Peltz oscillator with injection locking

Oscillator injection locking is an interesting subject; however, it seems to be a forgotten circuit concept that can be beneficial in some applications.

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This design idea shows an application of the built-in Bode capability within many modern low-cost DSOs such as the Siglent SDS814X HD using the Peltz oscillator as a candidate for investigating injection locking [1], [2], [3].

Figure 1 illustrates the instrument setup and device under test (DUT) oscillator schematic with Q1 and Q2 as 2N3904s, L ~ 470 µH, C ~ 10 nF, Rb = 10K, Ri = 100K and Vbias = -1 VDC. This arrangement and component values produce a free running oscillator frequency of ~75.5 kHz

Figure 1 Mike Wyatt’s notes on producing a Peltz oscillator and injector locking setup where the arrangement and component values produce a free running oscillator frequency of ~75.5 kHz.

Analysis and measurements

As shown in Figure 2, the analysis from Razavi [2] shows the injection locking range (± Δfo) around the free running oscillator frequency fo. Note the locking range is proportional to the injected current Ii. The component values shown reflect actual measurements from an LCR meter.

Figure 2 Mike Wyatt’s notes on the injection-locked Peltz oscillator showing the injection locking range around the free running oscillator frequency fo.

This analysis predicts a total injecting locking range of 2*Δf_o, or 2.7 kHz, which agrees well with the measured response as shown in Figure 3.

Figure 3 The measured response of the circuit shown in Figure 1 showing an injection locking range of roughly 2.7 kHz.

Increasing the injection signal increases the locking range to 3.7 kHz as predicted, and measurement shows 3.6 kHz as shown in the second plot in Figure 4.

Figure 4 The measured response of the circuit shown in Figure 1 where increasing the injection signal increases the locking range to 3.7 kHz.

Note the measured results show a phase reversal as compared to the illustration notes (Figure 2) and the Razavi [2] article. This was due to the author not defining the initial phase setup (180^o reversed) in agreement with the article and completing the measurements before realizing such!!

Injection locking use case

Injection locking is an interesting subject with some uses even in today’s modern circuitry. For example, I recall an inexpensive arbitrary waveform generator (AWG) which had a relatively large frequency error due to the cheap internal crystal oscillator utilized and wanted the ability to use a 10 MHz GPS-disciplined signal source to improve the AWG waveform frequency accuracy. Instead of having to reconfigure the internal oscillator and butcher up the PCB, a simple series RC from a repurposed rear AWG BNC connector to the right circuit location solved the problem without a single cut to the PCB! The AWG would operate normally with the internal crystal oscillator reference unless an external reference signal was applied, then the oscillator would injection lock to the external reference. This was automatic without need for a switch or setting a firmware parameter, simple “old school” technique solving a present-day problem!

 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/Exelis/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.

References

1. “EEVblog Electronics Community Forum.” Injection Locked Peltz Oscillator with Bode Analysis, www.eevblog.com/forum/projects/injection-locked-peltz-oscillator-with-bode-analysis. 
2. B. Razavi, “A study of injection locking and pulling in oscillators,” in IEEE Journal of Solid-State Circuits, vol. 39, no. 9, pp. 1415-1424, Sept. 2004, doi: 10.1109/JSSC.2004.831608. 
3. Wyatt, Mike. “Simple 5-Component Oscillator Works below 0.8V.” EDN, 3 Feb. 2025, www.edn.com/simple-5-component-oscillator-works-below-0-8v/.

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