This page is part of the N0NJY General Class self-study course for Technician operators upgrading to General.
G7 is where theory becomes hardware. This module covers the actual circuits that make up your station equipment — how receivers receive, how transmitters transmit, what amplifier classes mean in practical terms, how oscillators work and why they matter, how power supplies are built and what goes wrong with them, and how modern DSP and software-defined radio differ from traditional analog designs. If you ever build your own equipment, repair commercial gear, or simply want to understand why your radio does what it does, this material is essential.
The superheterodyne (superhet) receiver is the dominant architecture in virtually all modern HF transceivers. It was invented by Edwin Armstrong in 1918 and has remained the standard approach because it elegantly solves the fundamental challenge of radio reception: how do you build a high-selectivity, high-sensitivity receiver that works across a wide range of frequencies?
The answer is to convert the incoming RF signal to a fixed intermediate frequency (IF), do all the serious filtering and amplification at that fixed IF, and then detect the signal. Because the IF is fixed, you only need to build one excellent filter — and you can make it as sharp and precise as you want without having to rebuild it for every frequency. The variable-frequency local oscillator does the work of tuning.
1. Antenna input and bandpass filter (preselector): The signal from the antenna enters the receiver and passes through a bandpass filter (the preselector) that passes the desired frequency range and rejects strong out-of-band signals. On HF transceivers, this is typically a switched bank of bandpass filters — one per band — that the radio selects automatically when you change bands. The preselector prevents strong signals outside the amateur bands from overloading the first stage and producing spurious responses.
2. RF amplifier: A low-noise amplifier boosts the incoming signal before it reaches the mixer. The noise figure of this stage largely determines the receiver's sensitivity — noise added here cannot be separated from the desired signal at any later stage. High- quality HF receivers use very low-noise RF amplifiers (often GaAs FET devices) to maintain sensitivity. Some receivers include a switchable attenuator at this stage for use on very active bands where strong signals cause overload — counterintuitively, reducing gain at the front end sometimes improves the received signal quality.
3. Mixer: The mixer is the heart of the superheterodyne design. It combines the incoming RF signal with a locally generated oscillator signal (the local oscillator, or LO) and produces output signals at the sum and difference frequencies. If you are receiving 14.200 MHz and your LO is at 5.000 MHz, the mixer produces signals at 19.200 MHz (sum) and 9.200 MHz (difference). The IF filter selects one of these — say the 9.200 MHz difference product — and rejects the other. Now the 14.200 MHz signal has become a 9.200 MHz signal and can be processed by a fixed 9.200 MHz IF chain.
When you tune your radio from 14.200 to 14.205 MHz, the LO shifts from 5.000 to 5.005 MHz, keeping the IF product at 9.200 MHz. The IF chain never knows you changed frequency. This is the elegance of the superhet design.
4. The image frequency problem: The mixer is mathematically symmetric. A signal at 14.200 MHz with a 5.000 MHz LO produces a 9.200 MHz IF. But a signal at 4.200 MHz with the same 5.000 MHz LO also produces a 9.200 MHz difference product (5.000 − 4.200 = 0.800... wait, let me recalculate: if LO is at 23.200 MHz and desired signal is 14.200 MHz, the difference is 9.000 MHz. The image is at LO + IF = 23.200 + 9.000 = 32.200 MHz and LO − IF = 23.200 − 9.000 = 14.200, which is the desired signal. The image is at 14.200 + 2×9.000 = 32.200 MHz). In general, the image frequency is separated from the desired frequency by twice the IF. The preselector bandpass filter must reject signals at the image frequency before they reach the mixer.
This is one reason why a higher IF is generally better — a higher IF places the image frequency farther from the desired frequency, making it easier for the preselector to reject it. Modern double-conversion receivers use a high first IF (often 45 MHz or higher) for good image rejection, then convert down to a lower second IF (455 kHz or 9 MHz) for sharp audio filtering.
5. IF amplifier and filter: The IF amplifier boosts the signal at the intermediate frequency. The IF filter — typically a crystal filter or mechanical filter in traditional designs, or a DSP filter in modern radios — provides the selectivity that separates the desired signal from adjacent interference. This is the stage that gives your radio its ability to pull one signal out of a crowded band. Narrowing the IF filter bandwidth improves selectivity but reduces audio fidelity — a 250 Hz CW filter produces excellent signal-to-noise ratio for CW but makes SSB voice unintelligible.
6. AGC (Automatic Gain Control): The AGC samples the IF signal level and uses it to control the gain of the IF amplifier (and often the RF amplifier). When a strong signal arrives, the AGC reduces gain to prevent overload. When the signal fades, AGC increases gain to maintain a relatively constant audio output level. The attack and decay time constants of the AGC circuit determine whether it is suitable for SSB (slow decay preferred) or CW (fast attack and decay preferred).
7. Detector / demodulator: The detector extracts the audio (or data) from the IF signal. Different detectors are used for different modes:
8. Audio amplifier: The detected audio is amplified to a level suitable for headphones or a speaker. AGC-controlled audio stages maintain consistent volume across a wide range of received signal strengths.
An oscillator is an amplifier with positive feedback — it feeds some of its own output back to its input in phase, sustaining oscillation without any external signal. Every oscillator requires two things: an amplifying element (transistor, FET, op-amp) and a frequency-determining network that sets the oscillation frequency and provides the feedback.
Oscillators are everywhere in amateur radio equipment. The local oscillator in your superhet receiver, the carrier oscillator in an SSB transmitter, the frequency synthesizer in a modern transceiver, the clock in a microcontroller, the tone generators in a DTMF encoder — all are oscillators. If you are building your own equipment, oscillator design is one of the most critical skills you will need.
An LC oscillator uses an inductor-capacitor (tank) circuit as the frequency-determining element. The tank circuit resonates at a frequency determined by the L and C values (f = 1/(2π√LC)), and the transistor amplifier sustains the oscillation by replenishing the energy lost to resistance each cycle. Several circuit configurations are common:
Colpitts oscillator: The feedback is taken from a capacitive voltage divider in the tank circuit (two capacitors in series across the inductor). The transistor sees a low impedance feedback path, making the Colpitts reliable and easy to build. It is one of the most common oscillator circuits in amateur radio equipment.
Hartley oscillator: Similar to the Colpitts but uses a tapped inductor (inductive voltage divider) instead of a capacitive divider for feedback. The Hartley is slightly simpler to tune (vary one capacitor) but less common in modern designs.
Clapp oscillator: A modification of the Colpitts with an additional series capacitor in the tank circuit. The Clapp has better frequency stability than the basic Colpitts because the additional capacitor reduces the effect of transistor junction capacitance variations on oscillation frequency.
Quartz crystals exploit the piezoelectric effect — mechanical stress produces electrical charge and vice versa. A quartz crystal cut to a precise geometry resonates mechanically at a very specific frequency with extremely high Q (often 10,000 to 100,000 or higher). When used as the frequency-determining element in an oscillator, a crystal produces an output with frequency stability far superior to any LC circuit.
Crystal oscillators are used wherever frequency accuracy and stability are critical: the reference oscillator in a frequency synthesizer, the carrier oscillator in a CW transmitter, the IF filter in a receiver (multiple crystals in a ladder or lattice filter), and the clock in all digital circuits in your station. The frequency accuracy of a crystal oscillator is typically specified in parts per million (ppm). A 10 ppm crystal at 10 MHz will drift no more than 100 Hz from nominal frequency across its operating temperature range.
TCXO (Temperature Compensated Crystal Oscillator): Adds circuitry to compensate for the crystal's temperature coefficient, achieving stabilities of 1 ppm or better across a wide temperature range. Used in high-quality transceivers and GPS receivers.
VCXO (Voltage Controlled Crystal Oscillator): The crystal oscillator's frequency can be pulled slightly by an input voltage. Used in phase-locked loops where the reference oscillator must be adjusted to maintain lock.
A VFO is an LC oscillator with a variable capacitor (or varactor diode) that allows the frequency to be changed continuously across a range. In older transceivers, a mechanically tuned VFO set the operating frequency directly. The main challenge in VFO design is stability — keeping the frequency from drifting with temperature changes in the components. Good VFO design uses temperature-stable components (NPO/C0G capacitors, powdered iron cores with stable temperature coefficients) and mechanical layouts that minimize thermal stress.
Modern transceivers use PLL frequency synthesis rather than a traditional VFO. A PLL generates a stable, precise, digitally controlled frequency by comparing a voltage-controlled oscillator (VCO) to a stable reference oscillator (crystal) through a programmable frequency divider. When the VCO drifts, the phase detector senses the error and corrects the VCO. The result is the tuning flexibility of a VFO with the stability of a crystal reference. Every digital-display transceiver you have ever used contains a PLL synthesizer.
The class of an amplifier describes how much of the input signal cycle the transistor (or tube) is conducting. This determines the amplifier's efficiency, linearity, and appropriate applications. Choosing the wrong amplifier class for a given application produces either a distorted output, poor efficiency, or both.
| Class | Conduction Angle | Efficiency | Linearity | Typical Application in Amateur Radio |
|---|---|---|---|---|
| Class A | 360° (full cycle) | 25–30% | Excellent | Low-noise RF preamplifiers; audio amplifiers where distortion must be minimal |
| Class AB | 180°–360° | 50–70% | Very good | SSB linear power amplifiers; the standard for HF phone operation |
| Class B | 180° (half cycle) | ~78% | Good (push-pull) | Push-pull audio amplifiers; rarely used alone in RF due to crossover distortion |
| Class C | Less than 180° | Up to 90% | Poor (non-linear) | FM and CW transmitters; frequency multipliers; NOT suitable for SSB or AM |
In Class A, the transistor conducts throughout the entire 360 degrees of the input signal cycle. The bias point is set so the transistor is always in its active region, never cutting off. This produces the most linear amplification — the output faithfully reproduces the input waveform. The cost is efficiency: a significant DC current flows even with no input signal, and at best 25 to 30% of the DC input power is converted to useful RF output. The rest is dissipated as heat.
When to use Class A: Low-level, noise-sensitive stages where linearity matters more than efficiency. The first RF amplifier stage in a receiver (the LNA — low noise amplifier) is typically Class A. Driver stages in a transmitter signal chain are often Class A. Audio amplifiers where harmonic distortion must be minimized are Class A.
Class AB is the compromise between efficiency and linearity that makes it the standard for HF SSB linear power amplifiers. The bias point is set so the transistor conducts for slightly more than half the input cycle. The small amount of quiescent (idle) current prevents the crossover distortion that plagues pure Class B designs, while the efficiency is much better than Class A.
Every commercial HF linear amplifier you will encounter — the Icom, Yaesu, and Elecraft transceivers' final amplifier stages, and external amplifiers like the Ameritron AL-811 — operates in Class AB. The defining characteristic is that it is a linear amplifier: the output amplitude is proportional to the input amplitude. This is a strict requirement for SSB, where the amplitude variations in the modulated signal carry the audio information. A non-linear amplifier would distort the amplitude envelope and corrupt the audio.
DC input power at idle: A Class AB amplifier draws a small quiescent current even when no signal is present. In a 100-watt SSB transceiver, this idle current is typically equivalent to about 25 watts of DC input power. This is why your radio draws power even when you are listening but not transmitting.
A Class B amplifier biases the transistor at cutoff. It only conducts for half the input cycle (180 degrees). Using a single Class B transistor produces a half-wave output with severe distortion. The solution is push-pull: two Class B transistors are operated 180 degrees out of phase, each amplifying one half of the cycle. Their outputs are combined to reconstruct the full waveform. Push-pull Class B is common in audio power amplifiers and in some RF applications. The crossover distortion at the zero-crossing point is the main limitation of pure Class B, which is why practical amplifiers use Class AB rather than true Class B.
Class C biases the transistor well beyond cutoff. The transistor conducts for less than 180 degrees of the input cycle — only brief, high-amplitude pulses at the peaks of the input signal. This produces high efficiency (up to 90%) but severe distortion. The output waveform barely resembles a sine wave; it consists of short current pulses.
Class C is acceptable — and desirable — in applications where only the frequency of the output matters, not the amplitude. FM transmitters use Class C final amplifiers because FM is a constant-amplitude mode. CW transmitters use Class C because CW is simply on-off keying of an unmodulated carrier. Class C is also used in frequency multiplier stages, where the harmonics of the nonlinear output (which in a linear amplifier would be suppressed) are used intentionally to generate multiples of the input frequency.
Never use a Class C amplifier for SSB or AM. The non-linearity of Class C produces intermodulation distortion that would completely destroy the amplitude-modulated information in the signal.
Understanding the signal chain in an SSB transmitter tells you what each control does and what happens when something goes wrong. Here is the complete path from your voice to your antenna, stage by stage.
1. Microphone and microphone amplifier: Your voice produces acoustic pressure waves that the microphone converts to an electrical audio signal. The microphone amplifier (mic preamp) boosts this signal to the level needed to drive the balanced modulator. The microphone gain control on your transceiver adjusts how much gain this stage applies. Too little gain: weak modulation, low average power. Too much gain: clipping and distortion at the balanced modulator input.
2. Speech processor (optional): The speech processor compresses the audio dynamic range, bringing low-level audio components up in level relative to the peaks. This increases the average power of the transmitted signal without increasing the peak power. The result is a signal that sounds louder and cuts through noise better. Excessive compression degrades audio intelligibility and is instantly recognizable on the air as a harsh, compressed sound.
3. Balanced modulator: The balanced modulator combines the audio signal with a carrier frequency oscillator to produce a double-sideband suppressed-carrier (DSB-SC) signal. "Suppressed carrier" means the carrier frequency itself is cancelled in the output (the two inputs to the balanced modulator are fed 180 degrees out of phase, causing the carrier to cancel). The output contains only the two sidebands (upper and lower) with no carrier. The carrier suppression of a good balanced modulator is typically 40 dB or better.
4. Sideband filter: A crystal or mechanical bandpass filter selects one sideband and rejects the other. For upper sideband (USB), the filter passes frequencies above the carrier. For lower sideband (LSB), it passes frequencies below. The sharpness of the sideband filter determines how well the unwanted sideband is suppressed (the sideband suppression specification, typically 50 to 60 dB in a good transceiver).
5. Mixer and VFO/PLL: The filtered single-sideband signal at the IF frequency is mixed with the variable frequency oscillator (or PLL synthesizer) output to translate it to the desired transmit frequency. Tuning your radio changes this oscillator frequency, which shifts the transmitted frequency while keeping the signal chain ahead of the mixer at the fixed IF.
6. Driver amplifier: The translated SSB signal is amplified from the milliwatt level at the mixer output to the several-watt level needed to drive the final amplifier. The driver is a linear (Class AB) amplifier stage. Its output level is controlled by the ALC system.
7. Final power amplifier (PA): The linear final amplifier boosts the signal to the full output power level (typically 100 to 200 watts in a modern HF transceiver). The PA is Class AB. It is the stage most vulnerable to damage from high SWR, overdriving, inadequate heat sinking, or operation outside specified voltage ranges.
8. Low-pass filter: After the PA, a low-pass filter removes harmonics and other spurious products generated by the amplifier's non-linearities. The FCC requires harmonics to be suppressed to at least 43 dB below the fundamental. The low-pass filter provides this suppression. Without the LPF, your 14 MHz transmission would also radiate significant energy at 28 MHz, 42 MHz, and higher harmonics.
9. ALC (Automatic Level Control): The ALC system samples the PA output (or sometimes an internal point in the driver chain) and generates a control voltage that reduces the microphone amplifier gain when the output exceeds a threshold. This prevents overdriving the PA and keeps the output within the transmitter's linear range. The ALC meter on your transceiver shows the ALC action — it should deflect on audio peaks and recover between words. Continuous full-scale ALC deflection means the microphone gain is set too high and the transmitter is being driven into compression on every syllable.
A linear power supply converts AC to DC through rectification, filtering, and linear voltage regulation. The stages are:
Transformer: Steps AC mains voltage down to a level appropriate for the output voltage required. A 13.8V supply for amateur radio use typically uses a transformer with a 16 to 18V AC secondary.
Rectifier: Converts AC to pulsating DC. A full-wave bridge rectifier (four diodes) produces pulsating DC at twice the AC frequency — 120 Hz ripple on a 60 Hz supply. The peak voltage of the rectified output is VRMS × 1.414, minus the diode forward voltage drops (approximately 1.4V total for a silicon bridge).
Filter capacitors: Large electrolytic capacitors smooth the pulsating DC into something approaching steady DC. The larger the capacitance, the lower the ripple. A 13.8V/20A supply typically uses 20,000 to 50,000 microfarads of filtering capacitance. The ripple remaining after filtering is amplified by the audio stages in a receiver if it is not adequately suppressed, producing the classic 120 Hz hum in the audio.
Linear voltage regulator: A series pass transistor (or integrated regulator IC) continuously adjusts its resistance to maintain a constant output voltage regardless of load current changes or input voltage variations. The regulator dissipates the difference between input and output voltage as heat: Pdissipated = (Vin − Vout) × Iload. At 20 amps load current with 5 volts of headroom, the regulator dissipates 100 watts of heat. This is why linear supplies require large heat sinks and often run hot.
Advantages of linear supplies: Very low output noise (microvolts of ripple on the output), no switching-frequency interference, simple design, excellent for noise-sensitive applications. A high-quality linear supply is essentially transparent to an HF receiver. Many experienced operators prefer linear supplies specifically because they do not generate interference.
Disadvantages: Inefficient (40 to 60% efficiency at full load), heavy (the 60 Hz transformer is large and heavy), physically large. A 20-amp linear supply for a 100-watt HF transceiver might weigh 10 to 15 pounds.
A switching power supply (switch-mode power supply, SMPS) converts AC to DC using a fundamentally different approach. After an initial rectification stage, the DC is chopped into high-frequency AC (typically 20 kHz to several hundred kHz) by a switching transistor, transformed at that high frequency, rectified again, and filtered. Because the transformer operates at high frequency, it can be much smaller and lighter than a 60 Hz transformer. The switching transistor operates in saturation (fully ON) or cutoff (fully OFF), dissipating very little power in either state, which gives SMPS designs efficiencies of 80 to 95%.
Advantages: High efficiency (generates much less heat), compact and lightweight, wide input voltage range (many work from 90 to 240V AC), relatively inexpensive to manufacture. The majority of commercial station accessories, computers, and wall adapters use switching supplies.
The interference problem: The switching frequency and its harmonics appear at the output and radiate from the supply's wiring and case. A switching supply running at 100 kHz generates noise at 100 kHz, 200 kHz, 300 kHz, and harmonics extending into and through the HF amateur bands. A poorly designed or poorly filtered switching supply in your shack can raise the noise floor on HF significantly, making weak signals inaudible and degrading digital mode decoding. This is a real and growing problem as more switching supplies appear in the shack environment — laptop chargers, LED lighting drivers, USB chargers, and commercial station accessories all use switching supplies.
Diagnosis: Tune your receiver to a quiet part of the HF spectrum (40 meters in the evening, for example). Unplug suspected switching supplies one at a time and observe whether the noise floor drops. The noise from a switching supply typically sounds like a buzz or whine that moves with frequency or appears on multiple bands simultaneously. If you have an SDR receiver with a wide waterfall display, you can often see the switching frequency harmonics as a comb of equally spaced signals across the waterfall.
Suppression approaches in order of effectiveness:
Traditional superheterodyne receivers use analog hardware — crystal filters, ceramic filters, mechanical filters — to perform IF selectivity filtering. These filters are fixed at manufacture and cannot be changed without physically replacing components. A 2.4 kHz SSB filter is 2.4 kHz wide, period. DSP receivers replace these fixed analog filters with digital filters implemented in a processor. The incoming signal (at the IF or audio frequency) is converted to a stream of digital numbers by an analog-to-digital converter (ADC). The processor applies mathematical filtering operations to the digital stream. The results are converted back to analog by a digital-to-analog converter (DAC) for listening.
The practical advantages are significant: filter bandwidth is selectable in real time (from 50 Hz to 4 kHz or more, just by selecting a menu option), filter shapes can be optimized for different modes, the passband center frequency can be shifted without changing the transmit frequency (RIT in software), noise reduction algorithms can be applied, and the same hardware can implement any filter type imaginable. The disadvantage is processing latency (a small but measurable delay) and the potential for artifacts when aggressive processing is applied.
Software Defined Radio takes DSP to its logical conclusion: everything that can be done in software, is. In an SDR, the antenna connects to a wideband analog-to-digital converter that digitizes a very wide bandwidth of spectrum simultaneously — often several MHz at once. All tuning, filtering, demodulation, and display are performed in software running on a general-purpose computer or DSP processor.
The SDR receiver shows you the entire band simultaneously on a "waterfall" display, where frequency is on the horizontal axis, time on the vertical axis, and signal strength shown by color or brightness. You can see all signals on the band at once, click on any one to receive it, and zoom in or out as needed. This is fundamentally different from a traditional transceiver where you hear one signal at a time at the frequency you are tuned to.
Practical SDR for amateur radio: Inexpensive RTL-SDR dongles (originally designed for digital TV reception) receive from roughly 500 kHz to 1.7 GHz and cost $25 to $40. They are useful for monitoring, direction finding, satellite reception, and learning about propagation. For serious HF work, higher-quality SDRs (SDRplay RSP series, Airspy HF+, ANAN transceivers) provide better dynamic range, lower noise floor, and transmit capability.
Direct sampling SDR: High-quality HF SDRs sample the RF directly at the antenna (after appropriate filtering and gain) rather than down-converting to a lower IF first. This eliminates the image frequency problem of the superhet design but requires a very fast, high-quality ADC. The Flex Radio series and ANAN transceivers use this approach.
The G7 subelement covers practical circuits as tested in the 2023–2027 FCC General Class question pool. All pool questions are covered below.
Q1 (G7A01) — What is the purpose of a "bleeder" resistor in a power supply?
Q2 (G7A02) — What is the peak-inverse-voltage across the rectifier in a full-wave bridge power supply?
Q3 (G7A03) — What is the output waveform of an unfiltered full-wave rectifier connected to a resistive load?
Q4 (G7A04) — What is the output waveform of an unfiltered half-wave rectifier connected to a resistive load?
Q5 (G7A05) — What portion of the AC cycle is converted to DC in a half-wave rectifier?
Q6 (G7A06) — What portion of the AC cycle is converted to DC by a full-wave rectifier?
Q7 (G7A07) — What is the output voltage of an ideal full-wave bridge rectifier if the input is 120 VAC?
Q8 (G7A08) — Which of the following is an advantage of a switching power supply as compared to a linear power supply?
Q9 (G7A09) — What is the output voltage of an ideal half-wave rectifier if the input is 120 VAC?
Q10 (G7A10) — What is the DC output voltage of a full-wave bridge rectifier with a no-load input of 200 VAC (ignoring the rectifier voltage drop)?
Q11 (G7A11) — What is the ripple frequency of a full-wave bridge DC power supply operating on 60 Hz AC?
Q12 (G7B01) — What is the reason for neutralizing a triode RF amplifier?
Q13 (G7B02) — Which of the following is a characteristic of a Class A amplifier?
Q14 (G7B03) — Which of the following is a characteristic of a Class B amplifier?
Q15 (G7B04) — Which of the following is a characteristic of a Class C amplifier?
Q16 (G7B05) — How does a Hartley oscillator differ from a Colpitts oscillator?
Q17 (G7B06) — What is the advantage of using crystal oscillators in an HF receiver?
Q18 (G7B07) — For which of the following modes is a Class C power amplifier appropriate?
Q19 (G7B08) — What is the efficiency of a Class C amplifier?
Q20 (G7B09) — What is a characteristic of a Class AB amplifier?
Q21 (G7C01) — What is the function of a product detector?
Q22 (G7C02) — Which of the following is an advantage of a superheterodyne receiver?
Q23 (G7C03) — What is the purpose of the de-emphasis network in an FM receiver?
Q24 (G7C04) — What is the purpose of pre-emphasis used with FM transmissions?
Q25 (G7C05) — What is the purpose of a mixer in a superheterodyne receiver?
Q26 (G7C06) — What is the image frequency of a superheterodyne receiver with a 500 kHz IF frequency and a local oscillator frequency of 14.7 MHz?
Q27 (G7C07) — What are the main component parts of a receiver in order from the antenna connection to the output?
Q28 (G7C08) — What is the term for the signal that emerges from a product detector?
Q29 (G7C09) — Which of the following is characteristic of a direct-conversion receiver?
Q30 (G7C10) — How does a SDR (software-defined radio) differ from a conventional receiver?