Frans_M_Everaerts_Isotachophoresis_378342.pdf

162
DETECTION SYSTEMS
The modulation frequency must always be chosen to be greater than 100 Hz. A
preferable modulation frequency in our laboratory was found to be 125 Hz, at which
value the influence of possible electric currents in the phototube with a frequency of
50 Hz (mains frequency) or higher harmonics of ths frequency is minimal. These
electric currents may originate in the 100-Hz modulation in the electric lighting and in
disturbances in the mains. The circuit is shown in Fig.6.28.
The circuit with IClo is comparable with the part shown in Fig.6.26. The resistor R,
may not have a value greater than 1 G!2 because otherwise the band width of the circuit
is adversely affected. If this value is not exceeded, a source follower on the input of IClo
can be applied. This source follower can be a normal junction-FET, which is less noisy
than an MOS-FET. By means of ICll , the alternating voltage on the output of IClo is
amplified by a factor of 100. The circuit is operated such that the noise voltage on the
output of the synchronous detector is minimal. If the sensitivity is too great, the value of
R, and/or the amplification of ICll can be decreased. The output voltage of IClo should
not exceed * 1OV. In order to minimize the influence of the UVsource on the W detector,
the circuitry is surrounded with a metal frame and all connection cables between them are
screened. The output of ICI is connected with the input ‘Dem in’, as shown in Fig.6.29.
A synchronous detector is shown, combined with the oscillator that controls both the
detector and the W source. The oscillator is a symmetrical emitter coupled with a multivibrator,
which controls the transistor T4 such that it is periodically saturated and blocked,
The UV source, of which the modulation input ‘Mod’is connected with the collector of
T4, is thus periodically switched off. With the l-kn potentiometer, the modulation
frequency can be adjusted to 125 Hz.
All resistances : l/8 W
IClo, IC1, : PA 741
Tg : 2 N 5245
Fig.6.28. UV detector that can be used in combination with the modulated W source. This circuit is
applied in combination with the synchronous detector shown in Fig.6.29.