Page 243 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Multiple Channel Detection

236 Chapter Eleven

f 1
R L
PD
LED1 + A
l
-
f 2 1
Ch. A
l
LED2 2
f 1
Ch. B
Cuvette
f 2
Figure 11.4 Frequency-coding of two or more meas-
urement channels.

11.3 Two-Frequency Detection
Figure 11.4 shows a two-channel absorption measurement system where each
wavelength channel source is modulated at a different frequency. A single detec-
tor and transimpedance amplifier are used, which helps to optimize the track-
ing of detection gain of the channels. The photodiode’s temperature coefficient
of responsivity is a function of wavelength, so temperature variations will still
lead to some differential error, but in a low-tempco photodiode this differential
error will hopefully be very small, and certainly smaller than if two separate
photodiodes are used.
Path equality for the two wavelengths requires close juxtaposition of the two
LEDs or even interspersing of multiple LEDs in a single cluster, and this is gen-
erally straightforward to do. It is true that we have lost the advantage of using
the same current drive in both LEDs, but as the LED temperature coefficients
are quite different anyway, we haven’t lost much of an advantage. How should
the modulation frequencies be chosen? On one hand it would be better to keep
them bunched close together, in order to make best use of available bandwidth
in the photo-receiver, and to match the dynamic performance of the two signals.
On the other hand, if they are too close together, interchannel interference
will result.
We saw in Chap. 6 that the transmission frequency passband of the syn-
chronous detection process is centered on the modulation frequency f mod, with
a width proportional to the reciprocal of the postdemodulation low-pass filter
time-constant. However, the passband cutoffs are not infinitely steep; typically
they achieve only -20dB or -40dB/decade, perhaps necessitating two decades
of separation for good isolation. Hence with a 1Hz detection bandwidth, and as
long as the modulation frequencies are stable to the same order, separation of
the modulation frequencies by 100Hz should give negligible interference. If
higher detection bandwidths are needed to track fast-changing measurands,
the separation will need to be increased in proportion, which can become
inconvenient.

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