As mentioned in the Option 31 section, most broadband and mmWave configurations are compatible with dual source systems. The exception is that simultaneous drive, which includes true-mode stimulus, is not available on 2-port 3739x-based broadband and mmWave systems since the ports are linked by a transfer switch on that model. True mode stimulus is available with 4-port 3739x-based broadband and mmWave systems.

When in broadband/mmWave modular 3739 modes, the effective trace noise at higher frequencies can become important. While not ideal for low-signal-level measurements (which are not common in true mode stimulus applications), there is a frequency plan modification that can improve this trace noise particularly in phase above 54 GHz. This is selected with the TMS Coherence flag (available via System | Setup | Misc. Setup) and should be turned ON for both the 4-port calibration phase and for the TMS measurement. If one later decides to turn this off, the 4-port calibration must be redone.

The 4-port configuration has been the subject of most of this section. True mode stimulus is available with 2-port standard MS464xB VNAs for use in measuring balanced reflection.

True mode stimulus is available in 4-port broadband and mmWave configurations using Option 84 or Option 85. The required connections were described earlier in this chapter.