Why receiver secondary frequency (intermediate frequency) the significance of

What is a superheterodyne receiver?

The original radio Belong to direct put the radio, its characteristic is, receives the high frequency signal from the antenna, before detection, has not changed its original high frequency (that is, the high frequency signal amplification) directly.Its disadvantage is that in the high end of the receiving frequency and low magnification, the sensitivity of the whole band is not uniform.If it is a multi-band radio, this contradiction is more outstanding.Second, if you want to improve the sensitivity, it is necessary to increase the series high frequency amplifier, leading to all levels of the unity between the tuning of the difficulties, and high frequency amplifier gain do not high, easy to produce self-excited.  If you can receive the radio to the high frequency signal, are changed into fixed intermediate frequency signal amplification detection.

Due to low intermediate frequency than before the transform of signal frequency, and frequency is fixed, so any radio signal can be equal to put a lot of, the total amount at the same time can also be higher.Thus overcome the contradictions.

Typical superheterodyne receiver block diagram can be seen, oscillator to produce an always higher than receiving signals an intermediate frequency oscillation signal, using transistor in the mixer nonlinear oscillation signal and received signal subtraction to create a new frequency, intermediate frequency that is “heterodyne”.


 The advantages of superheterodyne receiver: 

1. Because after frequency conversion for fixed frequency, the frequency is low, easy to get a large amount, so the sensitivity of the radio can be doneVery high.

2. As the high frequency signal is turned into a fixed frequency, so it easy to solve the problem of uneven different radio signal amplification.

3. As a result of the difference frequency effect, external signals must be and oscillation are intermediate frequency can enter circuit for reservation, and resonance frequency selective circuit, intermediate frequency amplifier circuit is a good filter, other interfering signal is suppressed, thus improve the selectivity.

The main disadvantage of superheterodyne receiver:But superheterodyne circuit also has shortcomings, there may be the mirror frequency interference and intermediate frequency interference, the two interference is unique to the superheterodyne interference.  Superheterodyne frequency selective, the radio to 455 KHZ of intermediate frequency signal anti-jamming ability.

Due to the resonance frequency of the input circuit is higher than 455 KHZ, so the input circuit has larger inhibition on intermediate frequency interference.

According to the principle of superheterodyne frequency, when the difference between foreign signal frequency oscillation frequency is an intermediate frequency (455 KHZ), the signal can be smoothly through the intermediate frequency amplifier gain amplifier, formulization: f vibration – f = f in the letter, this is the signal frequency is lower than the oscillation frequency.

If the external signal frequency than a medium frequency oscillation frequency is high, and how?The gap between their f – f = f of the lens, their difference is intermediate frequency, intermediate frequency amplifier also can smoothly to let them by getting enlarged.Two type additive available f = f lens letter + 2 f, the diagram below: | – 455 KHZ – | — – | 455 KHZF the letter f f lens of vibration  Or for a specific receive frequency its mirror frequency is the frequency and 2 times of the intermediate frequency, just like for vibration f lens, f lens is f letter, like in the mirror.

Secondary frequency purpose: to improve the illusion mirror frequency suppression as frequency resistance ratio and improve the sensitivity

In short wave band, in order to make the input circuit to keep more evenly across the spectrum, the sensitivity of the resonance peak usually make them wide (namely, poor selectivity, the whole machine mainly by intermediate frequency selective circuit to guarantee), 15480 kHz, for example (intermediate frequency is 455 kHz) only, its mirror frequency is 16390 kHz, quite close to the frequency of the received, if there is a radio in 16390 kHz is easily mixed with frequency conversion circuit and become the mirror frequency interference, the ordinary radio frequency selective medium wave mirror > 20 db, and the short wave frequency selective > only 8 db, so less shortwave radio, especially only one or two short wave radio, 15480 kHz at 15480 kHz minus 14570 kHz is easy to receive 15480 kHz.But with the improvement of sensitivity, other 16390 KHZ weak radio signal will be received in 15480 KHZ, produce the noise and interference, which hinder the improvement of the radio sensitivity.Solution has two, one, increase the frequency high frequency amplifier tuned circuits, in front of the sameSample will bring unity tuning difficulty, as well as the sensitivity is not uniformity.Second, is now widely used in the secondary inverter.  What is second frequency conversion: By the second frequency conversion is the first radio frequency signal to the first intermediate frequency 10.7 MHz (e.g., 9702), then the first intermediate frequency by a second frequency conversion transformation to usually second intermediate frequency of 455 KHZ.

Mirror frequency rejection capability, and the first intermediate frequency inverter series and has a very complex mathematical relations, increase the first intermediate frequency inverter series and the use of high frequency to improve image frequency suppression.  In fact only from the point of view of principle also can understand, also in 15480 kHz, for example, the first intermediate frequency is 10.7 MHz, so the vibration frequency of 26180 kHz, mirror frequency is 36880 kHz, with 15480 kHzFrom far away (as opposed to a much greater a heterodyne), even 4 band of short-wave reception range II in 7 ~ 22 MHz up 14 MHz section also is bad, should be suppressed.

Due to improve the mirror frequency suppression, you can rest assured the use of a variety of means to improve sensitivity.Some professional receiver even have four frequency conversion.

Increased frequency series will greatly improve the cost, and the resulting performance improvement is not proportional to the, like HiFi, in order to finally improve sound quality, it takes almost all inputs before, therefore the secondary inverter using most.  Interference, of course, there are various, if is the same frequency interference, circuit of any technique is of no help, in addition to using a directional antenna or almost depend on the spread of the jamming signal.

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