ELEC 432

Background

Interconnections

A design which is correct on paper should work correctly when it is built if: (1) the components are sufficiently close to the ideal components assumed in the design, and (2) the connections between the components (i.e. the wires) are also sufficiently close to ideal. Item (1) is dependent on how well manufacturers can make their components and how much we are willing to pay for them. Fortunately, for the projects we are likely to undertake, very good components are available at quite reasonable prices, so component quality shouldn't be a serious problem, assuming we use them correctly. Which brings us to item (2).

There are two levels at which we will be concerned with interconnection: within a module, and between modules. We will consider intramodule connections in more detail a bit later when we take up prototyping and fabrication. For now, since we are already presented with a set of modules in the form of the instruments we will be using, we will concentrate on intermodule connections. In either case, the key to successful interconnection is essentially the same:

  1. Each signal must get from its source to all of its destinations with minimal attenuation, distortion, or interference.
  2. No fraction of any signal should reach any destination other than its intended one.
  3. The interconnections should not add any additional components to the circuit.
Or, more succinctly, we want to minimize loss, unintended coupling, and parasitic effects.

At RF, intermodule connections are usually made with coaxial cable (coax), at least until we reach sufficiently high microwave frequencies that waveguide is necessary. The advantages of coax over ordinary wire are (1) it is shielded, (2) it has a constant, accurately specified impedance.

We used coax interconnections in 241 lab, primarily for reason (1) (and of course, because most of the instruments had coax connectors), and this will be even more important now since we will be dealing with much fainter signals. We essentially ignored (2), and in fact, blithely connected 50 Ω cables to the 1 MΩ scope input with no apparent ill effect.

But we need to be more careful now. In addition to dealing with smaller signals, we have λ432<<λ241 This has two consequences:

  1. Conductor lengths which were extremely inefficient radiators are now sufficiently efficient to cause trouble.
  2. Wire lengths are now a large enough fraction of λ that transmission line effects must be considered.
The shielding that coax provides usually keeps (1) under control, but even with controlled impedance, we need to be concerned about (2) as we will see later in this exercise.