses and finite-height pulses, emitted from the modulator. Numerical examples show that the proposed method is practical in the sense that it can give an estimated stability condition, which is not excessively conservative, but still remains in the safety side. The fact that the finite-height pulses are taken into consideration as an approximation of the impulses gives another practicality to the approach.