G7C: Transceiver Design – Ham Radio General License Study Guide
G7C covers the architecture of SSB transceivers, filter terminology, and modern digital signal processing technology including direct digital synthesis (DDS), DSP filters, and software-defined radio (SDR). The fourteen questions in this group test your understanding of how signals are generated, selected, and received in a modern amateur radio transceiver.
Topics include what circuit selects a sideband from a balanced modulator, what output a balanced modulator produces, why an impedance matching transformer is used at a transmitter output, how a product detector is used, what characterizes a DDS, the advantage of a DSP filter over an analog filter, the meaning of insertion loss, ultimate rejection, and cutoff frequency, what affects receiver sensitivity, the phase relationship between I and Q signals in an SDR, the advantage of I-Q modulation, and what functions software performs in an SDR.
Transmitter Architecture
An SSB transmitter generates its signal in two stages before amplification:
Balanced Modulator
A balanced modulator combines the audio signal with a carrier frequency in a circuit that suppresses (cancels) the carrier itself. The output is double-sideband suppressed-carrier (DSB) RF — both the upper and lower sidebands are present, but the carrier is absent. The balanced modulator is the first stage of SSB signal generation.
Sideband Filter
A filter is used to select one of the two sidebands produced by the balanced modulator. It passes either the upper sideband (USB) or the lower sideband (LSB) and rejects the other. The combination of balanced modulator followed by a sharp bandpass filter is the standard method for generating a single-sideband signal in a modern transceiver.
Impedance Matching Transformer
At the transmitter output, an impedance matching transformer is used to present the desired impedance to the transmitter and the feed line. Most solid-state transmitters are designed to operate into a 50-ohm load. If the feed line or antenna system presents a different impedance, the transformer matches them so that maximum power transfer occurs and the transmitter operates within its design parameters.
Receiver Architecture
Product Detector
A product detector is used in an SSB receiver to extract the modulated signal. It works by mixing the received SSB signal with a locally generated carrier from the beat frequency oscillator (BFO), reinserting the suppressed carrier and producing the original audio. The product detector is the demodulation stage for SSB and CW modes. It is not used for FM detection.
Receiver Sensitivity
Receiver sensitivity — the ability to detect weak signals — is affected by all of the following parameters:
- Input amplifier gain — higher gain brings weak signals above the noise floor
- Demodulator stage bandwidth — a narrower bandwidth reduces noise, improving sensitivity
- Input amplifier noise figure — a lower noise figure means the amplifier adds less noise to the signal
The exam answer is all these choices are correct.
Filter Terminology
Four filter specifications appear in G7C. Each has a precise definition:
| Term | Definition |
|---|---|
| Insertion loss | The attenuation (signal loss) that the filter introduces inside its passband — ideally as small as possible |
| Ultimate rejection | The filter's maximum ability to reject signals outside its passband — how much it can suppress an unwanted signal at frequencies far from the passband |
| Cutoff frequency | The frequency above which a low-pass filter's output power falls below half the input power (the −3 dB point) |
| Bandwidth | For a bandpass filter, the range of frequencies between the upper and lower half-power (−3 dB) points |
DSP Filter Advantage
A digital signal processing (DSP) filter has a significant advantage over an analog filter: a wide range of filter bandwidths and shapes can be created using the same hardware simply by changing software parameters. An analog filter's characteristics are fixed by its physical components. A DSP filter can implement sharp skirts, adjustable bandwidth, notch filters, and other complex shapes that would require multiple analog components — all in software.
DDS, DSP, and SDR
Direct Digital Synthesis (DDS)
A direct digital synthesizer (DDS) generates a variable frequency signal digitally. It uses a numeric phase accumulator and a lookup table to produce a waveform that is then converted to analog. The key characteristic is that it provides variable output frequency with the stability of a crystal oscillator — the master clock is crystal-controlled, so all output frequencies inherit that stability. DDS synthesizers can be tuned continuously in very small steps and are found in most modern HF transceivers.
Software-Defined Radio (SDR) and I-Q Processing
Modern software-defined radios use I-Q (In-phase / Quadrature) signal processing. The I and Q signals are identical in frequency but are offset by exactly 90 degrees in phase. This 90-degree relationship allows the SDR to distinguish between signals above and below the center frequency, and to implement any modulation or demodulation scheme in software.
The advantages of I-Q modulation with SDR include:
- All types of modulation can be created with appropriate software processing — SSB, AM, FM, digital modes, and more are all implemented in software from the same hardware
Software in an SDR performs all of these functions:
- Filtering — replaces analog IF filters
- Detection — demodulates the signal
- Modulation — generates the transmitted waveform
The exam answer for which functions are performed by software is all these choices are correct.
G7C Practice Questions
Check Your Knowledge
G8: Signals and Emissions →
← G7B: Amplifiers and Oscillators