PicoQuant GmbH HydraHarp 400 Software V. 3.0.0.1
5.3. Time Tagged Mode Measurements
Time–Tagged Time–Resolved (TTTR) mode allows the recording of individual count events directly to hard
disk without on–board forming of histograms. It originates from the TimeHarp TCSPC systems and has been
further improved for the HydraHarp 400.
In TTTR mode, the timing of individual events is faithfully recorded to disk rather than just histogrammed. This
time-tagging is particularly interesting where the dynamics in a fluorescence process are to be investigated.
The availability of the time–tags permits photon burst identification, which is of great value e.g. for Single
Molecule Detection in a liquid flow. Other typical applications are Fluorescence Correlation Spectroscopy
(FCS) and Burst Integrated Fluorescence Lifetime (BIFL) measurements. Together with an appropriate scan
controller, TTTR mode is also suitable for ultra fast Fluorescence Lifetime Imaging (FLIM).
The HydraHarp 400 actually supports two different Time–Tagging modes, T2 and T3 mode, which will be
explained further below. When referring to both modes together we use the general term TTTR.
5.3.1. System Requirements
In cases where the time–tagging modes are to be used with high continuous count rates (> 1 Mcps) the PC
system must meet some special performance criteria, that usually are insignificant in standard interactive use
of the HydraHarp. The reason for this is the relatively large amount of data being generated in TTTR mode. In
order to prevent an overflow in the recording, the data must be transferred to the computer in real–time. This
requires a modern PC with a fast I ⁄ O subsystem. A recent CPU running at least at 1 GHz is necessary. For
the best possible performance in TTTR mode a modern SATA 6G hard disk with high throughput is
recommended. If it is intended to make use of the full bandwidth of a HydraHarp with USB 3.0 interface then
the hard disk must be able to handle sustained write rates of 160 MBytes/s. This can be achieved with RAID
arrays or modern solid state disks.
At the same time, the USB subsystem must be configured correctly. In order to deliver maximum throughput,
the HydraHarp 400 uses state–of–the–art USB 2.0/3.0 bulk transfers. This is why the HydraHarp must rely on
having a suitable host interface of corresponding speed rating. USB 2.0 host controllers of modern PCs are
usually integrated on the mainboard. USB 3.0 interfaces may need to be upgraded as PCIe cards. Throughput
is usually limited by the host controller and not the HydraHarp. Make sure that the host controller drivers are
correctly installed. This may require using the mainboard manufacturer's driver disk rather than relying on
Windows drivers. Do not run other bandwidth demanding devices on the same USB interface when working
with the HydraHarp.
5.3.2. T2 Mode
In T2 mode all timing inputs of the HydraHarp 400 and the SYNC input are functionally identical. There is no
dedication of the SYNC input channel to a sync signal from a laser. It may be left unconnected or can be used
for an additional detector signal. Usually all regular inputs are used to connect photon detectors. The events
from all channels are recorded independently and treated equally. In each case an event record is generated
that contains information about the channel it came from and the arrival time of the event with respect to the
overall measurement start. The timing is recorded with the highest resolution the hardware supports (currently
1 ps). Each T2 mode event record consists of 32 bits. There are 6 bits for the channel number and 25 bits for
the time–tag. If the time tag overflows, a special overflow record is inserted in the data stream, so that upon
processing of the data stream a theoretically infinite time span can be recovered at full resolution. Dead times
exist only within each channel (80 ns typ.) but not across the channels. Therefore, cross correlations can be
calculated down to zero lag time. This allows powerful new applications such as FCS with lag times from
picoseconds to hours. Autocorrelations can also be calculated at the full resolution but of course only starting
from lag times larger than the dead time.
The 32 bit event records are queued in a FIFO (First In First Out) buffer capable of holding up to 2 M event
records. The FIFO input is fast enough to accept records at the full speed of the time–to–digital converters (up
to 12.5 Mcps each). This means, even during a fast burst no events will be dropped except those lost in the
dead time anyhow. The FIFO output is continuously read by the host PC, thereby making room for fresh
incoming events. Even if the average read rate of the host PC is limited, bursts with much higher rate can be
recorded for some time. Only if the average count rate over a long period of time exceeds the readout speed of
the PC, a FIFO overrun could occur. In case of a FIFO overrun the measurement must be aborted because
data integrity cannot be maintained. However, on a modern and well configured PC a sustained average count
rate of 9 Mcps is possible with USB 2.0. If your device supports USB 3.0 and is connected appropriately the
Page 30
To Next Page
Loading...