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Super HETERODYNE


Communication Systems

Transfer information from one place to another

Transmission, reception, and processing of information between two or more locations.

 The Source of information can be analog or digital.

  All information is converted into Electromagnetic energy before propagation.

 

 

Diagram

 

What Heterodyning is:

 To heterodyne means to mix two frequencies together to produce a beat frequency, namely the difference between the two.

 

Example:

Amplitude modulation is a heterodyne process: the information signal is mixed with the carrier frequency to produce the sidebands. These sidebands occur at precisely the sum and difference of carrier frequency. These are beat frequencies (normally the beat frequency is associated with the Lower sideband (the difference between the two)).

 

What super heterodyning is:

When you are using the lower sideband (difference b/w two frequencies super heterodyning refers to creating a beat frequency that is lower than the original signal.

 

Transmission Modes:

Simplex, half-duplex and full-duplex are three kinds of communication channels in telecommunications and computer networking. These communication channels provide pathways to convey information.

 

Simplex:

 (one way only) A simplex communication channel only sends information in one direction. For example, a radio station usually sends signals to the audience but never receives signals from them, thus a radio station is a simplex channel. The good part of simplex mode is that its entire bandwidth can be used during the transmission.

 

Half-duplex:

 (two-way alternate) In half-duplex mode, data can be transmitted in both directions on a signal carrier except not at the same time. At a certain point, it is actually a simplex channel whose transmission direction can be switched. A walkie-talkie is a typical half-duplex device. It has a “push-to-talk” button which can be used to turn on the transmitter but turn off the receiver. Therefore, once you push the button, you cannot hear the person you are talking to but your partner can hear you. An advantage of half-duplex is that the single track is cheaper than the double tracks.

 

Full-duplex:

 (two way simultaneous) A full-duplex communication channel can transmit data in both directions on a signal carrier at the same time. It is constructed as a pair of simplex links that allow bidirectional simultaneous transmission. Take the telephone as an example, people at both ends of a call can speak and be heard by each other at the same time because there are two communication paths between them. Thus, using the full-duplex mode can greatly increase the efficiency of communication.

 

Three Important Frequencies in Heterodyne RX:  

 1 ) Radiofrequency(RF): The center frequency the signal is broadcast on.

 2) Intermediate Frequency (IF): Fixed frequency inside the RX. The RF signal is down-converted to this frequency.

 3) Local Oscillator(LO):  Tuneable frequency inside the RX used to translate the RF signal to the IF frequency.

 

Advantages of Using super heterodyning:

 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).

 It allows many components to operate at a fixed frequency (LF section) and therefore they can be optimized or made more inexpensively.

It can be used to improve signal isolation by arithmetic selectivity

 

Discussion on Arithmetic Selectivity:

The ability to isolate signals, or reject unwanted ones, is a function of the receiver bandwidth. 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 i.e. 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 super heterodyned to the range of 443 to 461 kHz will pass. Therefore, the LO frequency will be 398KHz. Taking this range back up into the carrier band, only carrier frequencies in the range of 841 to 859 kHz will pass.

 

An example might clear this up:

 Suppose there is an interfering signal at 863 kHz while you are tuned to 850 kHz. A conventional 2 % receiver will pass 833 to 867 kHz and so the interfering signal also passes. The superheterodyne receiver mixes both signals with 398 kHz to produce the desired signal at 452 kHz and the interference at 465 kHz. At 2 %, the IF section only passes 443 to 461 kHz, and therefore the interference is now suppressed. We say that the superheterodyne receiver is more selective. With a little thought, the reason is simple: it operates at a smaller frequency, so the 2 % actually involves a smaller range. That is why it is called arithmetic selectivity. Bandwidths that are expressed as a percentage are smaller when the center frequency is smaller (the same way that 2 % of ₹10 is less than 2 % of ₹10,000 ).

 

1. RF section: 

Preselector + Amplifier • Broad tuned bandpass filter with an adjustable center frequency that is tuned to the desired carrier frequency. • Provides enough initial band limiting, to prevent specific unwanted RF (image frequency) to enter the receiver. RF stage with the tuned circuit in such a way that it will amplify wanted signals.

2. Mixer/Converter :

 Local oscillator + Mixer • Bandwidth remains unchanged after heterodyning. • Although carrier and sideband frequencies converted from RF to IF, the shape of the envelope remains the same. • LO is a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor

3. IF section: 

Filter + Amplifier • Very narrow bandwidth Class-A amplifier capable of selecting particular frequency • This stage includes most of the amplification and filtering

4. Detector section :

Converts IF signal to original source information. RF is filtered to the ground and audio is supplied to audio stages for amplification. • Generally, a Linear diode detector is used.

5. Audio Amplifier section :

 O/P from the demodulator is recovered audio. Further, fed to speakers.

 

Image frequency:

In the superheterodyne radio, the received signal enters one input of the mixer. A locally generated signal (local oscillator signal ) is fed into the other. The result is that new signals are generated.

If a signal at 4 MHz enters the mixer it produces two mix products, namely one at the sum frequency which is 10 MHz, whilst the difference frequency appears at 1 MHz.

 This would prove to be a problem because it is perfectly possible for two signals on completely different frequencies to enter the IF. The unwanted frequency is known as the image.

 Fortunately, it is possible to place a tuned circuit before the mixer to prevent the signal from entering the mixer. These will be separated from the wanted channel by a frequency equal to twice the IF. In other words with an IF at 1 MHz, the image will be 2 MHz away from the wanted frequency.

 

 

Diagram

 


 

Example by taking two signals

take the example of two signals, one at 6 MHz and another at 6.1 MHz. Also, take the example of an IF situated at I MHz. If the local oscillator is set to 5 MHz, then the two signals generated by the mixer as a result of the 6MHz signal fall at 1 Mhz and 11 MHz. Naturally the 11 MHz signal is rejected, but the one at 1 MHz passes through the IF stages. The signal at 6.1 MHz produces a signal at 1.1 MHz (and 11.1 MHz) and thus falls outside the bandwidth of the IF so the only signal to pass through the IF is that from the signal on 6 MHz.

 

Receiver parameters:

1. Selectivity: Parameter that is used to measure the ability of the receiver to accept a given band of frequency and reject all others.

2. Sensitivity: Minimum Rf signal level that can be detected at the I/P to the receiver and still produce a usable demodulated information signal.

3. Dynamic Range: Difference in dB between the minimum I/P level necessary to discern a signal to the I/P level that will overdrive the receiver and produce distortion. Overdrive/needed

4. Fidelity: Measure the ability of the communication system to produce at the O/P of the receiver, an exact replica of original source information. Three types of distortion: Amplitude, frequency, and phase.

 

Radios use two kinds of coupling methods:

 • AF Amplifiers are used to amplify the signals lying in the audio range ( i.e. 20 Hz to 20 kHz)

• RF Amplifiers are used to amplify signals having a very high frequency.

 


RF amplifiers:

Greater gain provides better sensitivity

Improved image frequency rejection

Improved S/N ratio

 Better selectivity

Better coupling of the receiver to the antenna

Prevention of re-radiation of local oscillator voltage through the antenna.

 
Frequency Mixer:

The mixer stage is used to decrease the received frequency to an intermediate frequency. The mixer also receives input from the local oscillator. These two signals are beaten together to obtain the IF through the process of heterodyning.

Frequency of local oscillator + signal voltage  goes to the mixer as an input and in the output, we got low frequency as LO-SIGNAL VOLTAGE

Which is our IF ( INTERMEDIATE FREQUENCY)

 

IF Amplifier:

 An IF Amplifier is simply a high gain RF amplifier designed to function on one fixed frequency.

Simplicity: After initial alignment, no further adjustments to the tuned circuits are necessary.

Efficiency: High gain is obtainable owing to the low working frequency


LOCAL OSCILLATOR:

In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequency of the local oscillator and frequency of the input signal.

A crystal oscillator is one common type of local oscillator that provides good stability and performance at relatively low cost, but its frequency is fixed, so changing frequencies requires changing the crystal.




 Tuning to different frequencies requires a variable frequency oscillator which leads to a compromise between stability and tunability.

 A variable frequency oscillator (VFO) is an oscillator whose frequency can be tuned (i.e., varied) over some range. It is a necessary component in any tunable radio receiver or transmitter that works by the superheterodyne principle and controls the frequency to which the apparatus is tuned.

An analog VFO is an electronic oscillator where the value of at least one of the passive components is adjustable under user control to alter its output frequency. The passive component whose value is adjustable is usually a capacitor but could be a variable inductor.

  A loudspeaker is an electroacoustic transducer;

 a device that converts an electrical audio signal into a corresponding sound.

An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals such as the signal from the radio receiver or electric guitar pickup to a level that is high enough for driving loudspeakers or headphones.

  In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. After sound transmission began around 1920, the term evolved to mean a demodulator, which extracted the audio signal from the radio frequency carrier wave

 

 

 

MORE INFORMATION REGARDING DEMODUALTED WAVE AND DEMODULATORS :

 

Detector / demodulator stage:

Once the signals have passed through the IF stages of the superheterodyne receiver, they need to be demodulated.

Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered.

 Different demodulators used may include:

AM diode detector:

 This is the most basic form of detector and this circuit block would simply consist of a diode and possibly a small capacitor to remove any remaining RF.

 The detector is cheap and its performance is adequate, requiring a sufficient voltage to overcome the diode forward drop. It is also not particularly linear, and finally, it is subject to the effects of selective fading that can be apparent, especially on the HF bands.

Synchronous AM detector:

This form of AM detector block is used where Improved performance is needed. It mixes the incoming AM signal with another on the same frequency as the carrier.

 This second signal can be developed by bypassing the whole signal through a squaring amplifier.

The advantages of the synchronous AM detector is that it provides a far more linear demodulation performance and it is far less subject to the problems of selective fading.

SSB product detector:

The SSB product detector block consists of a mixer and a local oscillator, often termed a beat frequency oscillator, BFO, or carrier insertion oscillator. CIO. This form of detector is used for Morse code transmissions where the BFO is used to create an audible tone in line with the on-off keying of the transmitted carrier. Without this, the carrier without modulation is difficult to detect. For SSB. the CIO re-inserts the carrier to make the modulation comprehensible.

 

Basic FM detector:

 As an FM signal carries no amplitude variations a demodulator block that senses frequency variations are required. It should also be insensitive to amplitude variations as these could add extra noise. Simple FM detectors such as the Foster Seeley or ratio detectors can be made from discrete components although they do require the use of transformers.

 

PLL FM detector:

The phase-locked loop can be used to make a very good FM demodulator. The incoming FM signal can be fed into the reference input, and the VCO to provide the detected audio output.

Quadrature FM drive voltage used detector:

This form of FM detector block is widely used within us. IT is simple to implement and provides a good linear output.

Audio amplifier: The output from the demodulator is the recovered audio. This is passed into the audio stages where they are amplified and presented to the headphones or loudspeaker

 


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