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

What is a Blocking Oscillator?

A blocking oscillator is a type of electronic oscillator circuit that generates a periodic waveform consisting of narrow pulses. It is called a “blocking” oscillator because the active device, usually a transistor or tube, is driven into cutoff or saturation mode, effectively blocking the signal for a portion of each cycle. This results in the generation of sharp, narrow pulses rather than a continuous sinusoidal waveform.

The basic structure of a blocking oscillator includes an active device (transistor or tube), a transformer with a feedback winding, and a few passive components such as resistors and capacitors. The transformer plays a crucial role in the operation of the blocking oscillator by providing positive feedback and pulse shaping.

How Does a Blocking Oscillator Work?

The working principle of a blocking oscillator can be understood by examining the circuit’s operation during one complete cycle. Let’s consider a transistor-based blocking oscillator as an example:

1. Start-up: When power is applied to the circuit, a small current flows through the transistor’s base-emitter junction due to the presence of a resistor connected between the base and the positive supply.

2. Regenerative Feedback: This small current induces a voltage in the transformer’s primary winding, which is coupled to the feedback winding. The feedback winding is connected to the transistor’s base, providing positive feedback. The induced voltage in the feedback winding reinforces the base current, causing the transistor to conduct more heavily.

3. Transistor Saturation: As the base current increases, the transistor enters the saturation region, where it acts like a closed switch. This allows a large current to flow through the transformer’s primary winding.

4. Pulse Formation: The sudden increase in primary current induces a voltage spike in the secondary winding of the transformer. This voltage spike is the output pulse of the blocking oscillator.

5. Transistor Cutoff: The large current flowing through the primary winding causes the transformer’s core to saturate, limiting the magnetic flux buildup. As a result, the induced voltage in the feedback winding decreases, reducing the base current. This drives the transistor into the cutoff region, where it acts like an open switch.

6. Pulse Termination: With the transistor in cutoff, the current through the primary winding stops, and the magnetic flux in the transformer core starts to collapse. This induces a reverse voltage in the secondary winding, terminating the output pulse.

7. Recovery: The transformer’s core desaturates, and the circuit returns to its initial state. The small current flowing through the base resistor starts the cycle again, and the process repeats.

The frequency of the blocking oscillator is determined by the time constant of the transformer’s inductance and the capacitance in the circuit. The pulse width is primarily influenced by the saturation characteristics of the transformer core and the transistor.

Types of Blocking Oscillators

Blocking oscillators can be classified based on the active device used and the configuration of the circuit. Here are the main types of blocking oscillators:

1. Transistor Blocking Oscillator

Transistor blocking oscillators use a bipolar junction transistor (BJT) as the active device. They are further divided into two sub-types:

a. Emitter-Timed Blocking Oscillator: In this configuration, the transistor’s emitter is connected to ground through a capacitor. The capacitor charges during the pulse and discharges between pulses, determining the oscillation frequency.

b. Base-Timed Blocking Oscillator: Here, a capacitor is connected between the transistor’s base and ground. The capacitor’s charging and discharging action controls the base current and, consequently, the oscillation frequency.

2. Tube Blocking Oscillator

Tube blocking oscillators employ a vacuum tube, such as a triode or pentode, as the active device. The basic working principle is similar to that of transistor blocking oscillators, with the tube’s grid acting as the control electrode.

3. MOSFET Blocking Oscillator

MOSFET blocking oscillators use a metal-oxide-semiconductor field-effect transistor (MOSFET) as the active device. MOSFETs offer advantages such as high input impedance and low power consumption compared to BJTs.

Applications of Blocking Oscillators

Blocking oscillators find applications in various electronic systems where short, sharp pulses are required. Some of the common uses of blocking oscillators include:

1. Pulse Generation: Blocking oscillators are widely used as pulse generators in digital circuits, providing clock signals or trigger pulses for other components.

2. Time Base Generators: Blocking oscillators can be used to generate time base signals for oscilloscopes, radar systems, and other equipment that requires precise timing references.

3. Switching Power Supplies: In switching power supplies, blocking oscillators are employed to generate the high-frequency switching pulses that control the power conversion process.

4. Television Deflection Circuits: Blocking oscillators were commonly used in the vertical and horizontal deflection circuits of cathode ray tube (CRT) televisions to generate the sawtooth waveforms required for scanning.

5. Ignition Systems: In electronic ignition systems for internal combustion engines, blocking oscillators are used to generate high-voltage pulses for firing the spark plugs.

6. Pulse Transformers: Blocking oscillators are often used in conjunction with pulse transformers to provide isolation and impedance matching between circuits.

Advantages and Disadvantages of Blocking Oscillators

Blocking oscillators offer several advantages and have some limitations. Let’s examine them:

Advantages:

• Simple and compact circuit design
• Capable of generating narrow, high-amplitude pulses
• Wide frequency range, typically from a few hertz to several megahertz
• Efficient operation, as the active device is switched between saturation and cutoff
• Easy to synchronize with external signals

Disadvantages:

• Output pulse width and frequency are dependent on component values and can be affected by temperature and aging
• Limited output power due to the transformer’s saturation characteristics
• Requires a transformer, which can be bulky and expensive, especially at high frequencies
• Prone to jitter and noise due to the rapid switching of the active device
• Not suitable for applications requiring a continuous sinusoidal output

Comparison with Other Oscillator Circuits

Blocking oscillators differ from other common oscillator circuits in several aspects. Here’s a comparison table:

As evident from the comparison, blocking oscillators are uniquely suited for generating narrow pulses, while other oscillator circuits are better for producing continuous sinusoidal waveforms.

Frequently Asked Questions (FAQ)

1. What is the main difference between a blocking oscillator and a relaxation oscillator?
   A blocking oscillator generates narrow pulses by driving the active device into saturation and cutoff, while a relaxation oscillator generates a sawtooth or triangular waveform by repeatedly charging and discharging a capacitor.

2. Can a blocking oscillator be used to generate a continuous sinusoidal output?
   No, blocking oscillators are specifically designed to generate narrow pulses and are not suitable for producing a continuous sinusoidal output. For sinusoidal oscillations, circuits like the Hartley, Colpitts, or RC phase shift oscillators are more appropriate.

3. What determines the pulse width and frequency of a blocking oscillator?
   The pulse width is primarily determined by the saturation characteristics of the transformer core and the active device, while the frequency is governed by the time constant of the transformer’s inductance and the capacitance in the circuit.

4. Are blocking oscillators still widely used in modern electronics?
   While blocking oscillators were more common in the past, particularly in CRT television circuits, they are still used in certain applications that require simple, compact pulse generators. However, with the advent of integrated circuits and digital electronics, alternative pulse generation techniques have become more prevalent.

5. Can a blocking oscillator be synchronized with an external signal?
   Yes, blocking oscillators can be synchronized with an external signal by injecting the synchronizing signal into the feedback path or by modulating the bias voltage of the active device. This allows the blocking oscillator to lock its frequency and phase to the external signal.

Conclusion

Blocking oscillators are a unique class of oscillator circuits that generate narrow, sharp pulses by exploiting the saturation and cutoff characteristics of an active device and a transformer. They offer a simple and compact solution for pulse generation in various electronic applications, such as timing circuits, switching power supplies, and ignition systems.

While blocking oscillators have some limitations in terms of frequency stability and output power, they remain a valuable tool in the designer’s arsenal for specific pulsed signal requirements. Understanding the working principle, types, and applications of blocking oscillators is essential for engineers and hobbyists working with pulse circuits.

As electronic technology continues to evolve, blocking oscillators may find new applications or be replaced by more advanced pulse generation techniques. However, the fundamental concepts behind blocking oscillators will remain relevant, as they provide insights into the behavior of active devices and transformers in pulsed operation.