Concept:
Oscillators generate periodic waveforms and are used where repetitive signals are required.
Step 1: Signal generators use oscillators to produce test signals.
Step 2: Radio transmitters use oscillators to generate carrier waves.
Step 3: Digital clocks use oscillators to produce timing pulses.
Step 4: A rectifier does not generate oscillations. It converts AC into DC.
Therefore, the option which is not an oscillator application is:
\[
\boxed{\text{Rectifier}}
\]