Injection locking acts as a frequency divider and improves oscillator performance

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 (Figure 1) acting as a selective frequency divider in “Simple 5-component oscillator works below 0.8V” [4] and “Investigating injection locking with DSO Bode function” [5].

Figure 1.	The Peltz oscillator found in the [4] 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.

References

1. Razavi, B. "A study of injection pulling and locking in oscillators." Proceedings of the IEEE 2003 Custom Integrated Circuits Conference, 2003., pp. 305–312.
2. "EEVblog Electronics Community Forum." SMD Test Fixture for the Tektronix 576 Curve Tracer – Page 1.
3. "EEVblog Electronics Community Forum.” Injection Locked Peltz Oscillator with Bode Analysis.
4. Michael Wyatt. "Simple 5-component oscillator works below 0.8V."
5. Michael Wyatt. "Investigating injection locking with DSO Bode function."