G7: Practical Circuits – Ham Radio General License Study Guide

G7 covers the functional circuits that make up amateur radio equipment — from the power supply that converts AC to DC, to the amplifier stages that boost signals, to the transceiver architecture that combines oscillators, filters, and detectors to transmit and receive. Three exam questions come from this subelement, one from each group.

G7A addresses power supplies and schematic symbols: how bleeder resistors, filter networks, and different rectifier configurations (half-wave, full-wave, full-wave bridge) work, how switchmode supplies differ from linear supplies, and how to identify common electronic symbols including FETs, Zener diodes, NPN transistors, transformers, and inductors from Figure G7-1. G7B covers digital circuits, amplifiers, and oscillators: amplifier classes (A, B, AB, C) and their efficiency and conduction angle characteristics, what neutralization does, how AND gates and shift registers function, how many states a binary counter has, how oscillator frequency is determined, how amplifier efficiency is calculated, what a linear amplifier is, and which modes Class C is suited for. G7C examines transceiver design, filters, and software-defined radio: the role of the balanced modulator and product detector in SSB transceivers, DDS characteristics, DSP filter advantages, filter terminology (insertion loss, ultimate rejection, cutoff frequency, bandwidth), receiver sensitivity parameters, and SDR I-Q signal processing.

G7A: Power Supplies and Schematic Symbols

A power supply converts AC line voltage to the DC voltages required by radio equipment. The key stages are rectification (converting AC to pulsating DC using diodes), filtering (smoothing the pulsating DC into steady DC using capacitors and inductors), and regulation (holding the output voltage constant under varying load). A bleeder resistor connected across the output continuously draws a small current that discharges the filter capacitors when power is removed — preventing a shock hazard from stored charge. The filter network uses capacitors and inductors to smooth the rectifier's pulsating output.

Rectifier types differ in how much of the AC cycle they convert: a half-wave rectifier uses one diode and converts 180 degrees of the cycle; a full-wave rectifier uses two diodes and a center-tapped transformer and converts 360 degrees; a full-wave bridge uses four diodes and converts 360 degrees without a center-tapped transformer. The unfiltered output of a full-wave rectifier is a series of DC pulses at twice the frequency of the AC input. A switchmode power supply operates at high frequency, which allows the use of smaller transformers and filter components compared to a 60 Hz linear supply.

G7A also tests recognition of common schematic symbols using Figure G7-1: Symbol 1 = FET (field effect transistor); Symbol 2 = NPN junction transistor; Symbol 5 = Zener diode; Symbol 6 = solid core transformer; Symbol 7 = tapped inductor.

G7B: Amplifiers and Oscillators

Amplifier classes describe the portion of the AC input cycle during which the amplifying device conducts. Class A conducts 100% of the time — lowest distortion, lowest efficiency. Class B conducts 50% (180°). Class AB conducts more than 50% but less than 100%. Class C conducts less than 50% of the cycle — highest efficiency but produces significant harmonic distortion, making it suitable only for constant-envelope modes like FM, not for AM or SSB. A Class C stage used as a frequency multiplier is also acceptable. A linear amplifier preserves the shape of the input waveform in its output — required for AM and SSB. Amplifier efficiency is calculated by dividing RF output power by DC input power.

Neutralization of an amplifier feeds a signal back through the amplifier in opposition to the signal that is naturally feeding back through the transistor's internal capacitances, eliminating unwanted self-oscillation. An LC oscillator generates a sine wave using a tank circuit; its frequency is determined by the inductance and capacitance in that tank circuit. A direct digital synthesizer (DDS)... wait, that's in G7C. For G7B: a sine wave oscillator requires a filter and an amplifier operating in a feedback loop.

Digital logic: a two-input AND gate produces a high output only when both inputs are high. A 3-bit binary counter has 2³ = 8 states. A shift register is a clocked array of circuits that passes data in steps along the array.

G7C: Transceiver Design

In an SSB transmitter, a balanced modulator combines the carrier and audio to produce double-sideband suppressed-carrier (DSB-SC) RF output. A filter then selects one sideband from this output, producing the SSB signal. A product detector is used in the SSB receiver to extract the modulated signal — it multiplies the incoming SSB signal with a carrier injection signal to recover the audio. An impedance matching transformer at the transmitter output presents the desired impedance to both the transmitter and the feed line, maximizing power transfer.

A direct digital synthesizer (DDS) produces a variable output frequency with the stability of a crystal oscillator — it generates precise frequencies digitally using a phase accumulator and lookup table. A DSP filter implemented in software offers a wide range of filter bandwidths and shapes that would be impractical with analog components. Key filter terms: insertion loss = attenuation inside the passband; ultimate rejection = maximum ability to reject signals outside the passband; cutoff frequency = frequency above which a low-pass filter's output power falls below half the input power (−3 dB point); bandwidth of a band-pass filter = distance between upper and lower half-power frequencies. Receiver sensitivity is affected by all of: input amplifier gain, demodulator stage bandwidth, and input amplifier noise figure.

Software-defined radio (SDR) uses I-Q (in-phase and quadrature) signal processing. The I and Q signals are 90 degrees out of phase with each other. This quadrature arrangement allows all types of modulation to be created and decoded with appropriate digital processing. In an SDR, filtering, detection, and modulation are all performed by software.

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