HP 3048A Operating Manual page 173

Table 3-25. Numbered Error Messages (6 of 17)
Unable to close the Phase Lock Loop.
Action: There are several possible conditions that can prevent the HP 3048A from being
able to close the phase lock loop. These conditions and the actions recommended for
verifying them are listed below in the order of probable cause.
1. The system is not able to tune the beatnote to within the Capture Range set for the
measurement. This possibility can be minimized by carefully following the Checking
the Beatnote procedure provided in each measurement example in Chapter 3 of this
manual.
If an oscilloscope is available, you will find that connecting the scope to the AUX
MONITOR port on the HP 11848A allows you to continuously monitor the beatnote
throughout the measurement. By aborting this measurement and reinitiating a New
Measurement, you can observe the beatnote as the HP 3048A attempts to close the
phase lock loop (a message will appear in the HP 3048A's display to let you know
when this is happening). The beatnote frequency, as seen on the scope, should
decrease and then go to zero (dc). Watching the beatnote's response as the system
attempts to adjust it to 0 Hz can provide clues about tuning problems that may exist.
2. The beatnote drifted beyond the system's Capture Range before the HP 3048A was
able to acquire lock. For further information on verifying the frequency stability of the
beatnote, refer to Tracking Frequency Drift in Chapter 3 of this manual.
3. The VCO Tuning Constant entered for the measurement was not within a factor of 2
of the actual Tuning Constant, and the HP 3048A was not configured to measure the
Tuning Constant for verification. For details on verifying the Tuning Constant of your
VCO, refer to Estimating the Tuning Constant in Chapter 3 of this manual.
4. The Peak Tuning Range set for the measurement is > 10 MHz and is causing the
loop dynamics to be non-linear. When this condition exists, the non-linear response
of the beatnote as the system attempts to close the phase lock loop can often be
seen on an oscilloscope connected to the AUX MONITOR port on the HP 11848A.
5. One of the sources is being injection locked as the HP 3048A is tuning the beatnote
to 0 Hz. (Refer to Minimizing Injection Locking in Chapter 3 of this manual for
recommended actions.)
6. The modulation bandwidth of the VCO source's tuning port is not wide enough
to provide adequate phase margin for the system's phase lock loop (PLL). The
VCO's modulation bandwidth (the upper frequency limit of the VCO's tuning port)
should be greater than approximately ten times the PLL bandwidth set for the
measurement. PLL bandwidth is shown as a function of the Peak Tuning Range
(PTR) in Figure 3-35. (The PTR for the measurement is determined by multiplying the
VCO's Tuning Constant by its Voltage Tuning Range.)
3-126 Signal Source Applications rev.l2]UL88