Spurious parametric oscillation & cure

Post date: Apr 28, 2015 7:25:28 AM

Background

A 4 GHz power amplifier breaks into spurious parametric oscillation when simultaneously subjected to output mismatch and input overdrive. The amplifier utilizes a monolithic microwave integrated circuit (MMIC) integrating an enhancement-mode pseudomorphic high electron mobility transistor (ePHEMT), feedback, active bias and high pass filter (HPF) in a SOT-89 package [Avago]. The 0.25 x 800 um ePHEMT's quiescent operating conditions are: Vds = 5V and Ids = 50mA. The HPF attenuates signals below 1.8 GHz to aid stability at lower frequencies. The MMIC output is externally matched with lumped components (Z match) for broadband operation over 3.0-6.0 GHz (fig. 1). 

In the stability evaluation setup [Hickman], the amplifier output sees an infinite mismatch (VSWR = ∞) when the attenuation (att) is 0 dB (fig. 2). The sliding short varies the load phase over 360 degrees.  The 4.0 GHz input signal (fG) drives the amplifier (demoboard) to the threshold of gain compression. The amplifier oscillates over a portion of the sliding short's range. Reducing the load mismatch by inserting a 3 dB / 6 dB attenuator (att) between the amplifier output and the sliding short will suppress the oscillation. Reducing the input drive also will kill the oscillation.

The fundamental oscillation frequency (fo) is ~2.53 GHz and it can be varied over 3 MHz by adjusting the sliding short (fig. 3). During oscillation, the amplifier's spectral output consists of fo, fG, and their higher-order spurii.

The amplifier's small-signal stability factors (u and k) exceed 1 at 2.53 GHz (fig. 4). The small-signal stability factors have no predictive value because the equivalent electrical circuit is different under input overdrive [Bahl].

Oscillation suppression

Connecting a quarter wave open-circuit stub to the output can suppress the half-frequency (1/2f) oscillation [Uchida, Dearn]. Since this work's oscillation frequency is not half the input frequency, i.e. fo ≠ 0.5*fG, it is uncertain that the open-circuit (oc) stub can work. However, there is a possibility that it might work because the oscillation frequency is within 26% from half the overdrive frequency.

To rapidly evaluate the candidate solution, a makeshift stub is retrofitted to the amplifier (fig. 5a). The stub consists of a thin bare copper wire about 28mm long. One end of the wire is soldered to the amplifier output and the wire is dressed perpendicular to the printed circuit board (fig. 5b).

Results

No oscillation is observed over the sliding short's adjustment range; i.e. only the driving signal fG and its harmonic 2fG remained (fig. 6). Retrofitting a stub to the amplifier circuit does not degrade either gain or return loss at the signal frequency (fig. 7-8).

Discussion

For a permanent solution, a new PCB can be designed to replace the wire stub with a PCB trace. The trace will be shorter than the wire due to the PCB's higher dielectric constant, Dk over air. To save PCB space, the trace can be meandered. It may also be possible to replace the stub with a series LC between the output and ground.

Conclusion

The quarter wavelength open-circuit stub can suppress spurious parametric oscillation.

References

• 1. I. Hickman, Practical RF handbook, 4th ed., Newnes, 2006, pp.153

  2. Avago Technologies datasheet, "MGA-30989," Nov. 2013 [online] Available: www.avagotech.com.

  3. I. J. Bahl, Fundamentals of RF and Microwave Transistor Amplifiers, New Jersey: J. Wiley, 2009, sect. 17.4 spurious parametric oscillations.

  4. H. Uchida, Y. Itoh, M. Miyazaki, and S. Urasaki, ‘Conditions of half-frequency oscillation in high-power amplifier with FET and its suppression method using quarter-wavelength open-circuited stub’, Electron. Comm. Jpn. Pt. II, vol. 85, no. 6, pp. 11–19, Jun. 2002.

  5. A. W. Dearn and L. M. Devlin, ‘MM-Wave Super Harmonic Injection Locked Frequency Dividers’, IET Seminar MM-Wave Products &Technologies, 2006, pp. 63–65.