RF Electronics Chapter 6: Oscillators Page 197
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
The element OSCTEST is used to check if the oscillator has the correct gain and phase in
the feedback path. The phase shift around the loop should be exactly 0 degrees and the
gain should be greater than one. An excess gain of 3 dB ensures that the loop gain is
greater than one, even when different MAR 6 devices are used.
Tuning Step 2: The length of TL7 and TL8, which form part of the feedback signal path,
are both adjusted in order to obtain the correct phase shift of 0 º at 1 GHz. Having a loop
gain greater than 1 and a 0 º phase shift is a sufficient condition for oscillation.
Note that tuning capacitor C1 of figure 6.39 shows that this resonance at 1.3 GHz is due
to that capacitor, so that it is desirable to keep this capacitor as small as possible.
Figure 6.42. Single resonator oscillator open loop gain (OSCTEST) output.
Tuning Step 3: The design must now be arranged so that it fits on a PCB with all the
track ends joining in the correct manner. The lengths of all the tracks and the phase shift
produced by the MMIC must be kept exactly the correct length to satisfy the zero degree
open loop phase shift from OSCTEST and a gain slightly greater than one as shown in
figure 6.42.
Figure 6.43. PCB layout for the 1GHz oscillator.
RF Electronics: Design and Simulation
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