M5i Star-Hub

The Star-Hub modules allow the synchronization of up to 8 M5i 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 cososting of multiple shielded coax cables 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 M5i 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.

Star-Hub Mounting Position

The star-hub is mounted on the opposite site of the analog module and occupies a 3rd slot of the system. There is no additional PCIe connector but the star-hub has it's own front panel with a dedicated clock input. The right-hand picture shows the star-hub mounted on a single card without the connection cables. On top edge of the star-hub one can see the 8 connectors for the 8 cards that are suported. Star-Hub has a fixed connection with the base card and can only be installed in factory.

Ordering Options

To keep the cost down, here are three ordering options, each with a dedicated number of synchronization cables. In case a system should be extended at a later stage beyond the number of cables that have been purchased originally, one can order single synchronization cables:

• M5i.xxxx-SH8C2: star-hub for 8 cards, 2 synchronization cables included
• M5i.xxxx-SH8C4: star-hub for 8 cards, 4 synchronization cables included
• M5i.xxxx-SH8C8: star-hub for 8 cards, 8 synchronization cables included

Please take into account, that one synchronization cables is needed to connect the star-hub with the card the star-hub is mounted on.

Clock Skew Control

Clock Skew is the difference between the arrival of the clock signal at different components (See Wikipedia for more details). Although all traces and cables are of the very excat same length in our design, routing clock signals through multiple components and cables generates some skew between the clock edges. As sampling clock can be as high as 10 GS/s (100 ps between two edges) this skew can be a signifikant factor for signal quality, especially if using multiple signals for calculations. The M5i Star-hub allows to adjust teh clock skew between the connected cards by software commands. Depending on the clock source of the card, the clock skew can be adjusted in the range of:

• Sampling rate 3.2 GS/s: skew compensation up to 312 ps
• Sampling rate 6.4 GS/s: skew compensation up to 312 ps
• Sampling rate 10.0 GS/s: skew compensation up to 200 ps

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.