Blocking transducer

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A blocking oscillator is the electronic circuit of a feedback relaxation oscillation generator, which is used as an electrical oscillator circuit equipped with a transformer to generate pulses . In its simplest form it consists only of a magnetically coupled double coil or a pulse transformer and an amplifying component such as a transistor , previously also known as an electron tube . A distinction is made between blocking oscillators that are or can be operated self-oscillating and those that are triggered once or periodically. The characteristic that gives it its name is the non-linear operation of the amplifying element at the transition into the blocking range (e.g. blocking voltage of the electron tube).


By choosing the component parameters, the needle pulses, which are often short in time, can be selected within a very wide range of duty cycle and period duration. The electrical decoupling of the pulse signal can take place via the pulse transformer using an additional winding. The vibration is generated by positive feedback via the pulse transformer.

Depending on the circuit variant, the blocking oscillator can also be used to generate breakover oscillations as the form of the output signal. Since such a sawtooth oscillation contains all harmonics of the fundamental frequency, this type of oscillator was mainly used to generate sound in older electronic organs .

Areas of application

Electric clocks

Electronic clockwork with a blocking oscillator

During the technological transition in the middle of the 20th century from mechanical clockworks to today's common quartz clocks , there were also electronic clocks on the market that had a blocking oscillator as a drive device. In contrast to quartz watches with quartz clocks , where the electronic circuit itself represents a correspondingly precise time base and the clock mechanism only serves to move the hands, in this case the electronic circuit does not serve to improve the accuracy of the mechanical watch, but only to do so to replace the mechanical energy storage. This is necessary to compensate for friction losses in the movement , among other things .

Blocking oscillator in a clock circuit

In the case of purely mechanical watches, the drive device, including the associated elevator, usually consists of a spring accumulator. By replacing it with an electric drive and an electric energy store, consisting of the blocking oscillator and an energy store such as a battery , longer operating times can be achieved. There is no need to "wind up" the spring accumulator after only a few days, as with mechanical watches. In addition, there is no mechanically heavily stressed anchor escapement , which increases the service life . As with mechanical watches, the rate accuracy is set by a mechanical device on the balance wheel.

In this application, the blocking oscillator does not represent an independent oscillator circuit, but only becomes capable of oscillating as an overall electromechanical system through interaction with the mechanical clock system. The balance wheel is equipped with a permanent magnet with a high coercive field strength , which is attached directly next to two magnetically coupled air-core coils L1 and L2 . The coils are controlled by a bipolar transistor T , as indicated in the adjacent circuit diagram. To avoid excessive amplitudes on the balance wheel, there is an eddy current brake in addition to the permanent magnets . In order to set the electromechanical oscillator in motion, for example after replacing the battery, the balance wheel must be set in motion by a device with a mechanical impulse.

The capacitor C sets the approximate working frequency of the blocking oscillator, depending on the model, to approx. 2 Hz to approx. 5 Hz. The system is mechanically designed in such a way that, due to the interaction with the permanent magnet shortly before the balance wheel crosses zero (rest position), a short drive impulse emanates from the blocking oscillator, which ensures that the balance wheel moves until the battery is exhausted. Such clockworks are usually supplied with a voltage of 1.5 V from a mignon cell .

Blocking vibrator as a frequency divider 1: 2 in a tone generator of an electronic organ, the frequency is roughly set with the RC elements
Circuit diagram of a triggerable blocking oscillator

Synchronized oscillator

Blocking oscillators can be synchronized by coupling in another pulse signal so that they oscillate at the same frequency (see below for television applications) or in a fixed frequency ratio to the input signal ( frequency divider ).

They were used in older television sets to generate the tilting oscillations for vertical and horizontal deflection, synchronized by the sync pulses in the received television signal . In the horizontal deflection, however, the blocking oscillator was soon replaced by circuits that allow greater security of synchronization with the sender-side tilting oscillation (so-called flywheel synchronization ).

In older electronic organs (e.g. Dr. Böhm organ ) they were used as frequency dividers with a ratio of 1 to 2 for sound generation.

Radar technology

Self-oscillating blocking oscillators were used in old radar devices as a generator for the pulse repetition frequency .

Triggered blocking oscillators were used as frequency dividers in a ratio of 1 to 2 to 1 to 5, for example for the generation of distance markers in radar devices.

See also

Individual evidence

  1. Handbook for radio frequency and electrical technicians . tape V . Verlag für Radio-Foto-Kinotechnik, Berlin-Borsigwalde 1970 (specialist dictionary with definitions and images).
  2. ^ Meyer's Little Lexicon . VEB Encyclopedia Institute, Leipzig 1969.
  3. Junghans ATO-Mat , technical description
  4. ^ Rainer Böhm: Electronic organs and their self-construction . RPB 101/102. Franzis-Verlag, Munich 1963, 1973