Agilent Technologies HP 8642A/B Spécifications télécharger pdf (Page 421)

Table 8-3. Reference Messages (5 of 7)

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:

A large spur (> —40 dBc) on one of the input signals.

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 the Approximate HP 3048A Phase Noise Floor vs R

port Signal Level (See Graph 1 in Quick Reference Guide) graph. 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.

Reduce the Phase Detector Calibration Constant by reducing the signal level into the R

port of the Phase Detector. Then reinitiate a New Measurement.

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 the Operating 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.

12 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.

rev.05SEP89 Reference Messages 8-25

1 2 ... 416 417 418 419 420 421 422 423 424 425 426 ... 452 453

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