Aside from complete measurement triggering (as discussed earlier in the P2P example), most of this chapter has assumed the use of the internal pulse generators to perform the measurements. When using external pulse generators or when timing must be controlled entirely by an external system, there are some additional issues to consider:
• Recall from the data processing discussion that a central concept is the marking of the data stream with markers corresponding to T0 so that the correct data segments can be analyzed. Normally, that T0 timing is generated internally and also used to drive the pulse generators. The T0 timing signal is provided to the rear panel so it can be used on external equipment (Pulse Synch Out). The internally generated T0 signal is frequency locked to the data acquisition system, so jitter is minimized.
• If connecting the Pulse Synch Out to an external system, that system must be set for the same PRI for any reasonable timing and, for it to be precise, the 10 MHz timebases must also be linked. If timebases other than 10 MHz are used in the external system and are accessible, contact the factory for more information on how to link them to the MS464xB Series VNA.
• A Pulse Synch In connection is also available to allow an external system to provide the T0 timing. This can be used in two ways:
• If the VNA and external 10 MHz timebases can be linked, then the default mode is to let both systems run (set for the same PRI of course). The Pulse Synch In signal is used to phase align the T0 signals. This is enabled by selecting either EXT RISING EDGE or EXT FALLING EDGE on the PulseView™ Configuration dialog (under Synch). The internal pulse generators continue to be available with their timing controlled by the internal T0 system that has been aligned to the external system.
• If it is not practical to link timebases, then the external T0 signal can be used to directly mark the data. This is enabled by selecting either EXT RISING EDGE or EXT FALLING EDGE and clicking the Ext. Synch Marking checkbox on the Measurement tab of the PulseView™ Configuration dialog. Again, the external PRI should match the internal PRI setting, although the accuracy of this agreement is less important (accurate settings will help to avoid missing some pulses or being overly limited in profile or delay ranges). In this mode, the internal pulse generators are disabled (since the internal T0 system is disabled) and there may be increased jitter since the data marking is incoherent with the data acquisition system. As a result, this method is not recommended for very narrow pulses (< 200 ns).