Synchronizer

Overview

The Swabian Instruments’ Synchronizer allows for connecting up to 8 Time Tagger Ultra / Time Tagger X devices to expand the number of available channels. The Synchronizer generates a clock and synchronization signal to establish a common time-base on all connected Time Taggers. The Time Tagger software engine creates a layer of abstraction: the synchronized Time Taggers appear as one device with a combined number of input channels.

Key applications

The Synchronizer offers numerous advantages beyond its capability to extend the number of input channels up to 144, when connected to 8 Time Taggers, without introducing any additional time jitter.

Crosstalk elimination

Employing synchronized Time Taggers provides the benefit of effectively suppressing analog crosstalk in comparison to measurements involving a single device unit. In experiments such as interferometry, for instance, the crosstalk pickup in correlation measurements hinders the identification of the physical signal from the electronic noise around zero. A commonly adopted solution consists on delaying signals with respect to each other by using cables of different lengths. Nevertheless, this approach might introduce additional complexities, e.g. the length difference fluctuates over time due to temperature variations. In this regard, the Synchronizer represents a more robust solution.

High transfer rate

The data transfer rate from a single Time Tagger to the PC is limited to 80 MTags/s by the single-core CPU performance. Utilizing more than one Time Tagger allows to use multiple USB controllers and CPU threads, significantly increasing the total transfer rate (up to 80 MTags/s per device), as far as the Time Tagger and PC processing capabilities are not overcome.

Multi-room experiment

In some experiments, Time Taggers need to be placed far apart. The cable length between the Synchronizer and the Time Tagger can be as long as 50 m, without any additional time jitter caused. Different Sync cables result only in a constant time offset between the signals.

Synchronizer with a single Time Tagger

You can benefit from the Synchronizer with a single Time Tagger at least in the following two application scenarios:

  1. Long term clock stability

    The Synchronizer contains a highly stable clock oscillator which you can benefit from even when you have only one Time Tagger. Just connect any clock output of the Synchronizer to the CLK IN input of the Time Tagger and enjoy the clock stability provided by the Synchronizer, which matters especially measuring long time differences.

  2. Absolute clock timestamps

    By connecting all signals from the Synchronizer as shown in Cable connections the timestamps in the Time Tag stream will be referenced to the power-up time of the Synchronizer. Even when you disconnect the Time Tagger from your PC, e.g., in case of power down, USB timeout, or software restart, the time tags returned by the Time Tagger will remain referenced to the start time of the Synchronizer. To verify that this configuration is active, you will see a warning message in the console on createTimeTagger() that you are using the Synchronizer with only one Time Tagger.

Requirements

Successful synchronization of your Time Taggers requires:

  • You have obtained the Synchronizer hardware.

  • Your Time Tagger Ultra has hardware version 1.2 or higher. In case you have an older device and want to synchronize it with more units, please contact our support or sales team www.swabianinstruments.com/contact .

  • Your PC has a sufficient number of USB3 ports for direct connection of every Time Tagger. The Synchronizer itself does not require a USB connection.

  • You have a sufficient number of SMA cables of the same length. You need three cables for each Time Tagger. For more details, see in the section Cable connections.

  • You have installed the Time Tagger software version 2.6.6 or newer.

Cable connections

The Synchronizer provides a common clock signal for every Time Tagger as well as the synchronization signals. Furthermore, Time Taggers have to be connected to each other in a loop. The connection sequence in the loop defines the channel numbering order. An additional feedback signal is required to identify which of the Time Taggers in the loop is the first.

Using a single Time Tagger with Synchronizer is also possible and you shall connect LOOP IN and LOOP OUT together on the same device.

Note

After the release of the Synchronizer, we have changed the connector labels on the front panel of Time Tagger Ultra. In this section, we use the new labeling scheme, while showing the corresponding old labels in brackets: NEW_LABEL (OLD_LABEL).

Connections between the Synchronizer and Time Taggers

Synchronizer

Time Tagger

Description

CLK OUT <N>

CLK IN (CLK)

500 MHz clock

SYNC OUT <N>

SYNC IN (AUX IN 1)

Synchronization data

FDBK IN

FDBK OUT (AUX OUT 2)

Feedback from one Time Tagger

Every Time Tagger should have its LOOP OUT (AUX OUT 1) connected to the LOOP IN (AUX IN 2) of next Time Tagger, eventually forming a signal loop. The following diagram visualizes the connections required for the synchronization of three Time Taggers.

../_images/synchronization_loop.svg

Warning

For reliable synchronization, the cables for CLK and SYNC signals shall have a length difference below 4 cm. We recommend using the same cable type for these two signals.

Additionally, we recommend connecting every Time Tagger directly to a USB3 port on the same computer. If your computer does not have a sufficient number of USB3 ports, avoid using USB hubs as they limit the data bandwidth available for every Time Tagger. Instead, please install an additional USB controller card into your computer. While there is a wide variety of USB3 controllers, you have to look for one that can deliver full USB3 bandwidth at every USB port simultaneously. Typically, such USB controllers have an individual chip for each USB port and require a PCIe x4 slot on the computer’s motherboard..

Using an external reference clock

The Synchronizer has a built-in high accuracy and low noise reference oscillator and distributes the clock signals to all attached Time Taggers. In case you want to use your external reference clock, you have to connect it to the REF IN connector of the Synchronizer. Additionally, the Synchronizer can supply 10 MHz reference signal through its REF OUT output. Note that REF OUT is disabled when an external reference signal is present at the REF IN.

Requirements to the reference signal at REF IN.

Parameter

Value

Coupling

AC

Amplitude

0.3 … 5.0 Vpp

Frequency

10 MHz

Impedance

50 Ohm
Signal parameters at REF OUT.

Parameter

Value

Coupling

AC

Amplitude

3.3 Vpp (1 Vpp @ 50 Ohm)

Frequency

10 MHz

Software and channel numbering

The Time Tagger software engine automatically recognizes if a Time Tagger belongs to a synchronized group. It will also automatically open a connection to all other Time Taggers in the group and present all devices as a single Time Tagger. There is no specific “master” device, and the connection to the synchronized group can be initiated from any of the member Time Taggers.

The connection is opened as usual using createTimeTagger(), and optionally you can specify the serial number of the Time Tagger.

tagger = createTimeTagger()

The tagger object provides a common interface for the whole synchronization loop, and all programming is done in the same way as for a single Time Tagger. Note that, compared to a single Time Tagger, the channel numbering scheme is modified for easy identification by a user. The channel number consists of the Time Tagger number in the loop and the input number on the front panel. The channel number formula is

CHANEL_NUMBER = TT_NUMBER*100 + INPUT_NUMBER

As an example, let us assume we have three Time Tagger Ultra 18 in a synchronization loop. The Time Tagger that provides the feedback signal to the Synchronizer has sequence number 1, and its channel numbers will be from 101 to 118. The channels of the next Time Tagger will have numbers from 201 to 218, and so forth.

Note

In case the channel numbers on your Time Tagger Ultra start with 0, in the synchronized group, the channel 0 will appear as N01, where N is the Time Tagger number. See more about channel numbering scheme in the section Channel Number Schema 0 and 1.

You can request the complete list of available channels with the TimeTagger.getChannelList() method.

from TimeTagger import createTimeTagger, TT_CHANNEL_RISING_EDGES

# Connect to any of the synchronized Time Taggers
tagger = createTimeTagger()

# Request a list of all positive edge channels
chan_list = tagger.getChannelList(TT_CHANNEL_RISING_EDGES)
print(chan_list)
>> [101, 102, ... , 317, 318]

Incomplete cable connections

The software engine attempts to detect incorrect or incomplete connections of the cables in the synchronization loop. In case some connections are missing or were disconnected during operation, the software engine will show a warning and the data transmission from the disconnected Time Tagger will be filtered out until a valid connection is restored. Issues with the cable connections and synchronization status are indicated using the status LEDs on the front panel of the Synchronizer and the Time Tagger. See more in section Status LEDs and troubleshooting.

Buffer overflows

The synchronization loop also propagates the buffer overflow state from any Time Tagger to all members of the loop. On the software side, the buffer overflow has the same effect as for a single Time Tagger. See, Overflows.

Limitations

Conditional filter

The conditional filter cannot be applied across synchronized devices. However, it can still be enabled for each Time Tagger independently.

In case you want to use the conditional filter across devices, you have to send the signal to be filtered (for example, your laser sync) to every Time Tagger where trigger signals are connected. In software, you have to choose the corresponding input for time difference measurements.

Internal test signal

The internal test-signal generator is a free-running oscillator independent from the system clock. Therefore, the test signals are not correlated between different Time Taggers, even if the synchronization loop is set up correctly. If you try to measure a correlation with the internal test signal across two different Time Taggers, you will see a flat histogram. On the other hand, performing the same measurement with two input channels of the same Time Tagger will result in a jitter-limited correlation peak.

Status LEDs and troubleshooting

The front panel of the Synchronizer has several LEDs that indicate operation status.

LED

Color

Description

Power

dark

No power provided

–

solid green

Powered on

Status

dark

Warming up

–

solid green

Normal operation.

FDBK IN

solid green

Normal operation

–

solid red

Invalid feedback signal

REF IN

dark

No external reference signal

–

solid green

Valid 10 MHz reference signal

–

solid red

Invalid reference signal

REF OUT

dark

Output is disabled when using external reference signal

–

solid green

Output enabled

The LEDs of the Time Tagger Ultra also indicate the state of the synchronization loop. See more details in section LEDs.