The VectorStar MS4640A-Series VNA has two external analog inputs (Figure: Rear Panel External Analog I/O Ports) where the voltages may be plotted on the same graph types as S-parameters and un-ratioed parameters. Examples include the use of external power detectors or external current probes (to measure amplifier current draw as a function of frequency or power for example). Like everything else on this menu, it is a PER-TRACE selection.

These are selectable on the response menu as shown in Figure: RESPONSE Menu and Submenus below. The submenu allows a choice of which port is driving during that particular analog in measurement. This port selection may be important particularly with the use of external power detectors.

 

These rear panel ports route through a multiplexer to an additional A/D converter. Internal calibration standards are used to correct for offset and channel gain slope so the accuracies are on the order of 2 mV + 1% for absolute voltages under 5V and 2% for high voltages. The calibration is performed at power on and can be triggered from the diagnostics menus if necessary (see operation manual for details). This calibration is based on a pair of voltage references built into the instrument so a linear error model of the low frequency analog in path can be generated.

The maximum range for the inputs is ±10V and the nominal input impedance is 60 kΩ. While the input is primarily intended for DC or very low frequency measurements, it has a bandwidth of approximately 1 kHz.