User`s guide

120 Agilent E5505A User’s Guide
5 Absolute Measurement Fundamentals
Minimizing Injection Locking
Injection locking occurs when a signal feeds back into an oscillator through its
output path. This can cause the oscillator to become locked to the injected
signal rather than to the reference signal for the phase locked loop.
Injection locking is possible whenever the buffering at the output of an
oscillator is not sufficient to prevent a signal from entering. If the injection
locking occurs at an offset frequency that is not well within the PLL
bandwidth set for the measurement, it can cause the system to lose phase lock.
Adding Isolation
The best way to prevent injection locking is to isolate the output of the source
being injection locked (typically the DUT) by increasing the buffering at its
output. This can be accomplished by inserting a low noise amplifier and/or an
attenuator between the output of the source being injection locked and the test
set. (For information on determining the effect that the amplifier noise will
have on the measurement noise floor, refer to Inserting a Device in this
section.)
Increasing the PLL Bandwidth
If the injection locking bandwidth is less or equal to the PLL bandwidth, it may
be possible to increase the PLL bandwidth sufficiently to complete the
measurement. The PLL bandwidth is increased by increasing the peak tuning
range (PTR) for the measurement.
To estimate the PTR needed to prevent injection locking from causing the
system to lose lock:
1 Determine the injection locking bandwidth. Tune the beatnote toward 0 Hz
using the procedure described in the Checking the Beatnote section of each
phase lock loop measurement example in this chapter. When the injection
locking occurs, the beatnote disappears. The injection locking bandwidth is
the frequency of the beatnote just prior to where the injection locking
occurs as the beatnote is tuned toward 0 Hz.
2 Multiply the injection locking bandwidth by 2 to determine the minimum
PLL bandwidth required to prevent the injection locking from causing the
system to lose lock. (To prevent accuracy degradation, it may be necessary
to increase the PLL bandwidth to 4 X the injection locking bandwidth. The
NOTE
The PTR for the measurement is set by the tuning characteristics of the VCO source you
are using. Figure 78 on page 121 shows that increasing the PLL bandwidth can require a
substantially larger increase in the PTR. For information on the limitations of increasing
the PTR, refer to Changing the PTR in this section.