Ever questioned why the intermediate frequency (IF) in radios is mounted at 455kHz? The idea of IF isn’t solely fascinating however extremely related until date. To actually grasp how wi-fi communication has developed and the place it’s headed, understanding the significance of IF is important.
Intermediate frequency (IF) is an important idea within the subject of electronics and communications, particularly within the design and operation of radio, tv, and radar methods. Allow us to attempt to demystify the idea of intermediate frequency and see its use within the family AM radios.
Whereas the radio itself is an age-old idea, the idea of IF is a crucial subject in electronics and telecommunication engineering. So allow us to additionally see why we’d like the IF or how it’s useful within the context of creating a great radio receiver. It’s fascinating to see why the IF is mounted at 455kHz and never every other frequency in our AM radios! Although it’s not a complete doc regarding every little thing about radio engineering, it makes an attempt to function a great introduction to the basics of the IF idea.
In concept, an audio wave can vary from 20Hz to twenty,000Hz. For AM (amplitude modulation) primarily based radio communication, a service wave is required to transmit the audio wave. On the radio transmitter aspect, we take the audio wave, which has a altering frequency and amplitude, and modify the amplitude of the service wave in response to the audio wave, whereas preserving the frequency of the service wave unchanged. The ensuing output, or the modified service wave, is known as a modulated wave. Notice that solely the amplitude of the service is modified, therefore the title amplitude modulation (AM). Primarily, the modulated wave is then despatched to the antenna for AM broadcasting.
| How IF works in radio receivers |
| In a typical radio receiver, the incoming radio frequency (RF) sign is combined with a sign from a neighborhood oscillator. This mixing course of generates two new frequencies: the sum and distinction of the unique frequencies. The distinction frequency is often the intermediate frequency (IF). By changing the unique RF sign to this IF, it turns into simpler to course of – filter, amplify, and demodulate. |
Why do we’d like a service wave? It’s a good query. Regardless that it’s not very sensible, allow us to simply think about that we have been making an attempt to ship the audio sign straight for AM broadcasting. How can we then create a circuit that can decide up the radio sign from a specific radio station if there are dozens of them in an space? If all of the AM radio stations in a metropolis broadcast the audio straight, we won’t be able to differentiate them from one another, as a result of all can be transmitting wherever and in every single place between 20Hz and 20,000Hz! That might be like everybody in a room filled with 100 individuals shouting at one another within the loudest means and nobody with the ability to hear clearly what every other individual was saying.
To forestall this situation, it was thought within the early days that each AM radio station must be given a set frequency to transmit in order that any receiver tuning to that frequency would have the ability to hear solely that station. The frequency on which a specific radio station is transmitting is known as the service frequency of that station.
A radio station simply sending a service wave of a relentless amplitude and frequency is of no use as a result of the receiver won’t get any audio out of it. Keep in mind the modulation? Primarily, modifying the amplitude of such service wave with an audio sign after which sending that from the radio station will make it helpful (see Fig. 2). Sure, it’s nonetheless on the frequency mounted for the radio station as we modified solely the amplitude and never the frequency. Thus, we might ship the audio sign by the use of an amplitude change of the service wave to the receiving radios tuned to that service frequency.
For Europe, Africa, and Asia, the medium wave AM band consists of service frequencies from 531kHz to 1602kHz, with every station separated by 9kHz. (For a deeper take a look at the evolution of radio receivers, see the article titled ‘The Transformation of Radio Receivers’ at https://www.electronicsforu.com/market-verticals/radio-receivers-transformation hyperlink to grasp how the radio receivers developed.)
Challenges in AM reception
As we now know, every broadcasting station transmits its amplitude modulated service wave at its personal specified frequency, as mounted by the involved authorities authorities and laws.
That is good. Nevertheless, we nonetheless have an issue.
We now have to have a radio receiver designed with an amplifier for amplifying the acquired service wave. That can be not an issue. The actual drawback is designing an amplifier that can amplify such excessive frequency, however the response of the amplifier must also be good for a complete vary of doable service frequencies protecting all of the radio stations between 531kHz and 1602kHz!
Having an amplifier that may work nicely with such excessive frequencies and for the complete vary of frequencies (of varied service waves) with identical high quality may be very tough.
Then how about having many amplifiers, every designed for numerous service frequencies? That will work, however it could make the fee and dimension of such a receiver very excessive. Keep in mind, even such an answer calls for all of the amplifiers working for top frequencies as nicely. Think about how tough it might have been in these early days of AM broadcasting and reception the place realising excessive frequency circuits was nonetheless a problem. No surprise, the idea of IF emerged then.
The idea of IF
So, what’s the resolution for the restrictions described above? Properly, how about creating an amplifier that’s nicely fitted to a single frequency and changing the acquired modulated service wave of any station to the frequency to which the amplifier is nicely fitted to? It’s like, allow us to say we’ve got an amplifier designed to amplify a sign at 100kHz, and we convert the modulated wave acquired from 1200kHz to 100kHz in order that the amplifier can fortunately amplify it. Equally, if we have to hear the radio station transmitting the modulated service sign at 1400kHz, we merely convert the acquired modulated wave of 1400kHz to 100kHz after which ship it to the amplifier that’s designed to work nicely at 100kHz. So now we’re changing any modulated service frequency of any station in the complete AM frequency vary to a set 100kHz, after which such a sign at 100kHz is amplified, after which audio wave is extracted from that 100kHz. This frequency, that’s, 100kHz on this instance, is known as the intermediate frequency or IF.
How IF is achieved
Can it’s achieved? It seems that we will! How can we try this?
Allow us to see a good higher instance.
Think about a radio transmitter or broadcasting station sending the modulated service wave at 300kHz. Allow us to additionally think about that, on the receiver aspect, we’ve got an oscillator, and allow us to name this oscillator as ‘native oscillator’ or just LO (see Fig. 1). Assume the LO would produce any frequency we would like. Keep in mind, the LO’s output needn’t be an amplified sign. Because the acquired modulated service sign, which we’re going to combine it with, is weak anyway, and thus the weak output of LO is ok for our work right here.
Allow us to additionally think about we’ve got a sign mixer that can produce the addition and subtraction of the 2 indicators we offer. Allow us to say the acquired modulated service frequency is f1 and output of the native oscillator is f2. The mixer will take the sign of frequency f1 and sign of frequency f2, then it would produce addition and subtraction of f1 and f2 and produce its output as f2+f1, and f2-f1.
We are able to see an instance with numerous values of f1 and f2 in Desk 1.
| Desk 1: Varied values of f1 and f2 | |||
| Provider (f1) | LO (f2) | Mixer (f2+f1) | Mixer (f2-f1) |
| 600 | 700 | 1300 | 100 |
| 800 | 900 | 1700 | 100 |
| 1300 | 1400 | 2700 | 100 |
| 1400 | 1500 | 2900 | 100 |
| 1559 | 1659 | 3218 | 100 |
| 1600 | 1700 | 3300 | 100 |
Ensuing outputs f2+f1 and f2-f1 of the mixer are results of each f1 and f2, and therefore each f2+f1 and f2-f1 comprise the knowledge of the modulated service wave f1 and LO wave f2.
As you may see, the f2+f1 retains altering relying on the f1 and f2, whereas the f2-f1 is at all times fixed! That’s, it stays 100kHz on this instance. It means, if we apply a filter on the output of the mixer to pick solely f2-f1 (that’s 100kHz on this instance) and filter out all different frequencies, together with f2+f1, then we’ll find yourself solely with f2-f1 (with out the f2+f1 content material).
In different phrases, to extract any specific station from a bunch of stations, all we have to do is change the LO frequency (f2) such that the f2-f1 is 100kHz!
Thus, for any f1 (of station of our curiosity), we simply want to regulate native oscillator to supply a frequency f2 in such a means that f2-f1 will end in 100kHz. For instance, if we need to hear the radio station Akashvani Mumbai, which is transmitting at 558kHz (allow us to take it as f1), all we now have to do is simply alter the native oscillator to supply 658kHz (allow us to take it as f2), in order that the mixer will produce f2+f1 and f2-f1. Thereafter, when the mixer output is handed via a filter to permit solely f2-f1, we’ll get f2-f1, which is at 100kHz. The Vital factor to notice right here is, that the 100kHz is the intermediate frequency or IF. When this IF (see Fig. 3) is shipped to an envelope detector or demodulator (see Fig. 4), it would take away the service wave and provides the unique audio (baseband sign) out. This audio sign might be additional amplified suitably and heard via a loudspeaker.
Did you discover the benefit of the IF? As you may see, we simply want an audio amplifier that can amplify solely the audio sign. One also can have an amplifier earlier than the envelope detector in order that the mixer output is amplified a bit extra earlier than sending it to the envelope detector, if wanted. The principle benefit of the IF is, we will convert the incoming service frequency (f1) to a smaller frequency (f2-f1), which is the IF, and it turns into lot extra handy to design all of the circuitry after the mixer to work with a set frequency.
Why IF is mounted to 455kHz in AM radio
There’s a very fascinating background for this. Following are some causes for selecting the precise worth of 455kHz for IF:
1. Preserve it exterior the AM band
It was essential to make sure that any radio transmission was not precisely like IF. That’s, the IF must be exterior the AM band of 531kHz to 1602kHz. Which means the IF must be both decrease than 531kHz or increased than 1602kHz.
2. Be as much less as doable
Decrease frequency was higher as we didn’t have to make use of the costly circuitry similar to filters and IF amplifiers for working at excessive frequencies within the olden days.
When lively filters have been used, constructed utilizing vacuum tubes or transistors, they often had increased beneficial properties at decrease frequencies. So fewer amplifier levels have been required if the frequencies have been decrease resulting from excessive beneficial properties on the decrease frequencies. Thus, the selection of getting IF above the AM band (i.e., above 1602kHz) was dominated out.
3. Be as excessive as doable.
The upper IF offers a adequate bandwidth to accommodate the audio indicators with out distortion, guaranteeing clear and high-quality sound replica. After all, it ought to nonetheless be decrease than the bottom of the AM band (531kHz).
Additional, the upper the IF, increased the hole between tuned frequency (f2-f1) and the picture frequency. This increased hole helps in filtering out the picture frequency extra simply. (We’ll talk about the picture frequency quickly.)
| Benefits of IF |
| Whereas IF isn’t obligatory, listed below are some benefits of utilizing it:
Constant efficiency. Because the IF is mounted, whatever the precise broadcast frequency, the filters and amplifiers might be optimised for this particular frequency, guaranteeing constant efficiency. Simplified tuning. It permits using a single, fixed-frequency filter and amplifier levels, simplifying the design and tuning of the receiver. Within the early days, excessive frequency and wide-band amplifiers have been much more costly than they’re now. Improved selectivity and sensitivity. With a steady IF, the receiver can extra successfully choose the specified sign and reject others, bettering each selectivity and sensitivity. |
4. Not be matched by the distinction of any two channels.
Two robust stations whose frequencies occur to be separated by the IF might produce IF. This undesirable IF generated from such interference will get via filters, that are designed to permit IF to the subsequent stage, similar to detector. In different phrases, the IF shouldn’t be 10kHz or 9kHz, and even the multiples of 10kHz or 9kHz. Which means any quantity that’s divisible by 10 or 9 is dominated out.
Taking all these into consideration, many frequencies have been fashionable decisions throughout World Battle I timeframe, in early 1900’s.
As we will see, coping with increased frequencies, even within the vary of some kilohertz (kHz), was difficult in these early days resulting from unavailability of appropriate elements. This resulted in radio makers working with decrease frequencies for IF. Nevertheless, this resulted in poor audio high quality when the audio was extracted from very low IF, similar to 30 or 35kHz. Then the competitors started to construct higher methods with increased IF to generate higher audio qualities in radios.
Many radio producers began making radios in these days with increased and better IF, similar to 100, 155, 300, 455, 500, and even 700kHz! Whereas increased IF produced higher audio high quality, it clearly resulted in a better price.
After World Battle I got here the Nice Melancholy (1929-1939), simply earlier than the World Battle II. It was a interval of financial melancholy in the USA. Naturally, demand for low-cost methods, elements, and interchangeable components within the radios went up. The 455kHz appeared like a sensible choice contemplating the good-quality audio it might produce and the decrease price for elements in comparison with different increased IF decisions. So, then 455kHz was adopted as a typical by the business.
| What’s Heterodyning |
| The method of blending two indicators, i.e., modulated service wave (f1) and the sign generated by the native oscillator (f2), is known as heterodyning. Additionally referred to as frequency conversion and beat frequency technology, it is rather extensively utilized in communications engineering to generate new frequencies and transfer info from one frequency channel to a different.
The ensuing output of the mixer is 2 indicators: addition and subtraction of the frequencies of the 2 enter indicators (f1 and f2), one the sum of the 2 frequencies (f1+f2), and the opposite the distinction of the 2 frequencies (f1–f2). The brand new sign frequencies are referred to as ‘heterodynes.’ Sometimes, solely one of many heterodynes is required and the opposite sign is filtered out. The phrase heterodyne is coined from the Greek language ‘hetero’ (totally different) and ‘dyn’ (energy). The heterodyning was first developed in early 1900’s. Tremendous heterodyning or superhet. It’s principally identical as heterodyne, however all of the indicators concerned, extra notably the IF, are past the audible vary to stop any interference with the audio sign (increased than 20,000Hz or 20kHz), therefore the title ‘supersonic heterodyning’, which is usually referred as ‘tremendous heterodyning’ or ‘superhet.’ |
Picture frequency
Allow us to shortly perceive what picture frequency is, and why and the way it must be eradicated.
Allow us to say an AM radio receiver’s IF is 455kHz. Suppose we need to tune to a radio station broadcasting at 610kHz. Which means we have to get the native oscillator (LO) to supply 1065kHz in order that once we ship LO output of 1065kHz and broadcast frequency of 610kHz to a mixer, the mixer produces the distinction of 1065kHz and 610kHz as 455kHz, which is our desired IF.
We have now an issue right here.
If there may be one other radio station broadcasting at 1520kHz, then the identical mixer with its native oscillator supplying 1065kHz will produce the distinction of 1065kHz and 1520kHz as 455kHz, which is identical as 455kHz produced in earlier case!
| Typical IF values |
| The worth of the intermediate frequency varies primarily based on the appliance. For instance, in commonplace AM radios, it’s sometimes round 455kHz, whereas in FM radios, it’s round 10.7MHz. In tv receivers, the IF is perhaps round 38MHz for video and 33.4MHz for audio. |
This implies two stations shall be extracted on the identical time on 455kHz, one from 610kHz and one other from 1520kHz.
On this case 1520kHz sign is called ‘picture of the undesirable sign frequency,’ and plenty of occasions it’s merely known as ‘picture frequency.’
This picture frequency must be prevented from coming into the mixer.
A technique to do this is having a tuning circuit to pick the incoming RF indicators. The tuning circuit might use a variable capacitor (Fig. 5) to pick a desired frequency. Nevertheless, one should manually change the variable capacitor liable for frequency choice on the RF front-end in response to the frequency of the LO. That’s cumbersome.
Subsequently, we’d like a mechanism the place the capacitor of the tuning circuit ought to robotically be modified, thereby permitting a specific RF sign of our desired frequency in relative of the frequency produced by the LO. Subsequently, many methods have twin gang variable tuning capacitors, which include two separate variable capacitors whose worth might be diversified collectively (not independently) mechanically (see Fig. 6). Thus, one capacitor is utilized in RF tank circuit on the RF front-end, and one other is used to manage the frequency of the native oscillator (LO). When modified, capacitance of each the capacitors is modified concurrently.
Thus, when the RF front-end circuit is tuned for choosing the 610kHz with the assistance of 1 variable tuning capacitor, for instance, the native oscillator (LO) will robotically (being mechanically coupled) produce 1065kHz with the assistance of the second variable tuning capacitor. Because the RF front-end is tuned to select 610kHz, it’s not selecting different frequencies, together with the undesirable picture frequency 1520kHz!
Thus the picture frequency has no probability to enter the mixer.
| Picture frequency calculation |
| It’s nicely accepted that: Picture frequency = Receiving RF frequency + (2 x intermediate frequency) However how? Allow us to say we’ve got acquired an RF broadcasting of an AM sign at frequency R1 with native oscillator frequency L. Which means: IF=L–R1 Subsequently, L=R1+IF The picture frequency is the undesirable picture of the sign transmitted at frequency R2. When this R2 is combined within the mixer with the oscillator frequency L, this must also produce IF. IF=R2-L Subsequently, L=R2-IF Thus, R1+IF=R2-IF Fixing for R2 (picture frequency), we get, R2=R1+2 (IF) |
Future designs
Fashionable microprocessors permit constructing of a software-defined radio (SDR), also referred to as software program radio structure, the place the IF processing after the preliminary IF filter is carried out in software program. (See https://www.electronicsforu.com/electronics-projects/software-defined-radio-with-android-smartphones.)
Many low-cost FM radios have now integrated the SDR structure. With larger developments within the capabilities of the analogue-to-digital converters and the digital sign processing (DSP) capabilities of microprocessors or the devoted DSP processors, the incoming modulated service sign is usually straight sampled as an alternative of first getting transformed into the IF after which sampled.
Whatever the developments, the journey and the idea of IF and the picture frequency are fascinating. They’re elementary and related even in our fashionable telecommunications, each in audio and video.
Janardhana Swamy served as a Member of Parliament in fifteenth Lok Sabha (2009-2014). He holds an MSEE diploma from IISc, Bengaluru, and has labored in India and the US in numerous engineering and administration positions at Sasken, Cadence, Solar Microsystems, Dell, and Cisco Techniques. Swamy believes a great training in India may help the nation in constructing a stronger knowledge-based society and economic system
Neha JS is pursuing Bachelor’s Diploma in Electronics Communication Engineering at Reva College in Bengaluru. Her pursuits embody arithmetic, physics, electrical, electronics, and pc engineering
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