Superheterodyning is the technique of creating new frequencies from a set of Radio frequencies (RF) into Intermediate Frequencies (IF) for transmission. The system involves conversion of high-frequency signals into low-frequency signals. Superheterodyne architecture is commonly used in FM Radio and other communication systems.
How does Superheterodyne Receiver Works?
Superheterodyne architecture works on the principle of frequency mixing. A Frequency Mixer inputs Radio frequencies, from an antenna or any outside source, and frequency from a local source called a Local Oscillator and multiplies the two frequencies to get the Intermediate frequency. Later, the Intermediate Frequency is amplified and filtered for removing noise. The Intermediate frequency signal is then demodulated to recover the baseband signal.
Basic Components of Superheterodyne Architecture
The super heterodyning procedure involves multiple stages, some of the major stages and related components have been discussed below.
RF Amplifier and Filter
The first stage is amplification of the Radio Frequency input to the system. The amplifier aids in achieving a considerable Signal to Noise Ratio. If noise enters the system at the RF stage, it will be amplified later on by the rest of the circuit to prevent it low noise amplifiers are used.
The input RF signal is tuned to filter out any frequency except the Image Frequency of the system. It is essential that the filter keeps tracking the Local Oscillator so that when the Local Oscillator is tuned to a frequency, the RF Filter must stay on the required frequency for the mixer to correctly produce the IF signal. In Narrowband Radio Frequencies, the RF filter may have a fixed output frequency, in this case only the Local Oscillator frequency is changed.
The main task of the RF section is to pass f_RF±B_T/2, while rejecting the image frequency signal, with an RF Response of B_T< B_RF <2f_IF
Where B_T is the transmitted bandwidth and B_RF is the bandwidth of the RF Section.
Mixer and Local Oscillator
A Mixer performs frequency conversion by multiplying two frequencies. It uses RF and Local Oscillator frequency, to produce the Intermediate frequency (IF).
When two signals of different frequencies are multiplied, the result is both the sum and the difference of the frequencies that are the Intermediate Frequencies. Mathematically,
Sin(w_1 t)× Sin(w_2 t)= 1/2 [Cos(w_1-w_2 )t- Cos(w_1+w_2 )t]
If the Intermediate frequency (IF) is lower than the input RF (f_IF=f_RF-f_LO) then the intermediate frequency is called Low-Side Injection and vice versa, If the Intermediate frequency (IF) is higher than the input RF (f_IF=f_LO+f_RF) the intermediate frequency it is known as High-Side injection.
In Superheterodyne receiver, mixer lowers the frequency, while in case of transmission it boosts the frequency.
Mixers process both frequencies, making use of both side-bands, these are known as Double Side Band Mixers. If only one Intermediate Frequency is used then it needs filtering, these are called Single-Sideband Mixers. Single Sideband Mixers are sub-divided into High side mixer and Low Side Mixer, depending upon which frequency is being used.
Mixing and Image Frequency
Two output frequencies are generated at the mixing stage. These frequencies are mirrored about Local Oscillator frequency f_LO and hence call Image Frequency.
Consider a mixer with an Intermediate Frequency output requirement of 2Mhz, an input RF of 6Mhz and Local Oscillator frequency of 4Mhz. The Mixer processes 10Mhz and 2Mhz IF as outputs. However, if noise or unwanted signal Radio Frequency of 2Mhz enters the mixer, the sum will be 6Mhz and a difference of 2Mhz, which is the same as the required Intermediate Frequency. This unwanted frequency is called the image frequency.
In order to filter out the image frequency signals, the RF Filter and the Local Oscillator must be tuned
IF Filter and Amplifier
The IF filter is generally a Band Pass filter which allows only a specific frequency range to pass through. If there are any frequencies other than the allowed band of the filter, they are not allowed to pass through. It is essential because if an unwanted frequency from the adjacent channel enters the Intermediate Frequency filter, it will be transmitted along with intermediate frequency. The response of the filter must also be flat within the frequency range.
The Intermediate Frequency filter does not need to be tuned, it is tuned in the required center Intermediate frequency. If the received Radio signal’s frequency changes then it can be bought down to the Intermediate Frequency by tuning of the RF Filter and the Local Oscillator.
In case, if the Intermediate Frequency signal is weak, it is boosted with an Intermediate Frequency amplifier.
Demodulator recovers the original Baseband signal from the Intermediate Frequency signal. The demodulation may be different depending on the modulation.
In case, if the demodulated signal is attenuated, a baseband signal amplifier can be used.
Advantages of Superheterodyne Receiver
Low Cost Design With Variety of Supported Parts
Many Reduction of higher frequency sources is essential as several analog and digital components do not operate at high frequencies (GHz) hence the system design and development is expensive. Superheterodyne enables designers to design systems with comparatively lower frequencies and a vast range of components
Constant Gain With Wide Band
Designing a communication system with a wide band of frequency ranges with almost constant gain is another challenge. Superheterodyne architecture offers a solution to this problem by bringing down or upscaling the frequency to a desired Intermediate Frequency which is fixed throughout the system.
Majority of the components can be optimized at the Intermediate Frequency without the need of covering a wide range of frequencies.
Supports Arithmetic Selectivity
Superheterodyne Architecture supports Arithmetic Selectivity. Consider a Superheterodyne Communication System with RF signal of 850 KHz, fixed IF of 452 KHz and a Local oscillator of 398 KHz. With an RF filter bandwidth of 2%, means 833 KHz to 867 KHz will pass through the filter. If a noise signal of 863 KHz also enters the filter, then the mixer will create a 465 KHz interference signal. But the IF Filter is also tuned at 2%, meaning it will pass 443 to 461 KHz hence suppressing the interference frequency. The system becomes more selective as 2% of the Intermediate Frequency is less than the greater RF.
Disadvantages of Superheterodyne Receiver
Superhets are prone to Image Frequency with respect to spurious responses. Any distortion in the Local Oscillator generates harmonics that combine with spurious signals and pass through the Intermediate Frequency. This is why the Local Oscillator must be a pure sine wave without any noise