Agilent Technologies E1441A Output Amplitude Control, Figure C-4, Figure C-5, Appendix C

Phase Truncation Harmonics ≤ –20 x log10 (P) dBc

where “P” is the number of waveform points in RAM.

Output Amplitude Control

The Agilent E1441A uses a 12-bit digital-to-analog converter (DAC) to convert the digital representation of a signal to an analog output voltage. The DAC can create waveforms represented by 4,096 (212) discrete amplitude levels. All 4,096 amplitude codes are used for the built-in waveforms. Output levels from full maximum to minimum output are controlled by applying varying amounts of signal gain or attenuation to the signal created by the DAC as shown in the block diagram below. The output waveform is always described by the full 12-bit vertical resolution. You can download user-defined arbitrary waveforms using less than the full 12-bit vertical resolution; however, it is recommended that you always use the full DAC amplitude resolution to minimize amplitude quantization errors as previously discussed.

Figure C-4.

The Agilent E1441A has a fixed output source resistance of 50 ohms (see next diagram). During calibration, output amplitudes are calibrated for both the open-circuit voltage (no load) and the terminated output voltage (loaded). The terminated output amplitude is calibrated for an exact 50 ohm load. Since the function generator's output resistance and the load resistance form a voltage divider, the measured output voltage of the function generator will vary with load resistance value and accuracy. When the function generator's output is loaded with a 0.2% accuracy termination, an additional (negligible) 0.2% amplitude error is created. Using a 5% accuracy termination will add 5% additional error to specified output amplitudes.

Figure C-5.

If the function generator's output is measured with no load connected, the output will be approximately twice the displayed amplitude (Vgen instead of Vload). In some applications, you might continually use the function generator in a “no-load” conditions. In such applications, remembering to double the function generator's displayed amplitude can cause many errors. The Agilent E1441A allows you to specify the function generator's load condition using the OUTPUT:LOAD command; thus enabling the function generator to display the correct output amplitude.

156 Agilent E1441A Function Generator Tutorial Appendix C