Practical circuits: name the stage, then find the fault

This topic asks what a named stage of a radio does, why a station or a neighbour's equipment misbehaves, and which instrument answers a measurement. Each is settled by matching a job, a mechanism or a quantity to the box that owns it.

Three questions that settle the topic

Start from the job. A stage is named for what it does to a signal, and wrong answers are stages that do something adjacent. Sensitivity and selectivity are the pair most often traded: how weak a signal the receiver can still detect in its own noise, versus how well it holds back that signal's neighbours.

Trouble questions are solved by naming the mechanism before choosing a fix. A transmitter can be cleaned up only for energy it puts where it does not belong, such as harmonics and other spurious output. A signal legally on its own frequency cannot be filtered away at its source, so when another receiver hears it anyway the mechanism is overload of that receiver by the transmitter's fundamental signal, its front end being unable to reject a strong signal from outside its band, and the filter belongs at its antenna input. A shielded cable system is different again: the signal can only be getting in where the shield is broken, so the entry path is checked before any filter. Otherwise the first move on a complaint is to find which end owns it, and that enquiry starts at home. Problems of too much are cured by less: audio driven too hard into an FM modulator widens deviation until it splashes into the next channel.

For a measurement, name the quantity, then ask what the instrument must see and how it attaches. Voltage is a difference between two points, so the meter goes across the part; current must flow through the meter, so the circuit is opened and the meter inserted; resistance is different in kind, because the meter pushes its own small current through the part and reads what appears across it, so the circuit must be dead and circuit voltage on that range destroys the meter. Antenna resonance is a question of impedance across a band of frequencies, which wants an analyser rather than a counter. SWR is a ratio: a match reads 1:1, nothing reads below one, it is no decibel figure, and a radio that senses a mismatch folds its power back to protect its output devices. The rest is workmanship: line loss becomes heat, water kills coaxial cable and the jacket keeps it out until sunlight cracks it, and foam dielectric trades breakdown voltage and moisture resistance for lower loss.

Where the answers are lost

• Trading sensitivity for selectivity. Each word sits in the other's answer list, so decide first whether the question is about noticing a signal at all or about keeping other signals out.
• Expecting a filter or an amplifier to move a signal in frequency. A filter passes or blocks and an amplifier only raises the level; a new frequency needs a second frequency mixed in.
• Taking the option that endorses all the others as a safe default, or refusing it on principle. Test each listed item alone: where cooperating with a neighbour, keeping your own station clean and the rule against interference from unlicensed devices are each part of the response, the combined choice is right; one invented device or one wrong first step kills it.
• Blaming the transmitter for interference it is not causing. Answers that turn up microphone gain, overload an audio stage or filter the transmitter assume a fault at the transmitting end; if the transmission is where it may be, the mechanism lives in whatever cannot reject it.
• Answering an overload with more level. A preamplifier ahead of a swamped front end lifts the offender with the wanted signal, and talking louder or raising power makes an over-deviating signal worse.
• Attaching the instrument the way the last measurement wanted it. A meter reading a difference goes across the part, a meter carrying current goes in line with it, and the resistance range brings its own source, so it needs a dead circuit.

Try it yourself