Page 129 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

122 Chapter Six

Construction starts with your circuit diagram, making sure that all compo-
nents are present, including power supplies, decoupling capacitors, and offboard
connections, and that all IC pin numbers are correct. Draw the components in
pencil on coarse graph paper. I prefer something on a grid of about 5 to 6mm.
This can be bought through office supply and art shops. If it is a problem to
find, print something equivalent from the PC. Each square represents the 0.1
inch of the blob board. It’s not 1:1, but neither is it a problem to follow from
paper to board. The aim is to lay out the component positions to optimize per-
formance. Critical paths such as the feedback network and the photodiode con-
nection should be as compact as possible to minimize unwanted pickup. We have
seen how dependent performance can be on a few picofarads of extra capaci-
tance here. Component connections are drawn onto the coarse graph paper
using strict north, south, east, west routing. It might take a couple of tries to
get a neat layout throughout. With a little luck, the components themselves can
provide most of the flexibility needed, avoiding all but a few wire bridges. These
can be of bare tinned copper wire for rectilinear runs on top of the board, or
fine solid-core insulated wire for connections underneath. Tefzel or Kynar
30AWG (0.25mm) insulated wire-wrap wire is perfect for this, easily stripped
with the right tool and proof against overheating by the soldering iron. For off-
board connections you can use 1-mm-diameter pushpins (“Veropins”) made for
these boards, with soldered wire connections. Tiny screw terminals with con-
nections on 0.1-inch centers are also available, which work well with blob board.
Make sure you use sufficient pushpins to act as test points for all important
signals and a convenient ground pin or wire-loop for your scope probe ground.
Changes and errors are quickly corrected with an eraser. Figure 6.1b shows the
design layout. On this layer I have the components, wire connections, and all
the labels. This view will be used for debugging.
Once the layout is finished, I make a hand copy onto tracing paper (or matt
polycarbonate drawing sheet) again with a pencil. This is very quickly done,
taking only a minute or two. Only the connections are drawn on this layer not
the component bodies. You can avoid one step by drawing straight onto translu-
cent graph paper. Art shops sell this too, or you can print your own. The only
problem is that the underside view still includes all the component bodies,
which is too much information. Hence I prefer the two-sheet approach.
Working from the top view and following the layout, the components are now
inserted into the blob board exactly as in the paper layout. Count squares to
make sure everything looks right. Small placement errors can be trivial or di-
sastrous, depending on the local topology. Clamp the components with a cloth
or soft sponge, flip the board over, and solder the leads into place. Just one con-
nection per component will do. A soldering iron with a round 1.5-mm-diameter
tip is about right, allowing one pad to be heated without touching its neighbor.
Check that everything is held in place, and correct any components that are
falling out of the board or leaning over. Complete all the component soldering
and cut excess component leads flush with the solder bumps using sharp side-
cutters. Now the “wiring” can begin.

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