2-15
If the external load changes, there is a change in the
amplitude
or phase of the signal reflected back into the
oscillator. The
portion of that signal which reaches the oscillating loop
changes
the oscillation phase, and hence the frequency by
where G is the VSWR of the load, and q is the phase angle of the reflected wave; e.g., if Q ~ 106, and isolation ~40 dB
(i.e., ~10-4), then the worst case (100%
reflection) pulling is
~5 x 10-9. A VSWR of 2 reduces the maximum pulling by
only
a factor of 3. The
problem of load pulling becomes worse at
higher frequencies, because both the Q
and the isolation are lower.
Oscillator Instabilities - External Load