advantages of superheterodyne receiver

advantages of superheterodyne receiver

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advantages of superheterodyne receiver

Following are the benefits or advantages of superheterodyne Receiver and heterodyne receiver architecture types:
➨As it converts high frequency to low frequency, all processing takes place at lower frequencies. The devices are cheaper at such lower frequencies compare to higher frequencies.
➨It is easy to filter IF signal compare to RF signal.
➨It offers better sensitivity compare to homodyne receiver architecture.
➨Heterodyne uses single conversion and Superheterodyne uses double conversion. The Superheterodyne receiver prevents image noise foldover due to use of two IF frequencies before conversion to baseband.

1. The selectivity of this receiver is better since its IF amplifiers are narrow band, and the operate only at IF.
2. The design of IF amplifiers is relatively simple since they operate only at IF.

IF Requirement

Since IF is used in superheterodyne receiver, the IF amplifiers have to work only at one frequency. Hence their design is relatively simple. Adjacent channel selectivity, image frequency rejection, fidelity, selectivity etc performance parameters depend upon IF amplifiers. Since IF amplifiers work only at IF, their performance is improved. And IF is selected based on following factors.

1. IF should not fall in the tuning range of the receiver.
2. IF should be sufficiently high so that local oscillator does not have stability problems.
3. And IF should not be very low, because image frequency rejection is poor at low frequencies.
4. A high value of IF increases tracking difficulties.

Considering these points, IF is selected within the range of 438 to 465 kHz. Normally 465 kHz is most commonly used IF.

Superheterodyne Receiver

a radio receiver in which demodulation of an incoming signal is preceded by the conversion (lowering) of the signal’s carrier frequency without the modulation being changed. The superheterodyne receiver is the most common type of radio receiver. It has a comparatively simple and reliable design and provides high-quality signal reception. The superheterodyne circuit was invented in 1918 by E. Armstrong of the USA and L. Lévy of France.

Explanation: The main advantage of using high frequency signals is that the signal gets transmitted over very long distances and thus dissipates very less power. The antenna height required for transmission also gets reduced at high frequencies. And also it allows less noise interference and enables multiplexing.

History of the superheterodyne receiver

The superheterodyne receiver is still the most popular microwave receiver, and it was invented during and directly after the Great War and patented in 1918. Edwin Armstrong (see him in our Hall of Fame) was truly one of the great minds of the twentieth century. Unfortunately, he had a bad habit of inventing stuff independently of the corporate anti-Christ of the time, which was RCA (Radio Corporation of America); RCA and others pretty much ruined Armstrong’s life.

From this perspective, nothing’s changed, big companies ruin many engineers’ lives today, especially in arguments over inventions. As an engineer you’d best remember your place at the bottom of the corporate food-chain or you’ll be living in a cardboard house behind the Circle K pretty soon. Don’t ever think your services are as valuable as the work of more deserving corporate lawyers! But we digress….

The word “heterodyne” was coined by Reginald Aubrey Fessenden, also in our Hall of Fame. Heterodyning is the same as mixing, we offer a little history on the topic here. Although Fessenden was able to receive signals by mixing them, there is no evidence that he used a detector to sort out the baseband; therefore Fessenden only managed to put together just one of the five pieces of Armstrong’s superhet receiver (RF amp, mixer, local oscillator, detector and audio amp). Order “Man of Fidelity: Edwin Howard Armstrong” by Lawrence Lessing on our book page to corroborate that fact.

History

Prior to the superhet receiver, radio listeners had to constantly play with a set of knobs on a radio to keep locked on to radio programs of the day. Prior attempts at amplifying radio signals included the regenerative and super-regenerative receivers (both invented by Armstrong, but stolen by greedy RCA), which at least offered some gain to boost far-away signals, but were prone to drift.

The superhet solved this problem for all time by using frequency conversion that was controlled by a local oscillator. By tuning the LO frequency, the RF frequency that you received was thereby tuned. It turns out it is much easier to control the LO frequency and filter out unwanted channels at IF, rather than try to filter out unwanted channels at the RF input. This characteristic is known as receiver selectivity.

Advantages of Using Superheterodyning

Now, we easily see that this type of receiver can be constructed, but for what purpose? All we have accomplished is to reduce the frequency to the IF value. We still must process the signal as before. So why are so many receivers using the superheterodyne method? There are three main advantages, depending on the application used for:

• It reduces the signal from very high frequency sources where ordinary components wouldn’t work (like in a radar receiver).
• And allows many components to operate at a fixed frequency (IF section) and therefore they can be optimized or made more inexpensively.
• It can be used to improve signal isolation by arithmetic selectivity

Reduction in frequency

AT very extremely high frequencies, many ordinary components cease to function. Although we see many computer systems that work at previously unattainable frequencies like 166 MHz, you certainly never see any system that works at radar frequencies like 10 GHz (try that Intel!). There are many physical reasons for this, but suffice it to say, it can’t be done (yet). So the designer of a radar interceptor (fuzz-buster, et al.) is faced with a daunting circumstance unless he/she can use a superheterodyne receiver to knock down the frequency to an IF value. It is in fact, the local oscillator (a operating at radar frequencies) of the superheterodyne radar receiver that makes your radar detector detectable by the police (in VA for example, where the use of radar detectors are illegal).

Optimization of Components

It is a typical engineering dilemma: how to make components that have outstanding performance, but can also cover a wide range of frequencies. Again, the details aren’t important, but the problem is very real. A possible solution to this, is to make as much of the receiver as possible always work at the same frequency (the IF). This is accomplished by using the superheterodyne method. The majority of components can be optimized to work at the IF without any requirements to cover a wide range of frequencies.

Arithmetic Selectivity

The ability to isolate signals, or reject unwanted ones, is a function of the receiver bandwidth. For example, the band-pass filter in the tuner is what isolates the desired signal from the adjacent ones. In real life, there are frequently sources that can interfere with your signal. The FCC makes frequency assignments that generally prevent this. Depending on the application, you might have a need for very narrow signal isolation. If the performance of your band-pass filter isn’t sufficient to accomplish this, the performance can be improve by superheterodyning.

Frequently, the receiver bandwidth is some fraction of the carrier frequency. If your receiver has a bandwidth of 2 % and you are tuned to 850 kHz, then only signals within the range from 2 % above and below are passed. In this case, that would be from 833 to 867 kHz.

Arithmetic selectivity takes that fraction and applies it to the reduced frequency (the IF). For the fixed IF of 452 kHz, that means signals which are superheterodyned to the range of 443 to 461 kHz will pass. Taking this range back up into the carrier band, only carrier frequencies in the range of 841 to 859 kHz will pass. If this is confusing, recall that the local oscillator is set to reduce the 850 kHz to 452 kHz (i.e. must be set at 398 kHz). Thus, the 850 kHz is superheterodyned to 452 kHz. Any adjacent signals are also superheterodyned but remain the same above or below the original signal.

Where are superheterodyne receivers used?

6.3 Superheterodyne receiver. The superheterodyne receiver is the most common configuration for radio communication. Its basic principle of operation is the translation of all received channels to an intermediate frequency (IF) band where the weak input signal is amplified before being applied to a detector.

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