General Course, Module 5: Circuits Worth Understanding

5.1From Wall Socket to Clean DC

Every power supply is the same four-stage story: transform, rectify, filter, regulate. You know the transformer from Module 3. The next two stages carry the exam weight.

Rectifiers turn AC into pulsing DC, and the exam distinguishes them by diode count and by how much of the AC cycle survives:

• A half-wave rectifier uses only one diode and converts just 180 degrees of the cycle, half of it, throwing the rest away.
• A full-wave rectifier converts the entire 360 degrees. The classic version uses two diodes and a center-tapped transformer; the bridge version uses four diodes and a plain winding.
• The output of an unfiltered full-wave rectifier is a series of DC pulses at twice the frequency of the AC input, because both halves of each cycle arrive flipped upward. Ripple at 120 Hz instead of 60 is the full-wave signature.

Filtering smooths the pulses using capacitors and inductors, the energy-storage components filling the valleys between pulses. And lurking in that filter is a safety circuit you met in the Technician safety module from the other side: the bleeder resistor, whose function is to discharge the filter capacitors when power is removed. The charged-capacitor hazard and the bleeder resistor are the same fact, hazard and cure.

The modern alternative to this iron-and-capacitors design is the switchmode supply, and its defining advantage is that high-frequency operation allows the use of much smaller components. Chopping at 100 kHz instead of humming at 60 Hz shrinks the transformer from a brick to a cookie, which is why your 30-amp supply weighs three pounds.

This group also owns the exam's schematic figure, G7-1, a sheet of numbered symbols. The symbols to know cold, building on your Technician set: the FET (gate bar alongside the channel), the Zener diode (a diode with bent wings on its cathode bar), the NPN transistor (emitter arrow pointing out), the solid-core transformer (two windings with parallel lines between), and the tapped inductor (a coil with an extra lead partway along).

Website note: this page will include the G7-1 symbol sheet redrawn with a labeled study version and an unlabeled quiz version, as with the Technician symbols.

5.2Amplifiers, Oscillators, and a Little Logic

An amplifier's class describes how much of each signal cycle its device conducts, and the tradeoff is always the same: fidelity versus efficiency.

• Class A conducts 100 percent of the time. Cleanest, least efficient, the device is always on and always warm.
• Class C conducts less than half the cycle, and has the highest efficiency of the classes.

Here is the payoff question: which mode tolerates a Class C stage? FM. An FM signal carries its information in frequency, with a constant envelope, so brutal envelope distortion costs nothing. SSB, whose envelope is the information, demands a linear amplifier: one whose output preserves the shape of the input waveform. That single distinction (constant envelope versus information-bearing envelope) governs amplifier choice for every mode you will ever run.

Two amplifier housekeeping facts: efficiency is RF output power divided by DC input power, and neutralization is the cure for an amplifier's worst habit, eliminating self-oscillation. An amplifier with an accidental feedback path stops amplifying your signal and starts generating its own, which brings us to circuits that do that on purpose.

An oscillator is a filter and an amplifier operating in a feedback loop: the amplifier makes noise, the filter selects one frequency of it, the loop feeds it back, and a pure tone builds. In an LC oscillator, the frequency is set by the inductance and capacitance of its tank circuit, Module 3's resonance formula now employed as a heartbeat.

The group closes with three digital-logic spot checks:

• A two-input AND gate: output high only when both inputs are high.
• A 3-bit binary counter has 8 states (two to the third power).
• A shift register: a clocked array of circuits that passes data along in steps, the bucket brigade of digital electronics.

5.3Inside the Transceiver, and the Radio Made of Math

Now assemble the stages into the SSB transceiver you actually own.

On transmit, your microphone audio meets a carrier in a balanced modulator, whose output is double-sideband modulated RF: both sidebands, carrier suppressed. A filter then selects one sideband, and single sideband is born, exactly the "one sideband, carrier and other sideband suppressed" definition you will formalize in Module 6. At the output, an impedance matching transformer presents the desired impedance to the transmitter and feed line, Module 3 arithmetic guarding the back door.

On receive, the mirror image: a product detector extracts the voice from an SSB signal by mixing it with a locally generated replacement carrier, restoring what the transmitter suppressed.

Steering all of it is the frequency source, and modern rigs use a direct digital synthesizer (DDS): variable output frequency with the stability of a crystal oscillator, the free-spinning VFO knob backed by quartz discipline.

Filters get a vocabulary lesson, because you will read filter specifications for the rest of your ham life:

• Insertion loss: the filter's attenuation inside its passband, the toll for signals it is supposed to pass.
• Ultimate rejection: its maximum attenuation of signals outside the passband.
• Cutoff frequency: where output power falls to half the input power (the 3 dB point, Module 3 again).
• A band-pass filter's bandwidth is measured between its upper and lower half-power points.

Then the quiet revolution: the software-defined radio. An SDR digitizes the signal and performs filtering, detection, and modulation in software. All of these. Its native language is the I/Q signal pair, two copies of the RF signal 90 degrees apart in phase, and the payoff of I-Q processing is that all types of modulation can be created or decoded with appropriate software. The DSP filters this enables beat analog filters at their own game: a wide range of filter bandwidths and shapes from the same hardware, which is why your radio's filter knob now moves in 50 Hz steps instead of swapping crystals.

One receiver spec closes the module: sensitivity is affected by input amplifier gain, demodulator bandwidth, and input amplifier noise figure. All of them. Hearing weak signals is a system property, not a single component's virtue.