• The appearance of the table and the meaning of the band start and stop sometimes cause confusion. The band start and stop do not define the frequencies that you must sweep over, but rather define the range where the given set of equations will be applied. Thus multiple bands can be created with a different equation set for each. When multiple source is ON, the system frequency (i.e., that set on the frequency menu or the segmented sweep screen) must be between the band 1 start frequency (or equal to it) and the last band entered stop frequency (or equal to it). If the Frequency menu range was outside the multiple source range when editing started, when DONE EDITING is hit (or multiple source turned on if it was off), the start and stop frequencies will be coerced to the multiple source band range. One can always sweep any range within the span that has been defined.

• Activation of external synthesizers can sometimes be mysterious and part of this is related to the limitations in constantly polling the GPIB bus. To avoid a slowdown in sweep time, the bus is only polled when entering multiple source or the ext. source submenu (and certain other times related to mmWave modes). Thus the sources must be connected at their correct addresses (and of course turned on) when the menu is entered in order for it to be visible. If multiple synthesizers were connected and were accidentally set to the same address, it may be necessary to set the addresses correctly and then cycle power on the synthesizers for the bus communication to recover. Also do not forget to connect the 10 MHz references (often the VNA acts as the 10 MHz source for other instruments, but this is not required).

• When an external synthesizer is under remote control by the VNA, sometimes the display of the synthesizer will still show an old frequency (this depends on firmware revisions and the model of the synthesizer). This may not correspond to where the synthesizer actually has been programmed. That can best be verified with the VNA measurement itself or with a spectrum analyzer. The power readout, if present, will update real time and this is another way to check if control has been established (by changing the entry on the external source submenu of the Power menu).

• Complete control has only been established for the MG3690B (and beyond), MG362x1A, and MG37022A synthesizers. Certain older Anritsu and Wiltron synthesizers will work under some frequency plans, but may not always function correctly due to frequency resolution differences.

• The discussion in this chapter generally assumes the synthesizer was in its default state when control began. As the VNA only accesses the frequency and power lists, other aspects of the synthesizer may be manually configured and the VNA will not interfere. This can be useful for certain applications (e.g., performing flat power calibrations on the synthesizer itself using its software and then using the resulting improved power accuracy for VNA measurements), but may lead to confusing results in other cases (e.g., low rate modulation turned on).

• On the Ext. Module Control dialog, the maximum VCO overrange defaults to 250 MHz (this is the amount of overrange allowed on the base 2.5–5 GHz VCO, thus at the 20 GHz plane, it translates into 2 GHz), but this amount may not always be available depending on temperature and other variations. If one is using a lot of overrange and lock errors are visible on the message bar, the frequency plan of the setup should be re-arranged to require less overrange.

• When using RF leveling, less control range is available than when using IF leveling (only applies to the 374x modules) due to lower dynamic range detection. The IF leveling only works well when the net IF is less than a few hundred MHz so it can work well for IMD measurements, but not so well for mixer measurements. RF leveling should be used for the latter. Within RF leveling, there is a subset of using RF leveling detection and VNA leveling detection when the base instrument has the dual source option (default is RF). VNA leveling allows for more control range in exchange for a small degradation in level stability. For all leveling types when doing a user power calibration, the system automatically uses the very linear module receivers to improve accuracies at requested power levels below –25 dBm.

• The common offset selection (Ext. Module Control dialog) is valuable for high order multiplied measurements as has been discussed, but requires the base receiver and source (referenced to a 5 GHz plane) to be within about 80 MHz of each other. Thus at a 20 GHz max frequency plane, they must be within about 320 MHz. If lock errors are observed, consider unchecking this box.

• If the user needs to perform a user power calibration with a multiple source control setup, there can sometimes be ambiguity at which frequency the power calibration is being performed if the source equation(s) is(are) not at the default value(s). This is important since that frequency is sent to the power meter to use the appropriate correction factor and is used to determine if an appropriate power meter is connected. This is more likely to be an issue at mmWave frequencies when multiplication of the internal sources is more common.