Page 221 - Complete Wireless Design

Oscillator Design

220 Chapter Four

Figure 4.7 Perfect Bode plot of a correctly designed oscillator.

As mentioned above, employing a linear S-parameter-based program will
supply the designer with information on the open-loop input and output return
loss and impedance, as well as the gain and phase margins. These added func-
tions will permit the engineer to not only confirm gain peaking at zero phase
of the open loop, but also whether the input and output of the open loop are
both at, preferably, 50 ohms. This will result in maximum gain and more accu-
rate simulations. In addition, 50-ohm test equipment, such as a network ana-
lyzer, may be used on the physically completed 50-ohm oscillator for test
verification.
After this open-loop simulation procedure is accomplished successfully in
the linear S-parameter simulator, then the loop may be closed (Fig. 4.8),
and energy may then be tapped from the oscillator and placed into a load. The
energy tapped from the oscillator, however, will decrease the available feed-
back from the loop. A series X or X of approximately 100 ohms may be
L C
placed at the oscillator’s output, with a 50-ohm load attached, and the simu-
lation attempted with the Spice oscilloscope and fast Fourier transform (FFT)
tool. These software tools are connected to the 50-ohm load to confirm oscil-
lation, output voltage, starting, harmonics, etc. (Caution: A Spice frequency
source must normally be included somewhere in the Spice simulation, or the
oscillator will not function—just attach the Spice frequency source to the
input of the oscillator through a series 1 megohm resistor to “fool” the simu-
lator. Spice simulators may take 20 minutes or more of computer time for the
oscillator to reach full amplitude; so be patient when employing such simula-
tors within the time domain.) Figure 4.8 is using a series C to couple the
COUPLE
energy to the load from the oscillator. Coupling out the oscillator’s energy,
without decreasing the feedback to excessively low levels, will be discussed
later in this chapter.

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