Radio Receiver Superheterodyne

University of Malaya

Dr.Harikrishnan

Department of Electrical Engineering

e-mail: [email protected]

KEEE 2142Introduction to Communication System

Superheterodyne Radio Receiver

University of Malaya KEEE 2142 HRK 2/15

� 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 theoscillator and the incoming signal.

� The design of the receiver is such that when it is tuned to the frequency of another incomingsignal, the frequency of the local oscillator is also automatically changed so as to maintain thesame difference frequency as before, the process is called tracking.

� The circuits that amplifies the intermediate frequency (IF) are called IF amplifiers.

� After processing by the IF amplifier, the signal is delivered to the demodulator (sometimes alsocalled the second detector). Additional amplification follows the demodulator to provide thelevel of output desired.

� If the output is voice or music, audiofrequency (AF) amplifiers will be needed to obtain thepower to drive the loud speaker. If the output is the electrical equivalent of a television picture,video amplifiers are used.

Superheterodyne Radio Receiver (cont’d)


� LO radiation can be minimized by shielding the local oscillator circuits by using double-balanced mixer, by keeping the oscillator power level as small as possible and by using bufferamplifiers 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 befrom 540 kHz to 1600 kHz. A typical value of IF of 455 kHz with a tuning mechanism of downconversion, would result in an oscillator range 995 kHz to 2055 kHz, or from 85 kHz to 1145kHz.

� The frequency produced by the oscillator is determined approximately by the resonantfrequency of its LC circuit components, or:

� Suppose that it is the capacitance that can be changed from Cmin to Cmax and that theinductance L remains constant. Then an LC circuit will be tuned from:


1f

2 LC=

π


� Taking the ratio (1) to (2), the capacitance ratio required is :

max

min

1f (1)

2 LC=

π…

min

max

1f (2)

2 LC=

π…

2

max max

min min

C fC f

=

� If the oscillator uses high side injection then the tuning capacitor of the oscillator must have :

� If the oscillator uses low side injection, then the tuning capacitor of the oscillator must have :

� The conclusion that can be drawn from this analysis is that it is better to use HSI rather than LSIwhen tuning the AM broadcast band because the tuning capacitance range will be more


2

max

min

C 20554.27

C 995 = =

2

max

min

C 1145181.46

C 85 = =


reasonable.

Frequency Conversion


� When the output voltage of the local oscillator varies because of noise produced by theoscillator components, then there are undesired noise sidebands accompanying the desiredsinusoid in the oscillator output. The desired RF signal will have its own sidebands. When theoscillator noise sidebands mix with the undesired RF signals to produce additional noise in theIF passband, the process is called reciprocal mixing.

Reciprocal Mixing


� The weakest link in radio communications system is the receiver. When there are thousandsupon thousands of transmitting stations around the world, operating in numerous modes suchas TV, FM, AM, CW, teletype and facsimile, including military, commercial and telephoneservices, 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 allothers.

� When the receiver is tuned to a frequency of a desired station, the receiver should have


Superheterodyne Characteristic


� When the receiver is tuned to a frequency of a desired station, the receiver should havesufficient sensitivity to detect the presence of the desired signal, sufficient selectivity to acceptthe station selected and to reject all stations operating on other frequencies, sufficient fidelity ofreproduction of the demodulated output to be acceptable.

� Sensitivity refers to the ability of a receiver to respond to weak RF signal.

� Selectivity is a measure of how well a receiver can select a desired station to the exclusion ofall others. The early tuned-radiofrequency receivers did not have this desirable characteristic toa great extent, but in those days there were not many radio stations to select from.

Sensitivity

Selectivity


� 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 thedifference between the frequencies for which response is down by 3 dB from the maximumshown to be 0 dB.

� The response shape above and below the passband is called the skirt. The difference betweenthe frequencies for which the response is down by 60 dB is called the skirt bandwidth.

� Selectivity is often specified by a shape factor.


� Selectivity is often specified by a shape factor.


skirt bandwidthShape factor

nose bandwidth=

� Selectivity is considered to be poor if the shape factor is greater than 10, reasonably good if theshape factor is between 3 and 6, and very sophisticated if the shape factor is less than 1.2.

� If the incoming RF signal was at 1000 kHz and the IF was 100 kHz, the local oscillator could be

Image Response


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 tuningcapacitor which worked equally well. One of these was called the image setting of the localoscillator.

� If the incoming RF was from a weak distant station operating on 1000 kHz and the IF was 100kHz, then the local oscillator was automatically set to be 1100 kHz to produce the desired IFsignal. But suppose that an undesired signal from a strong local station on 1200 kHz also getsthrough the selectivity of the RF circuits and arrives at the mixer. It mixes to produce anundesired signal at 100 kHz. Both of this IF are demodulated and both of the station can beheard simultaneously. The undesired RF signal on 1200 kHz is called the image frequency.



� In general, when DC is used, the image frequency is given by:

� Interference due to image may be objectionable at one location and the frequency caused byimage 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 IFpossible. If IF is small, the image frequency falls higher on the RF selectivity curve than whenthe IF is large.

� Certainly an IF of 455 kHz is much better in improving image than an IF of 100 kHz.

image RFf f 2IF= +


� Adjacent channel selectivity refers to the ability to select a desired station and excludeundesired 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 1000kHz. Assume that an undesired station is in an adjacent channel on 1010 kHz. The desired andundesired frequencies are separated by 10 kHz out of a desired total of 1000 kHz, or 1 percent

Adjacent Channel Selectivity


separation. Superhet A has an IF of 100 kHz ( local oscillator frequency is 1100 kHz) andsuperhet 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 90kHz. Note that frequency conversion does not change the spacing between the desired andundesired signals. The desired and undesired IF signals are separated by 10 kHz out of thedesired total of 100 kHz, or 10 percent separation.

� Superhet B produces a desired IF signal at 500 kHz and an undesired interfering IF signal at490 kHz. The desired and undesired IF are still separated by 10 kHz, but now that 10 kHz is ofa desired total of 500 kHz, or 2 percent.


� Superhet A can provide much better adjacent channel selectivity than B. The next basic rule insuperhet design.

Double Conversion Superheterodyne


� In a double conversion receiver the first IF is made to be high for good image rejection and thesecond 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, producedby a difference mixer with HIS. So oscillator #1 tunes from 57 MHz to 85 MHz. The imagefrequencies range from 112 MHz to 140 MHz.


� If the second IF is 450 kHz, then the frequency of oscillator #2 can be either 55.45 MHz or54.55 MHz.

� The high first IF and low second IF should not be interchanged for a low first IF and a highsecond IF. If the interchange is made good adjacent channel selectivity is obtained from thelow first IF, but then image rejection is poor.

Example 1A superheterodyne FM receiver tunes 88-108 MHz. The IF is 10.7 MHz and high side injection of theoscillator is used. Is difference mixing or sum mixing is required?. Why?.


Example 2An 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 nofault at the transmitter.

Example 3The receiver in Example 2 is tuned near the high end of the AM broadcast band and a nonbroadcast bandstation is heard. If a local transmitter is known to operate on 2.45 MHz, explain how that station is heardthrough no fault at the transmitter and determine the frequency to which the receiver is tuned.

Example 4The receiver of Example 2 is tuned to a station on 1100 kHz. In addition to the desired signal, an interferingcode signal is heard and identified as a radio amateur operating in the frequency range 3.5 – 4.0 MHz. It isknown that spurious responses often occur in the receiver because harmonics of its own local oscillator are

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 5A certain superhet with poor image rejection is used for reception on the standard AM broadcast band. It isfound that each time the receiver is tuned to a station A at 1000 kHz, another station B at 1350 kHz will alsobe heard. Determine the intermediate frequency of the receiver.

Example 6An AM superhet for short-wave reception has an IF = 1.6 MHz. high side injection is used. The receiver istuned to an AM station operating on 30 MHz. (a) Determine the local oscillator frequency and the imagefrequency. (b) Draw a block diagram of the receiver and label the frequencies in each part of the receiver,



frequency. (b) Draw a block diagram of the receiver and label the frequencies in each part of the receiver,assuming that the station being received is amplitude modulated by a 5 kHz tone and that the bandwidth ofthe IF amplifier is 6 kHz.

Example 7Suppose you have a good quality AM radio receiver that tunes from 540 kHz to 1600 kHz. You wish to addsome 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 thereceiver because it is still under warranty. Draw a block diagram showing how you could do this. Name allblocks and mark all required frequencies.

Documents

Radio Receiver Superheterodyne