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3) Analog electronics.
The analog chain converts the photocurrent of the detector photodiodes to a voltage and
processes that signal (called the particle signal). The chain is repeated for the primary
and secondary detection systems.
After the photodiode transimpedence amplifier, the photo signal is mixed with a signal
derived from the reference detection system for noise cancellation. This cancels noise
fluctuations of the laser, lowering the noise floor and improving sensitivity.
The particle signal is fed into two different AC gain stages, differing in gain as specified
below. In total there are four gain stages: high and low for each of the primary and
secondary detection systems.
Gain stage labeling convention
High gain
Low gain
Primary detector
G3
G2
Secondary detector
G1
G0
Gain ratios:
G3/G2 = 50
G2/G1 = 20
G1/G0 = 20
Note that the gain ratios G3:G2 and G1:G0 are pure electrical amplification gain ratios.
The G2:G1 ratio is more complicated since it involves two independent photodetectors
with independent electronics and on opposite sides of the optical block. See the
discussion in the Calibration section.
The gain stages also provide low-pass filtering to the signal. Each gain stage then feeds
it’s own baseline restoration circuit, which restores the 0 Volt baseline which is disturbed
by frequent particle signals after AC coupling. The particle signal is then passed to a peak
hold circuit which tracks the rise of the photo-signal as a particle crosses the laser and
holds the peak value. The digital system then processes the signal and issues a reset. (See
Figure)
The noise-cancellation is not used on the secondary side. The reference detector is used
both for noise cancellation and for monitoring of the laser output power (which is directly
proportional to the laser cavity power.) At present there is no gain control implemented:
if the laser power drifts, the instrument must be calibrated. However, only large drifts in
power > 25% really need to be corrected, since the particle sizing sensitivity is a sixth-
root of the laser power.
