N5161A/62A/81A/82A/83A MXG Signal Generators User's Guide

Optimizing PerformanceUsing External Leveling (N5183A Only)Figure 5-19 ALC CircuitySignal GeneratorALC ModulatorOpt 1E1 OutputAttenuator(see page 107).RFOUTPUTComponent(Amp, Filter,Atten, etc.)Leveled OutputPower Splitteror CouplerDUTALCDriverInternal DetectorCablingExternal Detector(Negative output)ALC INPUTThe external detector outputs a negative voltage to the signal generator’s rear panel ALC INPUTconnector based on the power level at the detector. As the RF power level at the coupler’s/powersplitter input changes, the external detector returns a compensating negative voltage. The ALC circuituses this negative voltage to level the RF output power by raising and lowering the signal’s power,thus ensuring a constant power level at the point of detection (external detector). Since the point ofdetection does not occur at the output of the device to which the detector is connected, there issome power loss that is not compensated for by the external detector. For example on a coupler, thecoupled port siphons some of the signal’s energy to drive the external detector. In addition thecoupler experiences some insertion loss between the coupled port and the output.Figure 5- 21 on page 107 shows the input power versus output voltage characteristics for typicalAgilent Technologies diode detectors. Using this chart, you can determine the leveled power at thediode detector input by measuring the external detector output voltage. For a coupler, you must thenadd the coupling factor to determine the leveled output power.When using an external detector, the signal generator’s power range may vary from the values shownin the data sheet. This is primarily due to the efficiency of the detector. Always ensure that thedetector, coupler/power splitter are specified for the power and frequency range of interest. Todetermine the signal generator’s actual power range during external leveling, see “Determining theSignal Generator’s Amplitude Range” on page 109.With external leveling, the displayed amplitude value can vary significantly from the actual outputpower of the coupler/power splitter to which the external detector is connected (see Figure 5- 20).This is because the coupler/power splitter has it own signal characteristics (insertion loss, couplingfactor, and so forth), which are unknown to the signal generator, so it is typically unable to displayan accurate amplitude value. Also components between the signal generator and the external detectorcan affect the output power of the coupler/power splitter. You can compensate for this power displaydifference by using theExt Leveling Amptd Offset softkey or the Amptd Offset softkey. The difference between the two softkeys isthat the Ext Leveling Amptd Offset functions only while external leveling is active. For more informationon using the external leveling offset feature, see “Adjusting the Signal Generator Display’s AmplitudeValue” on page 111.Agilent N5161A/62A/81A/82A/83A MXG Signal Generators User’s Guide 105