wifimixerUpdated
HOW RECEIVERS TUNE
Your radio makes a signal of its own.
Inside most receivers, a mixer combines the incoming station with a signal from a local oscillator. Out comes one fixed, lower frequency that is easy to amplify and filter. Turning the dial just moves the oscillator.
A − B the difference wins
Tune a station
THE SHORT ANSWER
The trick is called heterodyning:
mixing two frequencies to make a new one.
MIXING
Mixing two frequencies produces their sum and their difference. Receivers usually keep the difference: the intermediate frequency.
Tune a stationONE KNOB
Every station is shifted to the same intermediate frequency, so the filters and amplifiers stay fixed. The superheterodyne could be tuned with a single knob.
See the numbersEVERYWHERE
The superheterodyne is used for AM, FM and TV broadcasting, mobile and satellite systems, radar and GPS.
Where it came fromTUNE A STATION
A textbook example: an AM receiver tuned to a station at 850 kHz, with an intermediate frequency of 455 kHz. Pick a part to see its frequency. The bar shows how high it is.
Worked example from University of Ottawa lecture notes (ELG3175), after Couch, Digital and Analog Communication Systems.
TWO NUMBERS WORTH KNOWING
One number on amplification,
one on a classic AM design.
up to 10⁶×
How much the IF stage can boost the signal
Because it always works at the same frequency, the IF amplifier can amplify the signal up to a million times and reject signals on neighbouring channels.
IF AMPLIFIER · UOTTAWA
455 kHz
Intermediate frequency in the textbook AM example
With a 455 kHz IF, a station at 850 kHz needs the oscillator at 1,305 kHz, and the unwanted image sits at 1,760 kHz.
AM RECEIVER · UOTTAWA
The first superheterodynes used much lower intermediate frequencies, between 30 and 90 kHz, because the valves of the day could not amplify higher frequencies well.
INSIDE THE RECEIVER
The main stages of a superheterodyne receiver, in the order the signal passes through them.
Boosts the weak signal from the aerial and rejects the image frequency.
Shift the chosen station down to the intermediate frequency. The oscillator does the tuning.
Amplifies the fixed-frequency signal and filters out neighbouring stations.
Recovers the sound, pictures or data. Automatic gain control keeps the level steady.
WHO INVENTED IT
Reginald Fessenden applied the heterodyne principle to wireless in 1901 and coined the word from the Greek for 'other' and 'force'. Lacking a good oscillator, he could not make it practical.
Lucien Lévy filed a French patent in 1917, Walter Schottky a German one in June 1918, and Edwin Armstrong a US application on 30 December 1918. Superheterodyne radios went on sale in the US in March 1924 and soon became the standard circuit.
Questions
What does a mixer do in a radio?
Together with the local oscillator, it shifts the incoming radio signal to the intermediate frequency. Mixing two frequencies produces their sum and difference, and the difference is usually the useful part.
Why not just amplify the station directly?
Early valves amplified high frequencies poorly. Shifting signals to a lower, fixed frequency made strong amplification and sharp filtering practical, and the design could boost weak signals significantly.
What is image interference?
A second station on the other side of the oscillator, twice the IF away from the wanted one, also mixes down to the IF. In one example a receiver tuned to 800 kHz with a 45 kHz IF would also hear a station at 890 kHz.
Does the mixer change the programme?
No. The signal and its sidebands are shifted in frequency but not otherwise altered, so the modulation carrying the sound is not affected.
Is the superheterodyne still used?
Yes. It is used across broadcasting, mobile and satellite systems, radar and GPS. A variant called zero IF, or direct conversion, mixes signals straight down to baseband.
ABOUT THIS NAME
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