G7C

Transceiver design, filters and SDR

Explain how SSB is made and received, what filter terms mean, and how DDS and software-defined radios work.

14 pool questions1 on the exam

Syllabus: Transceiver design; filters; oscillators; digital signal processing (DSP)

SSB: balanced modulator, filter, and a product detector to undo it

A balanced modulator mixes audio with a carrier and cancels the carrier. Its output is double-sideband RF. A filter then selects one sideband. In an SSB receiver a product detector mixes the signal with a carrier to get the audio back.

Balanced modulator: double sideband, no carrier. Filter: one sideband. Product detector: back to audio.

RememberBalanced modulator: double-sideband RF. Filter: picks the sideband. Product detector: SSB receiver, extracts the audio.

A matching transformer shows each side the impedance it wants

A transmitter output stage works best into one impedance, the feed line wants another. A transformer presents the desired impedance to the transmitter and the feed line. Impedance changes with the square of the turns ratio.

The transformer lets each side see the impedance it wants.

Turns ratio (primary : secondary)2 : 1

Transmitter side200Ω

RememberImpedance-matching transformer: the right impedance for both the transmitter and the feed line.

Sensitivity: gain, noise figure and bandwidth all matter

A receiver hears a signal only if it rises above its own noise. That noise floor depends on the bandwidth (wider lets in more noise) and the noise figure (how much noise the input stage adds). Input gain boosts the signal ahead of later, noisier stages. All three affect sensitivity.

A lower noise floor means weaker signals stand out. Narrow the bandwidth or lower the noise figure.

Bandwidth2400 HzNoise figure8 dB

Noise floor-132.2dBm

RememberInput amplifier gain, input amplifier noise figure and demodulator bandwidth all affect sensitivity.

Filter vocabulary: read it off the response curve

Pick a filter type and read each term off the curve. Half-power is the −3 dB point.

Low-pass: above the cutoff frequency the output power is less than half the input.

Term	Meaning
Insertion loss	attenuation inside the passband
Cutoff frequency	where a low-pass output power falls to half the input
Bandwidth (band-pass)	between the upper and lower half-power points
Ultimate rejection	the maximum attenuation outside the passband

Memory hookInsertion = inside. Ultimate = the most it can reject.

A DDS tunes digitally but is as stable as its crystal

A direct digital synthesizer counts with a fixed crystal clock. A tuning word sets how fast the count advances, and a table and DAC turn it into a sine wave. Change the word and the output frequency changes.

One crystal, many frequencies: the tuning word sets the output, the crystal sets the accuracy.

Tuning word2

Output frequency2× lowest step

RememberDDS = variable output frequency with the stability of a crystal oscillator.

SDR: two signals 90° apart, then software does the rest

An SDR mixes the signal with an oscillator at 0° and at 90°. These two are the I and Q signals. An ADC digitizes them and software does the filtering, detection and modulation.

Almost all of the radio after the ADC is software: filtering, detection and modulation.

I and Q together set amplitude and phase, so with the right processing all types of modulation can be created.

I and Q carriers are 90° apart. Changing I and Q gives any amplitude and phase.

Amplitude80%Phase40°

I and Q0.61 / 0.51

A DSP filter is just numbers, so one radio offers a wide range of bandwidths and shapes without changing any parts.

	Analog filter	DSP filter
Bandwidth and shape	fixed by its parts	set by software

RememberI and Q: 90° apart. Any modulation. Filtering, detection and modulation all in software.

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