RADIO RECEIVERS.pptx

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1. AVIONICS 04/05 MODE III PHASE I RADIO RECEIVERS BY woI(RTD) C N MAYABI

2. LESSON OBJECTIVE At the end of the lesson, the student should be able to correctly explain the principle operations of a radio receivers.

3. LESSON SCOPE Tuned radio frequency receivers Super heterodyne receivers Double Super heterodyne receivers Receiver Selectivity Image frequency rejection Automatic Gain Control

4. Tuned radio frequency receivers TRF receiver is a receiver where the tuning, i.e. selectivity is provided by the radio frequency stages. In essence the simplest tuned radio frequency receiver is a simple crystal set. Tuning is provided by a tuned coil / capacitor combination, and then the signal is presented to a simple crystal or diode detector where the amplitude modulated signal, in this case, is recovered. This is then passed straight to the headphones.

5. TRF receiver-----

6. TRF Receiver Typically a TRF receiver would consist of three main sections: Tuned radio frequency stages:    This consisted of one or more amplifying and tuning stages. Early sets often had several stages, each providing some gain and selectivity. Signal detector:    The detector enabled the audio from the amplitude modulation signal to be extracted. It used a form of detection called envelope detection and used a diode to rectify the signal.  Audio amplifier:    Audio stages to provide audio amplification were normally, but not always included.

7. TRF Receiver ----- The tuned radio frequency receiver was popular in the 1920s as it provided sufficient gain and selectivity for the receiving the broadcast stations of the day. However tuning took a little while as each stage in the early radios needed to be adjusted separately. Later ganged tuning capacitors were introduced, but by this time the superheterodyne receiver was becoming more widespread.

8. SUPERHEREDYNE RECIVER Super heterodyne receivers provide both improved sensitivity(ability to receive weak signals) and improved selectivity(the ability to discriminate signals on adjacent channels) when compared with TRF receivers. Superhet receivers are based on supersonic- heterodyne principle where the wanted input signal is converted to a fixed intermediate frequency ( IF ) at which the majority of the gain and selectivity is applied. The intermediate frequency chosen is generally 455khz or 1.6 MHZ for AM receivers and 10.7Mhz for communications and FM receivers.

9. SUPERHEREDYNE RECIVER The idea of the super heterodyne receiver revolves around the process of mixing. Here RF mixers are used to multiply two signals together. When two signals are multiplied together the output is the product of the instantaneous level of the signal at one input and the instantaneous level of the signal at the other input. It is found that the output contains signals at frequencies other than the two input frequencies. New signals are seen at frequencies that are the sum and difference of the two input signals, i.e. if the two input frequencies are f1 and f2, then new signals are seen at frequencies of (f1+f2) and (f1-f2). To take an example, if two signals, one at a frequency of 5 MHz and another at a frequency of 6 MHz are mixed together then new signals at frequencies of 11 MHz and 1 MHz are generated.

10. Parts of Communication System

11. SUPERHEREDYNE RECIVER---- In the superhet radio, the received signal enters one inputs of the mixer. A locally generated signal (local oscillator signal) is fed into the other. The result is that new signals are generated. These are applied to a fixed frequency intermediate frequency (IF) amplifier and filter. Any signals that are converted down and then fall within the pass-band of the IF amplifier will be amplified and passed on to the next stages. Those that fall outside the pass-band of the IF are rejected. Tuning is accomplished very simply by varying the frequency of the local oscillator. The advantage of this process is that very selective fixed frequency filters can be used and these far out perform any variable frequency ones. They are also normally at a lower frequency than the incoming signal and again this enables their performance to be better and less costly.

12. SUPERHEREDYNE RECIVER---- To see how this operates in reality 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 1 MHz. If the local oscillator is set to 5 MHz, then the two signals generated by the mixer as a result of the 6 MHz 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 this falls outside bandwidth of the IF so the only signal to pass through the IF is that from the signal on 6 MHz.

13. SUPERHEREDYNE RECIVER----

14. SUPERHEREDYNE RECIVER---- If the local oscillator frequency is moved up by 0.1 MHz to 5.1 MHz then the signal at 6.1 MHz will give rise to a signal at 1 MHz and this will pass through the IF. The signal at 6 MHz will give rise to a signal of 0.9 MHz at the IF and will be rejected. In this way the receiver acts as a variable frequency filter, and tuning is accomplished.

15. BLOCK DIAGRAM OF SUPER HET RECEIVER

16. Image frequency rejection The basic concept of the superheterodyne receiver appears to be fine, but there is a problem. There are two signals that can enter the IF. With the local oscillator set to 5 MHz and with an IF it has already been seen that a signal at 6 MHz mixes with the local oscillator to produce a signal at 1 MHz that will pass through the IF filter. However if a signal at 4 MHz enters the mixer it produces two mix products, namely one at the sum frequency which is 9 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 entering the mixer, or more correctly reduce its level to an acceptable value.

17. Image frequency Fortunately this tuned circuit does not need to be very sharp. It does not need to reject signals on adjacent channels, but instead it needs to reject signals on the image frequency. 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.

18. Image frequency----

19. Advantages of Using Superheterodyning 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 (IF section) and therefore they can be optimized or made more inexpensively. It can be used to improve signal isolation by arithmetic selectivity

20. ASSIGNIMENT A VHF Band II FM receiver with 10.7MHz IF covers the signal frequency range of 88MHz to 108MHz.Over what range should the local oscillator be tuned? Using f LO = frf+fif when frf =88MHz gives Flo= 88+10.7=98.7MHz Using f LO = frf+fif when frf =108MHz gives Flo= 108+10.7=118.7MHz

21. Double Super heterodyne Radio Receiver The double super heterodyne radio receiver improves the performance in a number of areas including stability (although synthesizers have largely overcome this problem), image rejection and adjacent channel filter performance. The double super heterodyne radio receiver is still widely used, especially at high frequencies where factors such as image rejection and filter performance are important.

22. Double Superheterodyne Radio Receiver--- The basic concept behind the double superheterodyne radio receiver is the use of a high intermediate frequency to achieve the high levels of image rejection that are required, and a further low intermediate frequency to provide the levels of performance required for the adjacent channel selectivity. Typically the receiver will convert the incoming signal down to a relatively high first intermediate frequency (IF) stage. This enables the high levels of image rejection to be achieved. As the image frequency lies at a frequency twice that of the IF away from the main or wanted signal, the higher the IF, the further away the image is and the easier it is to reject at the front end.

23. Block Diagram of Double superheterodyne receiver

24. PARAMETERS OF A RECEIVER There are a number of parameters which of great importance in specifying an AM receiver. They are : Sensitivity Selectivity Signal to noise ratio (S/N) Fidelity Image frequency rejection

25. PARAMETERS OF A RECEIVER Sensitivity of a receiver is the input required to produce certain audio power at the output. Selectivity is the ability to receive separately two adjacent channels. Signal to noise ratio (S/N) is the ratio of the signal at the output 6o noise at the output. Fidelity is the ability to reproduce accurately the information signal. Image frequency rejection is the ability of the receiver to reject signal frequency to a recceiver at image frequency.

26. RADIO RECEIVER SELECTIVITY Selectivity is one of the major specifications of any radio receiver for whatever application. There are two main functions of any receiver - one is to amplify the signal to the required level, and the other is to separate it from the other unwanted signals. These two requirements are the two major elements for any radio receiver. In view of this the selectivity performance and specification are form one of the major requirements for any radio receiver.

27. RADIO RECEIVER SELECTIVITY- ---- Radio receivers use tuned ccts in order to descriminate between incoming signals at different frequencies.Below are are two basic configuration for a tuned cct ; series and parallel.

28. WAVELENGTH AND FREQUENCY The impendance -frequency characteristics are shown against each.The impendance of the series tuned cct falls at resonant frequency while that of the parallel tuned cct increases to high value at resonant frequency.For this reason series tuned ccts are known as acceptor ccts.Parallel tuned cct on the other hand are referred to as rejectors .

29. IMPENDANCE-FREQUENCY CHARACTERISTICS

30. VOLTAGE AGAINST FREQUENCY The frequency response (voltage plotted against frequency) of a parallel tuned cct is shown below:

31. WAVELENGTH AND FREQUENCY This characteristic show that the signal developed across the cct reaches maximum at resonant frequency( fo ).The range of frequencies accepted by the cct is normally defined in relation to the half power –( 3dbpower) points which corresponds 70.7% of the maximum voltage and the frequency range between these two points is referred to as the bandwidth of the cct . An RF tuned cct will normally exhibit a quality factor (Q-factor) of about 100. The relationship between bandwidth(f),Q- factor,Q and resonant frequency,fo for a tuned cct is given by: F = fo /Q

32. FREQUENCY RESPONSE FOR TUNED CCT

33. EXAMPLE Consider a tuned cct which has a resonant frequency of 10MHz and a Q –factor of 100 its bandwidth will be F= fo /Q=10x10 6 /100=100KHZ

34. BLOCK DIAGRAM OF AM TRANSMITTER Clearily many strong signals will appear within this range and a significant number of them may be stronger than the wanted signal.With only a single tuned cct at the signal frequency,the receiver will be unable to differentiate between the wanted and the unwanted signals. Selectivity can be improved by adding additional tuned ccts at the signal frequency.Unfortunately the use of multiple tuned ccts brings with it the problem of maintaining accurate tuning of each cct throughout the ganged variable capacitors (or accurately) matched variable capacitance diodes) are required.

35. A band pass filter can be constructed using two parallel tuned cct coupled inductively(or capacitively ) The frequency response on this type of filter depends upon the decree of coupling between the two tuned ccts.Optimum results are obtained with a critical value of coupling as shown in fig below. Too great a decree of coupling will result in a double humped response whilst too little coupling results in a single peak in the in response curve accompanied by a significant loss in signal. Critical coupling produces a relatively flat pass band characteristics accompanied by a reasonably steep fall off either side of the pass band.

36. RADIO RECEIVERS Band pass filters are found in the IF stages of the superheterodyne receivers where they are used to define and improve the receivers selectivity. Where necessary a higher degree of selectivity and adjacent channel rejection can be achieved by using a multi-element ceramic,mechanical,or crystal filter.

37. IMAGE FREQUENCY In superhet receivers intermidiate frequency ( fIF ) is the difference between the signal frequency ( fRF ) and the local oscillator frequency ( fLO ). f IF = fLO – fRF fLO = fIF + - fRF EXAMPLE If fRF is 550khz and fIF is 455khz then fLO = fRF + fIF =550+455khz =1005khz

38. IMAGE FREQUENCY Therefore fRF at fLO +f IF f RF= 1005+455khz = 1460khz will also be able to give fIF with fLO . In other words there are two potential radio frequency signals that mix with the local oscillator signal in order to provide the required fIF.One of these frequency is the wanted signal( ie the signal present on the channel to which the receiver is tuned to while the other is referred to as the image frequency. A good superhet receiver should be able to reject the image frequency which is normally at twice IF away from the wanted signal.

39. AUTOMATIC GAIN CONTROL (AGC)     The signal level desired from the antenna fitted to an aircraft can vary from as little as 1micro V to more than 1000micro V.This case presents a problem when signal are to be amplified. The low level signals benefit from the maximum amount of gain present in the system while the larger signals require equally less gain in order to avoid non linearity and distortion of the signals and modulation.

40. AGC AM,CW and SSB receivers usally incorporate some means of automatic gain control (AGC) that progressively reduces signal gain as the amplitude of the input signal increases as shown below: In simple receivers ,the AGC voltage is(a dc voltage dependent on signal amplitude) is derived directly from the signal detector and is fed directly IF stages.

41. DELAYED AGC There is ,in fact no need to to reduce the signal gain for small RF signals. In more complex equipment, the AGC ccts may be designed to provide a ‘ delay’ so that there is no gain reduction until a predetermined thresh hold voltage is exceeded.

42. AGC

43. Q&A OK, what did I not make perfectly clear ?

44. LESSON OBJECTIVE AT THE END THE OF THE LESSON,THE STUDENT SHOULD BE ABLE TO CORRECTLY;- EXPLAIN WHAT IS RECEIVER WHAT IS RECEIVER SELECTIVITY WHAT IS IMAGE FREQUENCY WHAT IS AGC

45. REFERENCE STUDENTS NOTES