Injection locking [1] can not only improve oscillator frequency stability and phase noise, but act as a selective frequency divider as well [2][3].
You can find sample setups of a simple two-transistor LC-based Peltz oscillator acting as a selective frequency divider in “Simple 5-component oscillator works below 0.8V” and “Investigating injection locking with DSO Bode function”.
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The oscillator in the setup and shown in Figure 1 is just a pair of 2N3904s, a 10 µH inductor, a 2.6 nF capacitor, and a 1K bias resistor operating from -2 V. This produces a ~1 MHz oscillator output.
Figure 1 The Peltz oscillator found in the “Simple 5-component oscillator works below 0.8V” design idea (DI) consisting of only 5 components.
Signal injection is by means of a series 10 KΩ and 0.01 µF RC connected to the common emitters of the Q1 and Q2 2N3906 transistors. The subtle non-linearities within the oscillator allow selective frequency locking and division without additional active components. Figure 2 shows examples of frequency division by 2, 3, 5, and 10 respectively without any component values or circuit changes!
Figure 2 Examples of frequency division by 2, 3, 5, and 10 without any changes to the component values or circuit, this is due to the subtle non-linearities within the oscillator that allow selective frequency locking and division.
Injection locking also improves the oscillator phase noise even when acting as a divider. Figure 3 shows some results from the free running oscillator and when acting as a frequency-selective divider.
Figure 3 Spectrum analysis of the oscillator free running and when acting as a frequency-selective divider with an injection-locked division of 3 and 10. There is a marked improvement in phase noise when acting as a frequency-selective divider.
The test setup used a general-purpose AWG (SDG2042X) as the signal source, a DSO (SDS814X HD) and spectrum analyzer (SSA3021X Plus) for the displays. Of course this technique isn’t going to replace a proper digital divider, but might find use in a pinch when one needs frequency division, or improve a simple oscillators stability and phase noise.
Michael A Wyatt is a life member with IEEE and has continued to enjoy electronics ever since his childhood. Mike has a long career spanning Honeywell, Northrop Grumman, Insyte/ITT/Ex-elis/Harris, ViaSat and retiring (semi) with Wyatt Labs. During his career he accumulated 32 US Patents and in the past published a few EDN Articles including Best Idea of the Year in 1989.
Related Content
- Simple 5-component oscillator works below 0.8V
- Investigating injection locking with DSO Bode function
- Ultra-low distortion oscillator, part 1: how not to do it.
- Ultra-low distortion oscillator, part 2: the real deal
- The Colpitts oscillator
- A two transistor sine wave oscillator
- Clapp versus Colpitts
References
- Razavi, B. “A study of injection pulling and locking in oscillators.” Proceedings of the IEEE 2003 Custom Integrated Circuits Conference, 2003., pp. 305–312, https://doi.org/10.1109/cicc.2003.1249409.
- “EEVblog Electronics Community Forum.” SMD Test Fixture for the Tektronix 576 Curve Tracer – Page 1, eevblog.com/forum/projects/smd-test-fixture-for-the-tektronix-576-curve-tracer/.
- “EEVblog Electronics Community Forum.” Injection Locked Peltz Oscillator with Bode Analysis – Page 1, www.eevblog.com/forum/projects/injection-locked-peltz-oscillator-with-bode-analysis/. Accessed 25 Mar. 2025.
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