PicoQuant GmbH HydraHarp 400 Software V. 3.0.0.1
Unless you are sure what kind of signal your SYNC source delivers, use a fast oscilloscope (50 Ω input) to
check the pulse shape, polarity and amplitude. The leading edge should be steep (2 ns fall time or faster),
there should also be no excessive overshoot or ringing. The pulse width should be 0.5 ns … 30 ns. The upper
limit is not critical, however, the pulse width should be as small as necessary to fit in half the SYNC period.
If the signal is satisfactory, connect the source to the SYNC input and start the HydraHarp software if it is not
yet running. A detector signal (Channels 1..8) is not required at this point but it does no harm if it is also
connected. Open the HydraHarp control panel. Leftmost, there is a tab with a group of controls for the SYNC
input. Select the corresponding tab and find the edit box and spin control for the CFD level. The level should be
set to a value around half of the amplitude of the SYNC pulses. Note that the control panel shows absolute
numbers from 0 to 1000 mV despite the real signals are negative going. Next, set the Zero Cross level to
10 mV. Then look at the Sync Divider. It must be set so that the sync rate divided by the shown divider value
does not exceed 12.5 MHz, e.g.,for a diode laser with 80 MHz repetition rate it must be set to 8. For a slow
source such as a flash lamp it must be set to None.
If the SYNC source is active, the Sync rate will now be displayed at the bottom left rate meter in the HydraHarp
main window. Note that the rate meter internally corrects for the chosen divider setting, so that the meter
always shows the undivided input rate. The meter display should therefore match the rate delivered from the
source. The rate meter will be refreshed every 0.1 to 1 seconds, determined by the value in the General
Settings Dialog. The Sync rate should be displayed very accurately unless it is very low. Large fluctuations or
occasional zeros indicate an incorrect discriminator level setting or an unstable Sync signal. Try to vary the
discriminator level to obtain a stable Sync rate display. If the rate is stable and as expected, you can proceed to
the set–up of the other input. A last fine tuning of the discriminator and zero cross level can be done when the
detector inputs are up and running.
Setting up the Photon Detector Inputs (Channel 1…8)
Dependent on the number of installed TDC modules your HydraHarp may have 2, 4, 6 or 8 input channels. As
noted before, the HydraHarp 400 input channels are designed electrically identical, each with a CFD. The
signals must consist of pulses with falling active edge. The pulse edges must be steep (2 ns rise / fall time or
faster). The inputs can handle a voltage range of 0 to −1 V. Note that some SPAD–detectors (e.g. SPCM–
AQR) deliver positive pulses of ~3 V (TTL) and must therefore be connected through a pulse inverter with
attenuation (PicoQuant SIA 400). Connecting them directly will cause damage. PMTs should be connected
through a preamplifier (10 dB … 20 dB). MCP–PMT detectors should be connected through an amplifier with
slightly higher gain. All accessories are available from PicoQuant. Be sure to switch the high voltage supply of
PMTs off and allow their electrodes to discharge before connecting/disconnecting them. Their high voltage
charge may damage the preamplifier. Observe the allowed input signal levels including those of the pre–
amplifier. Again, in a new experimental setup, to be absolutely sure, please check your detector pulses as well
as the preamp output with a fast oscilloscope. Start timing is on the leading edge, so it should be steep.
Ringing and overshoot should be as small as possible. Do not over–illuminate the detector to avoid damage.
If the signals are appropriate, connect a detector at channel 1. Starting with only one detector makes the first
steps easier. Preliminary adjustments can actually be done with uncorrelated light, e.g. daylight. For safety of
your detector, use strongly attenuated light, even when the detector is off. Start the HydraHarp software and
open the Control Panel. In the Control Panel section for the chosen Channel, initially set the CFD Discriminator
Level to 30 mV and the CFD Zero Cross level to 10 mV.
Using the input selector at the right of the rate meters, select the channel you are currently using. The rate
meter next to the sync rate meter will now show the input rate at that channel. Make sure that there is actually a
signal coming in (some strongly attenuated light on the detector) and try to adjust the CFD discriminator level.
To monitor this, watch the count rate meter. The adjustment range is 0 mV … 1000 mV for the CFD
discriminator level and 0 mV … 40 mV for the CFD zero cross level. Note that a precise tuning of the CFD
settings is only critical with PMT or MCP–PMT detectors. For such detectors the initial setting of the
discriminator level of 30 mV should be approximately right to suppress any electrical noise. If you lower it, you
should see an increase in count rate (probably mostly noise). If you raise it you will see a reduced rate.Thus,
you should stay just slightly above the noise level. If you still see millions of counts per second at levels over
50 mV, reduce the detector illumination. Your detector may be at risk at such high count rates. You can then
also try out responsivity to illumination changes. The Zero Cross level is not yet meaningful. It can be adjusted
for optimum timing later. For SPAD detectors that typically deliver pre–shaped pulses of constant amplitude the
CFD set–up is very simple. Just set the level to approximately half of the pulse amplitude. Again, the Zero
Cross level is not yet critical. It can be adjusted for optimum timing later.
To actually collect histograms, select a measurement range large enough (determined by the chosen
resolution) and a display range to cover your SYNC interval (i.e. 1 / sync) if possible. Set Offset = 0 and
StopAt = 4,294,967,295. Start a measurement in oscilloscope mode with e.g. 1 second acquisition time (see
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