If you've ever designed an RF oscillator, you've probably encountered squegging. Sometimes called "motor boating," squegging causes oscillators to start and stop at frequencies much lower than the frequency of interest. Viewed on an oscilloscope, squegging looks like bursts of oscillations. On a spectrum analyzer, it looks like a Christmas tree. In some designs, such as super-regenerative receivers or wildlife radio tags, this might be a desirable side effect. In most cases, though, it's a nuisance.
Squegging is inherently a nonlinear effect. As such, it's difficult to model mathematically, although it can be effectively simulated in your favorite flavor of Spice. The cause of squegging is usually a shift in the bias of a transistor or other active element responsible for providing positive feedback.
As oscillation starts, nonlinearities cause bias voltages to shift extensively, to the point where oscillation cuts off. The bias voltage then returns to its quiescent value and oscillation starts again. The cycle repeats at a frequency related to time constants within the bias network. Poor circuit layout or inadequate power-supply decoupling can also cause squegging. These causes are readily addressed, but the solution to the shifting bias problem is sometimes elusive. Changing bias points can help, but it's not always clear whether or not squegging is effectively squashed.
One way to avoid squegging is to use a current-mirror bias arrangement rather than the resistors found in traditional designs. As shown in the figure, the circuit is essentially an overtone Colpitts oscillator. Assuming transistors Q1 and Q2 are a matched pair, the current through Q2 is nearly identical to that of Q1, which is essentially the current through R1. As long as the supply voltage is constant, the bias current will not shift, since the base-emitter voltage is essentially constant.
Choke L1 isolates Q1 from Q2 at RF; at dc, it's a short circuit. L2 serves two purposes: It keeps Q2's emitter at ground and, along with C2, determines the crystal overtone frequency. Q2's base presents a negative resistance to the crystal at frequencies above the resonance of C2-L2. Reactance values of 100 Ω or so work well. Larger reactances increase negative resistance and reduce bias current requirements. Choose the resonance of C2-L2 to be between the desired overtone and the next lower crystal resonance. Bias currents on the order of a milliamp or less are sufficient, given the reactance values above.
The only critical components are the matched transistors. Transistor arrays, such as the LM3046, are adequate for VHF. Transistor pairs like the Panasonic XN6537 or XN6543 are usable up to several gigahertz. The crystal can be replaced by an inductor, SAW resonator, strip line, or other resonator. Bias current depends only on the current through R1, so it's easy to control the transistor operating point.
IDEAS FOR DESIGN WANTED
Send us your Ideas for Design. We'll pay you $150 for every Idea for Design that we publish. In addition, this year's top design as selected by our readers will earn an additional $500, with two runners up each receiving $250. You can submit your Ideas For Design via:
• Postal mail to: Ideas For Design Electronic Design 45 Eisenhower Dr., Suite 550 Paramus, NJ 07652 Go to www.electronicdesign.com for our submission guidelines.
"Squegging" is the uncontrolled, unwanted, or parasitic oscillation, varying in amplitude from some peak value to very low, or completely off. The frequency of this oscillation is high compared to the rate at which its varying. It's as if an HF sinusoidal oscillator were being modulated by an LF relaxation oscillator.
Mark Barner -June 25, 2007
thank u for urs ideas but i want to ask question what is Squegging In RF Oscillators
eng hisham -June 25, 2007
Great idea. I will try it for a stripline oscilator.
Eduard Richter -June 17, 2007
Have seen it in a JSSC or EE Times article two years ago...
Robert Sanders RF -June 14, 2007
Very clever idea. More ideas like this, please!
Scott Miller -June 07, 2007 (Article Rating: )
Your Comments:
Enter the text from the image below
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
)
