Page 145 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

138 Chapter Six

the sharpness of the bandpass characteristic is given by the resonance Q
factor:

1 L
Q = (6.2)
R C
If we want to connect this filter to an opamp output then its impedance at res-
onance should be high enough to avoid overload, say, 1kW for a 1V output. This
means choosing the filter resistance greater than 1kW. For measurements in
the audio range, a few trials of component values show that we need a rather
large inductor and small capacitor, and this necessitates using a ferromagnetic
core for the inductor. Ferrite pot-cores are about right for this, which are char-
acterized by a value A L . This is the inductance in nanohenry for a single turn
2
on the core, whereby the inductance goes as the number of turns N . A L is a
characteristic of the actual component, not only of the ferrite material, and
depends most strongly on the air-gap dimension. Small gapped cores, such as
the RM-series from MMG-Neosid have A L values from 160 to 400nH. Ungapped
RM-series cores in F44 or F9 materials have A L = 2000 to 4300. These are the
ones to use for a simple audio filter. For example N = 250 turns on an RM10
pot-core with A L = 4300 should give L = 270mH. With a 1nF capacitor this gives
a resonance at 9700Hz. With R = 1kW, Q ª 16. This would be useful to perform
noise measurements in the audio band, with the response 3dB down at 9420
and 10010Hz. The RM10 core is about 30mm diameter and 20mm high and
comes with a coil former making winding simple. Again, a scrap of blob board
does a good job (Fig. 6.12c). A wide variety of toroidal cores is also available,
but these are tedious to wind without elaborate jigging. It is easy to see why
we have chosen an ungapped core; the much smaller A L would necessitate an
inconvenient number of wire turns.

6.10 Metal Box for Testing

All optical receivers need very effective electrostatic screening to reduce pickup
of line-frequency interference. This means mounting them in a conducting
enclosure such as of die-cast aluminum or metal-sprayed plastic. The conduct-
ing surface must be connected to the circuitry ground. Small enclosure holes
and even the photodiode package window can give problems with high-
sensitivity receivers. The same difficulties arise during testing, and so another
useful tool is a metal box. This should offer excellent electrical and optical
screening, allow quick mounting of circuits under test, and have a range of feed-
throughs for power leads and input/output signals. I use a large (300 ¥ 300 ¥
150mm) diecast aluminium box. The lid is used as the base, mounted on tall
rubber feet or standoffs to make room for a collection of right-angle BNC
sockets and power leads. The deep lid should be easily removable but needs to
be bonded to the base with a heavy ground lead. Halfway across the lid/base is
an aluminum plate wall with a 5mm hole in it. This is to allow separation of

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