Transient Recorder - Data Sheet |
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Concept
The Licel transient recorder is a powerful data acquisition system, especially
designed for remote sensing applications. To meet the demanding requirements of
optical signal detection, a new concept was developed to reach the best dynamic
range together with high temporal resolution at fast signal repetition rates.
For the first time analog detection of the photomultiplier
current and single photon counting is combined in one acquisition system. The
combination of a powerful A/D converter (12 Bit at 40 MHz) with a 250 MHz fast
photon counting system increases the dynamic range of the acquired signal substantially
compared to conventional systems. Signal averaging is performed by specially
designed ASIC's which outperform any CISC- or RISC-processor based solution.
A high speed data interface to the host computer allows readout of the acquired
signal even between two laser shots.The implementation of this concept makes
the Licel transient recorder the state of the art solution for all applications
where fast and accurate detection of photomultiplier, photodiode or other electrical
signals is required at high repetition rates.
Applications:
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LIDAR (Light Detection and Ranging)
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LIF (Laser induced fluorescence)
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TDLAS (Tunable Diode Laser Absorption Spectroscopy)
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and other pulsed optical signal techniques.
Principle of operation
The Licel transient recorder is comprised of a fast transient
digitizer with on board signal averaging, a discriminator for single photon detection
and a multichannel scaler combined with preamplifiers for both systems.For analog
detection the signal is amplified according to the input range selected and digitized
by a 12-Bit-20/40 MHz A/D converter. A hardware adder is used to write the the
summed signal into a 24-Bit wide RAM. Depending on whether trigger A or B is used,
the signal is added to RAM A or B, which allows acquisitions of two repetitive
channels if these signals can be measured sequentially.
At the same time the signal part in the high frequency domain is amplified
and a 250 MHz fast discriminator detects single photon events above the selected
threshold voltage. 64 different discriminator levels and two different settings
of the preamplifier can be selected by using the acquisition software supplied.
The photon counting signal is written to a 16-Bit wide summation RAM which allows
averaging of up to 4094 acquisition cycles.
Detection of light signals ranging over 5 orders of magnitude
In remote sensing applications like LIDAR (Light Detection
and Ranging) a photomultiplier looking at a laser pulse sent out into the atmosphere
is driven close to saturation by the backscattered light from the vicinity, but
a few hundred microseconds later it is required to detect single photons returning
from a distance of 40 to80 km. This high dynamic range of up to five orders of
magnitude is one of the challenges in the detection of LIDAR signals as well as
in other techniques like LIF (Laser Induced Fluorescence) or CARS (Coherent Anti
Raman Scattering).
Usually, the detection systems are optimized for measuring low level light
intensities using the single photon counting technique but at higher light levels
this approach results in nonlinear signal response.An analog measurement of
the photomultiplier current is therefore necessary to increase the dynamic range.
Combined analog and photon counting acquisition
Analog and photon counting detection techniques require different signal conditioning.
A high speed and high gain amplification is needed for photon counting, whereas
a strictly linear amplification below the Nyquist frequency of the A/D converter
is necessary for analog measurement. Only the integration of two complete acquisition
chains from the preamplifier to the summation memory will therefore enable you
to combine both techniques for increased linear dynamic range. This has been
done in the Licel transient recorder.
High end analog acquisition
The best 12-Bit A/D converter available at 40 MHz sampling frequency
was used to build a high end transient digitizer with a signal to noise ratio
greater than 74 dB up to the Nyquist frequency of 20 MHz. Carefully shielded preamplifiers
for three different input ranges and a fast hardware adder can average up to 4094
shots of 200 µs or 400 µs (800µs at 20 MHz) signals to attain a
maximum resolution of 0.03 µV (500 µV input range). Each sample is checked
for overrange to control clipping in the average signal.
Fast Photon Counting
The photon counting acquisition system includes a fast three-stage
preamplifier and a discriminator with 64 threshold levels, controlled by the host
computer. With a maximum count rate of 250 MHz, single photon counting is pushed
to new limits when the selected photomultipliers are used. A time resolution of
50 ns without any dead time or overlap between two memory bins is reached by using
a continuous counter together with a multichannel scaler burnt into the silicon
of a custom designed ASIC.
Complete acquisition software
The Licel transient recorder is completely software controlled.
Input ranges for analog and photon counting acquisition, discriminator levels
and the Number of active bins can be selected. The acquired analog and photon
counting signals for both summation memories can be read out separately.Data are
transferred via a 2 x 16 Bit interface to a National Instruments DIO-32-HS family
(PC) interface card.
We had to stop the support of NuBus-cards, as they are not listed anymore
in the NI catalog.
The data transfer rates are 800 KB (PC/486
DX2-66) using DOS and 500 KB using LabVIEW. Up to 16 Transient recorders can
be controlled by one interface card. A ready to use LabView Interface comes
with each Licel transient recorder. It can be run on PC/Windows
platforms. Software drivers and acquisition programs for PC/Linux are
supplied.
Specifications
| In/Outputs: | |
| Signal input: | BNC, 50 Ohm, frontpanel |
| Trigger A: | BNC, 50 Ohm, frontpanel |
| Trigger B: | BNC, 50 Ohm, frontpanel |
| Host I/O: | 50-pol bus., backpanel |
| Indicators: | -Analog input range |
| -Signal overrange |
| -Trigger and Host I/O |
| Environmental: |
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| Power: | 230V/50 Hz or 110V /60Hz, 12 W per transient recorder. |
| Operating temperature: | +10º to + 40º C |
| Storage Temperature: | -30º to + 70º C |
| Humidity: | 0 to 95%, non condensing. |
| Dimensions: |
Each transient recorder is mounted in a RF-shielded cassette 50.4x262x232mm. Systems up to 6 channels are mounted in a 448.6x 311.5 x 361mm housing for 19" rack mounting. |
| Analog acquisition: |
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| Signal input range: | 0... -20 , -100, -500 mV |
| A/D Resolution: | 12 Bit |
| Sampling rate: | 20 MSPS/40 MSPS |
| Lidar spatial resolution: | 7.5 m / 3.75 m |
| Bandwidth: | DC-10/20 MHz |
| A/D differential nonlinearity: | typ. 0.65 LSB |
| max. 1.25 LSB @25º C |
| A/D integral nonlinearity: | typ. 1 LSB @25º C |
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