M4i Star-Hub

The Star-Hub modules allow the synchronization of up to 8 M4i cards in one system. Please contact us if you need more cards in one system. The synchronization option was designed for system setup with a minimum phase delay and phase skew between channels. The connection of the boards is automatically recognized and checked by the driver at load time. The programming of the Star-Hub is included in the standard card interface and consists of only a few additional commands.

It is possible to synchronize boards of the same type with each other. The module acts as a Star-Hub for clock and trigger. Each board is connected with a small cable of the same length, even the master
board. That minimizes the clock skew between the different boards. The Star-Hub carrier board is the clock master. All clock sources of this board can be used, be it internal clock, reference clock or external clock. Synchronized cards all run with the same sampling rate. Any one or even several boards can be used as trigger sources for the complete system. The trigger source of all boards can be combined with logical OR. All trigger modes that are available on the master boards are also available if the synchronization Star-Hub is used.

Background

Theoretically, there are two issues when synchronizing instruments. The first is to arrange for a common trigger. The second is to have both instruments operate from a synchronized clock. As simple as this seems there are issues that arise when attempting to synchronize multiple digitizers.

The clock can be synchronized by using an external clock at the desired clock rate. A second method is to supply an external reference such as 10 MHz, this is then applied to a phase locked loop (PLL) which is used to multiply the frequency of the reference clock to the desired clock rate. The Spectrum M4i series digitizers used in this article handle both types of external clock through a common external clock input. The external clock input is connected to an internal PLL and this is set by the user to either multiply a reference clock or to phase lock to the external clock and pass it through without changing the frequency. This guarantees the correct frequency for the clock but does not guarantee that the clock in each digitizer has the identical phase.
On the trigger side of the synchronization process we have to consider that each digitizers external trigger input uses a separate comparator to detect the trigger level crossing. Small differences in reference level and differences in setup and hold times can result is discrete changes in the trigger point location in time, a form of trigger jitter.

The only way to guarantee exact synchronization of multiple digitizers is to distribute the clock to each module and to synchronize the trigger event to the system clock. In the Spectrum digitizers this can be done with the optional Star-Hub module.

Family Synchronization Possibilities

This list will be updated with each new M4i family release and shows how different families can be synchronized:

• M4i.44xx series: only synchronization with M4i.44xx cards at the moment
• M4i.22xx series: only synchronization with M4i.22xx cards at the moment
• M4i.66xx series: only synchronization with M4i.66xx cards at the moment
• M4i.77xx series: only synchronization with M4i.77xx cards at the moment

Star-Hub Mounting Positions

There are two different mounting positions of the star-hub. The star-hub mounting is defined with the order of the options. Please check with the support team if you need to change the mounting position after delivery.

Option M4i.xxxx-SH8tm: top mount

The star-hub is mounted on top of the M4i card. The length of the card stays the same but the star-hub needs an additional slot width as it doubles the width of the card. There don't need to be a physical slot for the star-hub nor does it need a bracket. A short length card or a bracket with connectors can still be placed before the star-hub.

Option M4i.xxxx-SH8ex: extension

The star-hub is mounted as an extension to a M4i card. The width of the card stays the same and it only needs one PC slot but the length of the card is extended to full PCIe length. Please make sure that your system can handle full length PCIe cards. The star-hub has a retainer mounted that will slide into a matching holding system inside the PC.

Later extension of system

Any system setup can later be extended up to the maximum number of cards supported. All cards delivered are ready to be connected to a star-hub. If you start with one card you can simply start with a single plain card and buy a second card together with the star-hub option for extension later.