Superheterodyne receiver
| Superheterodyne receiver | |
|---|---|
| Also called | Superhet |
| Invented | 1918, by Edwin Howard Armstrong |
| Key idea | Convert every incoming signal to one fixed intermediate frequency (IF) |
| Main stages | Front-end filter, mixer, local oscillator, IF filter and amplifier, detector |
| Design | |
| Image frequency | Twice the IF away from the wanted signal, on the far side of the local oscillator |
| Common IFs | 455 kHz, 9 MHz and 10.7 MHz; first IFs of several tens of megahertz in up-conversion radios |
| Variants | Single, double and triple conversion |
| Main alternative | Direct-sampling software-defined radio |
A superheterodyne receiver, usually shortened to superhet, converts every incoming signal to a single fixed frequency, called the intermediate frequency or IF, before amplifying, filtering and detecting it. Because the work of picking out one signal and rejecting its neighbors happens at a frequency that never changes, the filters and amplifiers can be optimized for it. Edwin Howard Armstrong developed the design in 1918, and for most of the following century nearly every broadcast radio, television set and amateur transceiver was a superhet. Many current radios still are.
How it works
A superhet passes the signal through a chain of stages:
- A front-end filter passes the band in use and rejects signals far from it.
- The mixer combines the incoming signal with the output of the local oscillator (LO), a tunable oscillator inside the radio. A mixer's output contains new signals at the sum and the difference of its two input frequencies.
- The IF filter passes only one of those products, and only as wide as the mode needs, such as about 2.4 kHz for SSB or 500 Hz for CW.
- The IF amplifier supplies most of the receiver's gain, under the control of the AGC.
- The detector recovers the audio: an envelope detector for AM, a product detector fed by a beat frequency oscillator (BFO) for SSB and CW, or a discriminator for FM.
Tuning a superhet means moving the LO so that the difference between it and the wanted signal always equals the IF: IF = LO frequency − signal frequency (or the reverse when the LO is below the signal). With a 9 MHz IF, receiving 14.200 MHz takes an LO of 23.200 MHz, because 23.200 − 14.200 = 9.000 MHz. The sum, 37.400 MHz, is discarded by the IF filter. A transmitter works the same way in reverse, generating SSB at the IF and mixing it up to the operating frequency.
The image frequency
For any LO setting, two different input frequencies produce the IF: the wanted one, and another the same distance away on the opposite side of the LO. The second is the image, and it lies twice the IF away from the wanted signal: image frequency = signal frequency ± (2 × IF).
Suppose a receiver with a 455 kHz IF is tuned to 14.200 MHz with its LO at 14.655 MHz. A signal on 15.110 MHz is also 455 kHz from the LO, so it too produces the IF. The two are only 910 kHz apart, too close for simple front-end filters to separate well. With a 9 MHz IF, the image of 14.200 MHz falls at 32.200 MHz, far enough away for a simple filter to remove. A receiver's image rejection, in decibels, measures how well it ignores the image.
Single and double conversion
A single-conversion receiver uses one mixer and one IF. Many classic SSB transceivers and homebrew designs used a single IF around 9 MHz.
A double-conversion receiver uses two mixers. The first converts to a high first IF for good image rejection, and the second converts to a lower second IF, often 455 kHz, where narrow filters are practical. Each extra mixer and oscillator adds the chance of internally generated signals, called birdies.
From the 1980s, many general-coverage HF radios used up-conversion, putting the first IF well above 30 MHz so that every image falls far above the HF spectrum. The weakness is that filters at such high frequencies cannot be made very narrow, so strong signals a few kilohertz away reach later stages, a problem covered under Receiver performance.
History
Armstrong devised the superheterodyne while serving with the US Army Signal Corps in France during World War I. The name comes from supersonic heterodyne, because the IF was above the range of hearing. Engineers in France and Germany worked on similar ideas at about the same time. Superhet broadcast receivers reached the market in the mid-1920s and were the standard design by the 1930s.
Compared with direct-sampling SDR
A direct-sampling software-defined radio skips the analog mixers. After a front-end filter, an analog-to-digital converter (ADC) digitizes the whole HF spectrum at once, and software does the rest. The superhet idea survives in the code, which multiplies the samples by a numerically generated oscillator, mathematically a mixer and LO.
Direct sampling has no image in the analog sense, but it has a close cousin, aliasing. An ADC must sample at more than twice the highest frequency it handles, and a signal above half the sampling rate folds back onto a lower frequency unless a low-pass filter removes it first. Seeing the whole band at once also makes a panadapter display easy.
Many current transceivers are hybrids: an analog superhet front end, often with a narrow roofing filter at the first IF, feeding an ADC that does the final filtering and detection. Either approach can make an excellent receiver; measured performance matters more than the label.