HP 3048A Operating Manual page 192

Table 3-26. Reference Messages (5 of 7)
1 1 The LNA of the HP 11848A has been bypassed. This will result in a
degradation of the system noise floor. The HP 3048A evaluates the total noise
from the Phase Detector to determine whether the Low-Noise-Amplifier (LNA) should
be switched IN or OUT. The LNA is normally switched in to provide the maximum
measurement sensitivity for the system.
If the system determines that the noise level from the Phase Detector will overload the
LNA, the LNA is switched out. When the LNA is switched out, the system's measurement
sensitivity is degraded by 20 to 30 dB depending on the offset frequency. If you are
measuring a device that has a very low noise level (near the system's noise floor) this
degradation may affect the measurement results.
Action: Proceed with the measurement. When the measurement is complete, evaluate
the results to determine the possible causes of the high noise level at the Phase Detector
output. A few possible causes are:
1. A large spur (> -40 dBc) on one of the input signals.
2. A high noise pedestal on one of the sources.
3. Spurs at the output of the Phase Detector.
To determine whether bypassing the LNA has affected your noise measurement results,
compare your results to the noise floor provided by the system when the LNA is OUT.
Use the Phase Detector Constant value measured by the HP 3048A to determine the
system's noise floor with the LNA OUT from Detector Sensitivity graph (Figure 4-7). If it
appears that your measurement may have been limited by the system's noise floor with
the LNA bypassed, it may be necessary to take one of the following actions to prevent
the LNA from being switched out.
1. Reduce the Phase Detector Calibration Constant by reducing the signal level into the
R port of the Phase Detector. Then reinitiate a New Measurement.
2. Suppress the noise and spurs at the Phase Detector output by increasing the Phase
Lock Loop (PLL) bandwidth. The PLL bandwidth is increased by increasing the
Peak Tuning Range (PTR) for the measurement. Details on increasing the PTR are
provided in Changing the PTR in Chapter 3 of this manual.
If you wish to further evaluate the LNA decision, abort the measurement and access the
Special Functions display. Select the PULSED Carrier Type and then initiate a Repeat
Measurement. When the LNA decision is displayed again, connect an oscilloscope to the
AUX Monitor port or the HP 11848A. Observe the LNA output and toggle the LNA IN. If
the peak output level exceeds -0.5V or +1V, the noise may be clipped or compressed
so the measurements must be made with the LNA OUT. If the noise is not significantly
clipped, the measurement may be made with the LNA IN.
1 2 A comparison of theoretical and actual PLL suppression indicates a
degradation of the accuracy specified for the HP 11848A. The Phase Lock Loop
(PLL) Suppression is verified by measuring the actual loop suppression at frequencies
near the predicted PLL bandwidth. This message is displayed when the measured loop
suppression differs from the theoretical by > 1 dB.
3-142 Signal Source Applications
rev.UJUL88