AP 3302 Pt. 3
Section 2
CHAPTER 9
Ringing and Blocking Oscillators
The ringing oscillator needs to be checked frequently to ensure that its operating frequency is correct. In most radar systems a crystal-controlled oscillator is supplied for use as a check on the frequency. The crystal-controlled oscillator cannot itself be used in place of the ringing oscillator, because of difficulties in triggering.
Transistor Ringing Oscillator
Like most circuits, we may also have a transistor version of the ringing oscillator. A common-base arrangement of a ringing oscillator, together with associated waveforms, is shown in Fig 8. For a p-n-p transistor a negative-going input to the base or a positive-going input to the emitter is required to cut the transistor on. In Fig 8, where the input is applied to the emitter, when the square wave rises at A the transistor cuts on and the circuit rings. This train of oscillations is quickly damped by the conducting transistor. When the square wave input cuts the transistor off at B the circuit again rings-this time with very little damping. The resulting train of oscillations may be used to produce cal pips, as explained earlier.
Ringing Oscillator as Pulse Generator
The ringing oscillator may be modified to produce a series of single pulses (instead of a train of pulses), each pulse being tied to the trailing edge of the input square wave. The required modifications to the circuit are indicated in Fig 9a:
a. The inductor
L may contain a variable core to provide adjustment of frequency.
b. The
capacitor C is reduced to a very small value, very often only the self-capacitance
of the coil.
c. A resistor R may be inserted
across the coil.
d. A diode D may be connected
across the coil in the direction indicated.
