G2 · Published 23 October 2026 · By The Ham Desk

Inside the Transceiver: Mixing, Filtering and Modulation

This unit opens the box. It follows a signal from the microphone to the antenna and back again, and explains why the choices made along that path - where to convert, what to filter, how to generate a sideband - determine how clean the transmitted signal is.

Band 15 m
Study time 18
Level Stretch
questions 10

Waveform and antenna diagram for Signals, Modulation and Digital Modes — Signals, Modulation and Digital Modes — waveform overlay and radiation pattern

Pre-flight briefing

The superheterodyne idea

Almost every transceiver converts incoming signals to a fixed intermediate frequency, where a single set of filters and amplifiers can serve every band. A mixer multiplies the incoming signal by the local oscillator and produces the sum and the difference; one is selected as the IF and the other is filtered out. When the local oscillator sits above the signal, the difference gives the IF and reception is called high-side injection; the same arithmetic means a signal on the other side of the oscillator also converts to the IF - the image frequency, and the reason a sharp front-end filter precedes the mixer.

On transmit the process runs backwards: the signal is generated at a low level on the IF, filtered to remove unwanted products, then mixed up to the operating frequency and amplified. Choosing a high first IF makes the image far enough away to be filtered cleanly, which is one of the design decisions that separates a good receiver from a poor one.

Filters, and the shape of a signal

A band-pass filter passes a range and rejects everything outside it; that is how a receiver hears one signal while a broadcast station a few hundred kilohertz away is ignored. Crystal and mechanical filters achieve narrow bandwidths with steep skirts, and the shape of that response - how quickly it falls away outside the passband - decides whether a strong nearby signal can be heard through the filter's skirts.

The transmitter needs the same care in reverse. A low-pass filter after the final amplifier removes harmonics, and a pi-network or similar tank circuit both matches the final to the feed line and attenuates the harmonic energy that the non-linear final stage produces. Section 97.307(b) sets the resulting limits: spurious emissions must be at least 43 dB below the fundamental for a transmitter below 250 watts.

Modulation is the other half of the design. Amplitude modulation varies the carrier's amplitude and produces a carrier plus two sidebands; single sideband suppresses the carrier and one sideband to put all the power into information; frequency modulation varies frequency, and its bandwidth follows Carson's rule - roughly twice the sum of the deviation and the highest modulating frequency, so 5 kHz deviation with 3 kHz audio needs about 16 kHz.

Levels, ALC and the things that go wrong

Every stage has an optimum level. Too little drive and the stage contributes noise; too much and it compresses, generating intermodulation products that appear as splatter across the band and as a distorted signal on the air. The ALC circuit controls transmit gain to hold the final amplifier just below the point where distortion rises, and heavy ALC action is a warning that the microphone gain or the drive is set too high.

Two failures are worth recognising. A stage that is oscillating will produce a signal with no input and a loud hum or buzzing note; a stage that is overdriven will produce a broad, dirty signal with a ragged envelope. Both are visible on a monitor oscilloscope or on the waterfall display of an SDR receiver, and both are the operator's responsibility under the requirement to use the minimum power necessary and to avoid interference.

Terms you must be able to define
Term	What the examiner wants to hear
Intermediate frequency	A fixed frequency to which all incoming signals are converted for filtering and amplification.
Image frequency	The frequency on the opposite side of the local oscillator that also converts to the IF.
Carson's rule	FM bandwidth is roughly twice the sum of deviation and highest modulating frequency.
ALC	Automatic level control: holds transmit gain below the distortion threshold.
Spurious emission	Unwanted output at frequencies outside the intended signal, limited by §97.307.

Question deck 10 questions

G5B01 Core

Why do most receivers convert incoming signals to an intermediate frequency?

G5B02 Core

What does a mixer produce from an incoming signal and a local oscillator?

G5B03 Stretch

A receiver uses a 9 MHz intermediate frequency and is tuned to 14.200 MHz. What is the image frequency if the local oscillator runs above the signal?

G5B04 Core

What is the function of the band-pass filter at a receiver's front end?

G5B05 Core

What is the purpose of the low-pass filter following a transmitter's final amplifier?

G5B06 Core

What is the minimum required suppression of spurious emissions for a transmitter running less than 250 watts?

G5B07 Stretch

Using Carson's rule, what bandwidth does an FM signal with 5 kHz deviation and 3 kHz of audio occupy?

G5B08 Core

What is the effect of driving a transmitter's final amplifier into heavy ALC action?

G5B09 Stretch

What does a filter's shape factor describe?

G5B10 Stretch

A transmitter produces a signal with a loud hum and a rasping note even with no microphone connected. What is the most likely cause?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

G5B01 Explanation 1

Key B — So one set of filters and amplifiers can serve every band, with the frequency selection done by the local oscillator

A superheterodyne receiver does its selectivity at a fixed frequency, where sharp crystal or mechanical filters are practical, and tunes by moving the local oscillator. Without that, every band would need its own set of tuned circuits.

Rule Fundamentals - superheterodyne receivers

G5B02 Explanation 2

Key A — The sum and the difference of the two frequencies

A mixer multiplies, and multiplying two sine waves produces sum and difference components. Both are present at the output; the receiver's filters decide which one becomes the intermediate frequency and reject the other along with the unwanted products.

Rule Fundamentals - mixers

G5B03 Explanation 3

Key D — 32.200 MHz

High-side injection puts the local oscillator at 14.200 + 9 = 23.200 MHz, and a signal at 23.200 + 9 = 32.200 MHz also converts to the same IF. The image always sits twice the IF away from the wanted signal, on the opposite side of the oscillator.

Rule Fundamentals - image response

G5B04 Explanation 4

Key C — It passes the wanted band and rejects signals outside it, including the image frequency

Filtering before the mixer is the only way to remove the image and strong out-of-band signals, because once they have been converted they are indistinguishable from the wanted signal. This is why a high first IF, which places the image far away, makes the filter's job easier.

Rule Fundamentals - receiver front end

G5B05 Explanation 5

Key B — It attenuates harmonics generated by the non-linear final stage

A Class AB or Class C final generates harmonics of the operating frequency, and the output filter removes them so the transmitter meets the spurious emission limits. The matching network does the impedance transformation; the two functions are often performed by the same pi-network.

Rule 47 CFR §97.307(d) - harmonic suppression

G5B06 Explanation 6

Key A — 43 dB below the fundamental

Section 97.307(b) requires spurious emissions to be at least 43 dB below the mean power of the fundamental for transmitters below 250 watts, and 60 dB for higher power. This is why an amplifier with a poorly tuned output network is a rule violation and not merely a poor design.

Rule 47 CFR §97.307(b) - spurious emission standards

G5B07 Explanation 7

Key D — About 16 kHz

Carson's rule gives bandwidth as twice the sum of the deviation and the highest modulating frequency: 2 x (5 + 3) = 16 kHz. This matches the standard amateur 2 metre FM channel spacing of 15 to 20 kHz and the emission bandwidth designator used for FM phone.

Rule 47 CFR §97.307(f) - emission bandwidth

G5B08 Explanation 8

Key C — Intermodulation products increase, producing a distorted, splattering signal

ALC exists to keep the final below the point where non-linearity rises sharply. Pushing past it produces intermodulation products that spread either side of the signal and interfere with nearby stations, which is exactly what the requirement to use minimum necessary power is meant to prevent.

Rule 47 CFR §97.313(a) - minimum power necessary

G5B09 Explanation 9

Key B — How quickly the response falls away outside the passband

Shape factor compares the bandwidth at a wide attenuation point to the bandwidth at a narrow one, so a small number means steep skirts. Steep skirts are what let a receiver separate two signals that are close together, and what let a transmitter keep its energy inside the allowed bandwidth.

Rule Fundamentals - filter response

G5B10 Explanation 10

Key B — A stage in the transmitter is oscillating

An output with no input is the signature of self-oscillation, often caused by excessive feedback, a decoupling failure, or a stage being tuned outside its intended range. It must be fixed before operating: an oscillating transmitter puts energy wherever it likes, and the minimum-power requirement does not permit it.

Rule 47 CFR §97.307(b) and §97.313(a)

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.