University of Malaya KEEE 2142 Introduction to Communication System Dr.Harikrishnan
Department of Electrical Engineering e-mail:
[email protected]
Superheterodyne Radio Receiver
The incoming radiofrequency (RF) signal is combined with sinusoid output of a local oscillator (LO) in a mixer, which is sometimes referred as first detector. The output circuit of the mixer is usually tuned to the difference between the frequencies of the oscillator and the incoming signal. The design of the receiver is such that when it is tuned to the frequency of another incoming signal, the frequency of the local oscillator is also automatically changed so as to maintain the same difference frequency as before, the process is called tracking. The circuits that amplifies the intermediate frequency (IF) are called IF amplifiers.
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Superheterodyne Radio Receiver (cont’d)
After processing by the IF amplifier, the signal is delivered to the demodulator (sometimes also called the second detector). Additional amplification follows the demodulator to provide the level of output desired. If the output is voice or music, audiofrequency (AF) amplifiers will be needed to obtain the power to drive the loud speaker. If the output is the electrical equivalent of a television picture, video amplifiers are used.
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Superheterodyne Radio Receiver (cont’d)
LO radiation can be minimized by shielding the local oscillator circuits by using doublebalanced mixer, by keeping the oscillator power level as small as possible and by using buffer amplifiers between the antenna and the mixer/oscillator circuit.
High Side Injection (HSI) & Low Side Injection (LSI)
Suppose the RF frequencies for a standard AM broadcast band had been standardized to be from 540 kHz to 1600 kHz. A typical value of IF of 455 kHz with a tuning mechanism of down conversion, would result in an oscillator range 995 kHz to 2055 kHz, or from 85 kHz to 1145 kHz.
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Superheterodyne Radio Receiver (cont’d)
The frequency produced by the oscillator is determined approximately by the resonant frequency of its LC circuit components, or:
f =
1 2π LC
Suppose that it is the capacitance that can be changed from C min to Cmax and that the inductance L remains constant. Then an LC circuit will be tuned from:
f max
=
f min
=
1 2π LC min 1 2π LC max
… (1) … (2)
Taking the ratio (1) to (2), the capacitance ratio required is :
C max C min University of Malaya
f max = f min KEEE 2142
2
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Superheterodyne Radio Receiver (cont’d)
If the oscillator uses high side injection then the tuning capacitor of the oscillator must have :
C max C min
2
=
4.27
If the oscillator uses low side injection, then the tuning capacitor of the oscillator must have :
C max C min
2055 = 995 1145 = 85
2
=
181.46
The conclusion that can be drawn from this analysis is that it is better to use HSI rather than LSI when tuning the AM broadcast band because the tuning capacitance range will be more reasonable.
Frequency Conversion
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Superheterodyne Radio Receiver (cont’d) Reciprocal Mixing
When the output voltage of the local oscillator varies because of noise produced by the oscillator components, then there are undesired noise sidebands accompanying the desired sinusoid in the oscillator output. The desired RF signal will have its own sidebands. When the oscillator noise sidebands mix with the undesired RF signals to produce additional noise in the IF passband, the process is called reciprocal mixing.
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Superheterodyne Radio Receiver (cont’d) Superheterodyne Characteristic
The weakest link in radio communications system is the receiver. When there are thousands upon thousands of transmitting stations around the world, operating in numerous modes such as TV, FM, AM, CW, teletype and facsimile, including military, commercial and telephone services, operating on so many different frequencies and with such a variety of power levels, the receiver has its job cut out for itself in selecting the one that is desired and rejecting all others. When the receiver is tuned to a fre uenc of a desired station the receiver should have sufficient sensitivity to detect the presence of the desired signal, sufficient selectivity to accept the station selected and to reject all stations operating on other frequencies, sufficient fidelity of reproduction of the demodulated output to be acceptable.
Sensitivity
Sensitivity refers to the ability of a receiver to respond to weak RF signal.
Selectivity
Selectivity is a measure of how well a receiver can select a desired station to the exclusion of all others. The early tuned-radiofrequency receivers did not have this desirable characteristic to a great extent, but in those days there were not many radio stations to select from.
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Superheterodyne Radio Receiver (cont’d)
Selectivity is often expressed by the shape of the IF filter response. The filter will have an attenuation or insertion loss. The nose bandwidth or passband is the difference between the frequencies for which response is down by 3 dB from the maximum shown to be 0 dB. The response shape above and below the passband is called the skirt. The difference between the frequencies for which the response is down by 60 dB is called the skirt bandwidth. .
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Superheterodyne Radio Receiver (cont’d) Shape factor =
skirt bandwidth nose bandwidth
Selectivity is considered to be poor if the shape factor is greater than 10, reasonably good if the shape factor is between 3 and 6, and very sophisticated if the shape factor is less than 1.2.
Image Response
If the incoming RF signal was at 1000 kHz and the IF was 100 kHz, the local oscillator could be tuned to either 1100 kHz or 900 kHz to produce the desired IF from the mixer. Thus for every received RF signal, there were found two settings of the oscillator tuning capacitor which worked equally well. One of these was called the image setting of the local oscillator. If the incoming RF was from a weak distant station operating on 1000 kHz and the IF was 100 kHz, then the local oscillator was automatically set to be 1100 kHz to produce the desired IF signal. But suppose that an undesired signal from a strong local station on 1200 kHz also gets through the selectivity of the RF circuits and arrives at the mixer. It mixes to produce an undesired signal at 100 kHz. Both of this IF are demodulated and both of the station can be heard simultaneously. The undesired RF signal on 1200 kHz is called the image frequency.
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Superheterodyne Radio Receiver (cont’d)
In general, when DC is used, the image frequency is given by:
f image
=
f RF + 2IF
Interference due to image may be objectionable at one location and the frequency caused by image response may not be noticeable at a location far removed from the other. The best way to improve image response is to design the receiver initially to have as high an IF possible. If IF is small, the image frequency falls higher on the RF selectivity curve than when the IF is large. Certainly an IF of 455 kHz is much better in improving image than an IF of 100 kHz.
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Superheterodyne Radio Receiver (cont’d) Adjacent Channel Selectivity
Adjacent channel selectivity refers to the ability to select a desired station and exclude undesired stations that may be operating in adjacent channels. Superheterodyne is capable of very good adjacent channel selectivity. Let us compare two superhets, A and B, each with HSI/DC, tuned to a desired station on 1000 kHz. Assume that an undesired station is in an adjacent channel on 1010 kHz. The desired and undesired frequencies are separated by 10 kHz out of a desired total of 1000 kHz, or 1 percent separation. Superhet A has an IF of 100 kHz ( local oscillator frequency is 1100 kHz) and superhet B has an IF of 500 kHz ( local oscillator frequency is 1500 kHz). Superhet A produces a desired If signal at 100 kHz and an undesired interfering signal at 90 kHz. Note that frequency conversion does not change the spacing between the desired and undesired signals. The desired and undesired IF signals are separated by 10 kHz out of the desired total of 100 kHz, or 10 percent separation. Superhet B produces a desired IF signal at 500 kHz and an undesired interfering IF signal at 490 kHz. The desired and undesired IF are still separated by 10 kHz, but now that 10 kHz is of a desired total of 500 kHz, or 2 percent.
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Superheterodyne Radio Receiver (cont’d)
Superhet A can provide much better adjacent channel selectivity than B. The next basic rule in superhet design.
Double Conversion Superheterodyne
In a double conversion receiver the first IF is made to be high for good image rejection and the second If is made to be low for good adjacent channel selectivity. For example, a certain superhet tunes from 2 MHz to 30 MHz. The first IF is 55 MHz, produced by a difference mixer with HIS. So oscillator #1 tunes from 57 MHz to 85 MHz. The image frequencies range from 112 MHz to 140 MHz.
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Superheterodyne Radio Receiver (cont’d)
If the second IF is 450 kHz, then the frequency of oscillator #2 can be either 55.45 MHz or 54.55 MHz. The high first IF and low second IF should not be interchanged for a low first IF and a high second IF. If the interchange is made good adjacent channel selectivity is obtained from the low first IF, but then image rejection is poor.
Example 1 A superheterodyne FM receiver tunes 88-108 MHz. The IF is 10.7 MHz and high side injection of the oscillator is used. Is difference mixing or sum mixing is required?. Why?. Example 2 An AM superhet has an IF=455 kHz, and it uses high side injection. When the receiver is tuned to 540 kHz, a local AM station operating on 1450 kHz is heard. Explain how this image response can occur through no fault at the transmitter. Example 3 The receiver in Example 2 is tuned near the high end of the AM broadcast band and a nonbroadcast band station is heard. If a local transmitter is known to operate on 2.45 MHz, explain how that station is heard through no fault at the transmitter and determine the frequency to which the receiver is tuned. Example 4 The receiver of Example 2 is tuned to a station on 1100 kHz. In addition to the desired signal, an interfering code signal is heard and identified as a radio amateur operating in the frequency range 3.5 – 4.0 MHz. It is known that spurious responses often occur in the receiver because harmonics of its own local oscillator are University of Malaya
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Superheterodyne Radio Receiver (cont’d) also present in the mixer. Explain how the code signal is also heard through no fault at the transmitter and determine the fundamental frequency of the code station. Example 5 A certain superhet with poor image rejection is used for reception on the standard AM broadcast band. It is found that each time the receiver is tuned to a station A at 1000 kHz, another station B at 1350 kHz will also be heard. Determine the intermediate frequency of the receiver. Example 6 An AM superhet for short-wave reception has an IF = 1.6 MHz. high side injection is used. The receiver is tuned to an AM station operating on 30 MHz. (a) Determine the local oscillator frequency and the image . , assuming that the station being received is amplitude modulated by a 5 kHz tone and that the bandwidth of the IF amplifier is 6 kHz. Example 7 Suppose you have a good quality AM radio receiver that tunes from 540 kHz to 1600 kHz. You wish to add some circuitry external to the receiver so that you can listen to the AM short-wave broadcast band 6000 – 6200 kHz by tuning the receiver only from 1000 kHz to 1200 kHz. No modifications are made internal to the receiver because it is still under warranty. Draw a block diagram showing how you could do this. Name all blocks and mark all required frequencies.
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