Blocking Oscillators: An Introduction Into its Working, Types, and Uses

What are Blocking Oscillators?

Blocking oscillators are a type of relaxation oscillator that generates a periodic output waveform consisting of narrow pulses. They are widely used in various electronic applications, such as Pulse Generators, timing circuits, and voltage converters. The key characteristic of blocking oscillators is their ability to produce short duration, high-amplitude pulses followed by a relatively long off-time.

Components of a Blocking Oscillator

A basic blocking oscillator consists of the following components:

1. Transformer: A transformer with a primary and secondary winding is used to provide positive feedback and coupling between the input and output.
2. Active device: A transistor (BJT or FET) or vacuum tube is used as the active device to amplify the signal and control the oscillation.
3. Resistor: A resistor is connected in series with the transformer’s primary winding to limit the current and control the pulse width.
4. Capacitor: A capacitor is connected across the transformer’s secondary winding to store energy and shape the output pulse.

Working Principle of Blocking Oscillators

The working principle of a blocking oscillator can be divided into two phases: the pulse generation phase and the recovery phase.

Pulse Generation Phase

1. When power is applied, the transistor is initially in the cut-off state, and no current flows through the transformer’s primary winding.
2. A small base current starts to flow due to the transistor’s leakage current or noise, causing the transistor to conduct slightly.
3. As the transistor conducts, the current through the transformer’s primary winding increases, inducing a voltage in the secondary winding.
4. The induced voltage in the secondary winding is applied to the transistor’s base through the feedback connection, causing the base current to increase further.
5. This positive feedback rapidly increases the transistor’s conduction, resulting in a sudden rise in collector current.
6. The rising collector current induces a higher voltage in the secondary winding, driving the transistor into saturation.

Recovery Phase

1. As the transistor reaches saturation, the transformer’s primary current reaches its maximum value, and the magnetic flux in the core starts to saturate.
2. The saturated core cannot support further changes in magnetic flux, causing the induced voltage in the secondary winding to collapse.
3. With no further base drive, the transistor turns off, and the collector current falls to zero.
4. The energy stored in the transformer’s core and the capacitor connected across the secondary winding starts to dissipate.
5. The capacitor discharges through the transformer’s secondary winding, causing a reverse current flow in the primary winding.
6. This reverse current helps to reset the transformer’s core, preparing it for the next cycle.
7. Once the capacitor is discharged and the transformer’s core is reset, the blocking oscillator remains in the off state until the next trigger pulse or noise initiates the pulse generation phase.

Types of Blocking Oscillators

Blocking oscillators can be classified based on the type of active device used and the configuration of the feedback network.

Transistor-based Blocking Oscillators

1. NPN Blocking Oscillator: Uses an NPN transistor as the active device. The transformer’s secondary winding is connected between the base and emitter of the transistor.
2. PNP Blocking Oscillator: Uses a PNP transistor as the active device. The transformer’s secondary winding is connected between the emitter and base of the transistor.
3. FET Blocking Oscillator: Uses a field-effect transistor (FET) as the active device. The transformer’s secondary winding is connected between the gate and source of the FET.

Vacuum Tube-based Blocking Oscillators

1. Grid-leak Blocking Oscillator: Uses a vacuum tube as the active device. The transformer’s secondary winding is connected between the grid and cathode of the tube, and a grid-leak resistor is used to provide bias.
2. Cathode-coupled Blocking Oscillator: Uses a vacuum tube as the active device. The transformer’s secondary winding is connected between the cathode and ground, and the tube’s grid is driven by a separate trigger pulse.

Applications of Blocking Oscillators

Blocking oscillators find applications in various electronic systems, including:

1. Pulse Generators: Blocking oscillators are used to generate short duration, high-amplitude pulses for triggering other circuits or devices.
2. Timing Circuits: The periodic nature of blocking oscillators makes them suitable for generating timing signals or clock pulses in digital systems.
3. Voltage Converters: Blocking oscillators can be used in voltage converter circuits to step up or step down voltages by exploiting the transformer’s turns ratio.
4. Switching Power Supplies: Blocking oscillators are employed in switching power supplies to generate the required high-frequency switching pulses for efficient power conversion.
5. Ignition Systems: In automotive and other ignition systems, blocking oscillators are used to generate high-voltage pulses for spark plug ignition.
6. Radar and Sonar Systems: Blocking oscillators are utilized in radar and sonar systems to generate short, high-power pulses for transmitting signals.

Advantages and Disadvantages of Blocking Oscillators

Advantages

1. Simple and compact design: Blocking oscillators require few components and can be implemented in a small form factor.
2. High output voltage: The transformer’s turns ratio can be used to step up the output voltage, making blocking oscillators suitable for generating high-voltage pulses.
3. Fast rise time: The positive feedback mechanism allows for fast rise times, resulting in sharp output pulses.
4. Wide pulse width range: The pulse width can be easily controlled by adjusting the resistor value or the transformer’s turns ratio.

Disadvantages

1. Limited duty cycle: Blocking oscillators have a relatively low duty cycle due to the long off-time required for the transformer’s core to reset.
2. Frequency instability: The oscillation frequency of blocking oscillators can be affected by factors such as temperature, supply voltage, and component tolerances.
3. Inefficient at high frequencies: As the operating frequency increases, the transformer’s core losses and switching losses in the active device become more significant, reducing efficiency.
4. Noise sensitivity: Blocking oscillators are sensitive to noise and may require additional filtering or shielding to ensure stable operation in noisy environments.

Designing a Blocking Oscillator

When designing a blocking oscillator, several factors need to be considered:

1. Transformer Selection: Choose a transformer with the appropriate turns ratio, inductance, and core material to achieve the desired output voltage and pulse characteristics.
2. Active device selection: Select a transistor or vacuum tube with suitable voltage and current ratings, gain, and switching speed.
3. Resistor value: Determine the resistor value based on the desired pulse width and the active device’s characteristics.
4. Capacitor value: Select the capacitor value to achieve the required pulse shape and duration.
5. Trigger mechanism: Decide on the triggering method (self-starting or externally triggered) and design the appropriate trigger circuitry.
6. Layout considerations: Ensure proper layout techniques to minimize stray inductance and capacitance, which can affect the oscillator’s performance.

Troubleshooting Blocking Oscillators

If a blocking oscillator fails to operate as expected, consider the following troubleshooting steps:

1. Check the power supply: Ensure that the power supply is providing the correct voltage and current to the oscillator circuit.
2. Verify component values: Double-check the values of the resistor, capacitor, and transformer windings to ensure they match the design specifications.
3. Inspect the transformer: Check for any physical damage to the transformer, such as broken windings or a damaged core.
4. Test the active device: Replace the transistor or vacuum tube with a known good component to rule out any issues with the active device.
5. Check the feedback connection: Verify that the transformer’s secondary winding is correctly connected to the active device’s input (base or grid) to ensure proper feedback.
6. Monitor the waveforms: Use an oscilloscope to observe the waveforms at various points in the circuit, such as the active device’s input and output, to identify any anomalies or distortions.
7. Adjust the trigger signal: If the oscillator is externally triggered, ensure that the trigger signal has the correct amplitude, duration, and polarity.

By following these troubleshooting steps and understanding the working principle of blocking oscillators, most issues can be identified and resolved.

Frequently Asked Questions (FAQ)

1. Q: What is the main difference between a blocking oscillator and other types of oscillators?
   A: The main difference is that blocking oscillators generate narrow, high-amplitude output pulses followed by a relatively long off-time, while other oscillators typically produce continuous sinusoidal or square wave outputs.

2. Q: Can a blocking oscillator be used as a frequency multiplier?
   A: Yes, blocking oscillators can be used as frequency multipliers by exploiting the transformer’s turns ratio. By selecting the appropriate turns ratio, the output frequency can be a multiple of the input trigger frequency.

3. Q: How does the resistor value affect the pulse width in a blocking oscillator?
   A: The resistor value in series with the transformer’s primary winding controls the pulse width. Increasing the resistor value will increase the pulse width, while decreasing the resistor value will decrease the pulse width.

4. Q: What is the purpose of the capacitor in a blocking oscillator circuit?
   A: The capacitor connected across the transformer’s secondary winding serves two purposes: it stores energy during the pulse generation phase and helps to shape the output pulse by controlling the discharge rate during the recovery phase.

5. Q: Are blocking oscillators still relevant in modern electronic systems?
   A: While blocking oscillators have been largely replaced by more advanced oscillator circuits in many applications, they still find use in specific areas where their simplicity, high output voltage, and fast rise times are advantageous, such as in ignition systems and high-voltage pulse generators.

Conclusion

Blocking oscillators are a simple yet effective type of relaxation oscillator that generates narrow, high-amplitude output pulses. By understanding their working principle, types, and applications, designers can leverage the unique characteristics of blocking oscillators in various electronic systems. When designing or troubleshooting blocking oscillators, careful consideration of component selection, layout, and trigger mechanisms is essential to ensure reliable and efficient operation.

As technology continues to advance, blocking oscillators may find new applications in emerging fields, such as pulsed power systems and high-energy physics experiments. By combining the fundamental principles of blocking oscillators with modern components and design techniques, engineers can continue to push the boundaries of pulse generation and high-voltage electronics.

In conclusion, blocking oscillators offer a unique set of characteristics that make them suitable for specific applications requiring high-voltage, fast-rising pulses. By understanding their working principle, types, and design considerations, engineers can effectively utilize blocking oscillators in a wide range of electronic systems, from pulse generators and timing circuits to voltage converters and ignition systems.