Pulse wave  transducer has one Doppler  crystal. This crystal emits a short burst of ultrasound at a certain frequency [PULSE REPETITION FREQUENCY (PRF)].  The ultrasqund is reflected from moving rbc's  and is received by  the same crystal. 

The pulse wave trbnsducer will not send out a new  burst of waves until  it has received returning echoes from previous pulse, which must travel to and Go  from the  location  indicdted by  the sample volume. The pulse wave Doppler gives velocity of blood now at the level of  sample volume placed along ultrasound beam.

Pulse repetition fitquency,  as well as transducer frequency, plays a role  in determining the maximum velocity  that can be measured using a pulsed Doppler method. These rellationships come into play  especially when  attempting to record or to measure  incteased velocities  that might occur in  the case of obstructive lesions  such as vilvular  stenosis. This maximal measured velocity  is called NYQUIST LIMIT.

The NYQUIST LIIMIT-It  is a sampling phenomenon, which limits the maximum  fiequemcy shift measurement to one half of  the  sampling frequency (PRF).

By  its nature, pulse wave Doppler is a sampling system while continuous wave Doppler is not. The nyquist limit  is  a theoretical maximum frequency that a sampling system can  accurately measure.