Am And Fm Receivers

AM and FM Receivers Design Description and Comparison By Ashar Salman CE04-0383

Project Report of Communication System

INDEX Modulation ¾ Amplitude Modulation ¾ Frequency Modulation Demodulation AM Receiver Superheterodyne Receivers Circuit for Superheterodyne Receiver ¾ Local Oscillator Stage ¾ Mixer Stage ¾ Coupling Capacitor ¾ Intermediate Frequency Transformer/Filter (IFT) ¾ Detector Stage ¾ Audio Amplifier Stage FM Receiver ¾ (a) F.M. Discriminator (figure 12) ¾ (b) Ratio Detector ¾ (c) Crystal Discriminator ¾ (d) Phase Lock Loops FM receiver with TDA7088T Comparison between AM and FM References

1 1 2 2 2 3 5 7 7 8 8 9 9 10 10 11 11 11 11 14 16

Modulation Modulation is a technique for transferring information or message of lower frequency by riding it on the higher frequency carrier. In other words, the process by which some characteristic of a higher frequency wave is varied in accordance with the amplitude of a lower frequency wave.i This solves the major problem of antenna size and signal distortion (or noise) in communication system. There are two types of modulation: 1 AM 2 FM

Amplitude Modulation The basic idea of AM is that “vary the amplitude of carrier wave in proportion to the message signal”. For this purpose message is multiplied with a sinusoidal of frequency ωο. The highest frequency of the modulating data is normally less than 10 percent of the carrier frequency. The instantaneous amplitude (overall signal power) varies depending on the instantaneous amplitude of the modulating data.ii Figure below shows an AM signal.

Figure 1: (a) Carrier signal. (b) message (c) AM signaliii

Frequency Modulation In Frequency Modulation, the frequency of carrier wave is varied with respect to the amplitude of message signal. Another definition could be Frequency modulation (FM) is a form of modulation, which represents information as variations in the instantaneous frequency of a carrier wave.iv In figure below a FM signal shown.

Figure 2: (a) Message signal. (b) FM signal

Demodulation Demodulation is the reverse of modulation that is a process for retrieving an information signal that has been modulated onto a carrier.

AM Receiver For extracting the message signal back from the carrier wave we “demodulate” the RF signal. For AM demodulation we have different methods: v ¾ Early Receiver Architectures o 4.1.1 Tuned RF Receivers o 4.1.2 Regenerative Receivers o 4.1.3 Super-Regenerative Receivers ¾ Superheterodyne Receivers o Modern Single Conversion Implementations o Multiple Conversion Implementations o Up Conversion Implementations o Designs with Ultra-Low IFs o Designs with Image Rejection Mixers o Designs with Selective Demodulators

¾ Direct Conversion Receivers ¾ Digital Receivers ¾ Ideal Low-Power Receivers Keeping in mind the limited pages of this report, I will discuss only superheterodyne receiver.

Superheterodyne Receivers The concept of heterodyning an incoming signal to convert it to a lower frequency was developed by Armstrong and others in 1918. Armstrong's original design, shown in Figure, was intended to allow low frequency radiotelephone receivers to be adapted for use at newer HF frequencies being used in Europe. Figure 3: Original Superheterodyne design

However, it was quickly recognized that the basic approach offered many additional benefits, including: ¾ The low-frequency receiver (typically a high quality tuned-RF design) could be adjusted once, and thereafter all tuning could be done by varying the heterodyne oscillator. ¾ Amplification could be provided primarily at a lower frequency where high gains were easier to achieve. ¾ Amplification was split between two frequencies, so that the risk of unwanted regenerative feedback could be reduced. ¾ Narrow, high-order filtering was more easily achieved in the low frequency receiver than at the actual incoming RF frequency being received. Eventually, the separate tuned-RF receiver was replaced by the dedicated IF section of the modern superheterodyne design, in which pre-tuned fixed-frequency filters are employed. The result became the well-known architecture used today with high quality channel-select filtering and no adjustments aside from volume and tuning controls.vi Two demodulation techniques are used with superheterodyne receivers, Synchronous and Asynchronous.

Figure 4: Synchronous and Asynchronous superheterodyne receivers.

Again for simplicity, I will stick to only with Asynchronous Superheterodyne model. Below in the figure is shown a more general block diagram of superheterodyne receiver.vii

Figure 5: block diagram of superheterodyne receiver

Circuit for Superheterodyne Receiver Although superheterodyne radio receivers looks not very complicated but for practicable purposes there must be additional circuitry involved in the design. One of them is Automatic Gain Control (AGC).viii The AGC circuit keeps the receiver in its linear operating range by measuring the overall strength of the signal and automatically adjusting the gain of the receiver to maintain a constant level of output. When the signal is strong, the gain is reduced, and when weak, the gain is increased, or allowed to reach its normal maximum.ix For simplicity of circuit, I will present a circuit without AGC. The complete circuit at next page appears complicated, that is why I have decided to explain it systematically.

Local Oscillator Stage In most of AM receivers, local oscillator (LO) is designed with the help of a special component, known as oscillator coil. They are not more than ordinary transformer but with an additional capability, that their core is movable between the coils. The main purpose of having a moveable core is to tune the oscillator at desire band. The top side of LO is colored white in order to distinguish it from intermediate frequency transformers. They come in metal housing and there are five pins plus two pins of metal housing. The pin configuration of LO is shown in figure 7.

Figure 6

Figure 7: Three different views of LO Figure 8

Mixer Stage Multiplying the RF signal from the antenna with the frequency of LO is an essential part of demodulation. Different methods are employed for this purpose, transistors, diodes, transformers or other electronic components may be used. But I prefer IC NE612 for this purpose in my circuit for many reasons. The main reason is that using IC instead of other component is that the need of RF stage amplifier is reduced very much, because NE612 takes very little power from input signal. Moreover, other important reason is that the quality of mixing is very good and output signal is very much close to the intermediate frequency (IF). Another good reason is that as we all know that for mixer circuit the

Figure 9: Block Diagram and pin configuration of NE612

supply voltage should be very constant, and NE612 has its own voltage regulator, that mean that we don’t have to implement one by our self. And the biggest advantage is that its use is very simple, attach antenna to pin 1 or 2, ground pin no. 3 and 6 volt to pin no. 8. Then connect LO between pin 6 and 7, and get IF frequency out from pin 4 and 5.

Coupling Capacitor As we know that in superheterodyne design our RF stage and LO should oscillate in such a way that their difference is always 455 kHz (IF frequency). In order to get simultaneously tuning of both circuits, we use coupling capacitor. They are just pair of two capacitors connected parallel to each other. One is for main tuning and other is for fine-tuning. In the case of FM, there are four capacitors. There block diagram and pin configuration is shown bellow.

Intermediate Frequency Transformer/Filter (IFT) Intermediate frequency filter is made with the help of transformer similar to the LO stage, so it is called IFT. They too came in metal housing as LO. The only difference is that they also have a capacitor built in them. The capacitor can be seen in the following figure. As you can see it in figure, the IFT is, in fact, a parallel oscillatory circuit with a leg on its coil. The coil body has a ferrite core (symbolically shown with single upward straight dashed line) that can be moved (with screwdriver), which allows for the setting of the resonance frequency of the circuit, in our case 455 kHz. The same body contains another coil, with fewer quirks in it. Together with the bigger one it comprises the HF transformer that takes the signal from the oscillatory circuit into the next stage of the receiver. Both the coil and the capacitor C are placed in the square-shaped metal housing that measure 10x10x11 mm. From the bottom side of the housing you can see 5 pins emerging from the plastic stopper, that link the IFT to the PCB, being connected inside the IFT. Besides them, there are also two noses located on the bottom side, which are to be soldered and connected with the device ground. Japanese IFT's have the capacitor C placed in the cavity of the plastic stopper, as shown in figure. The part of the core that can be moved with the screwdriver can be seen through the eye on the top side of the housing, figure 10-d. This part is colored in order to distinguish the IFT's between themselves, since there are usually at least 3 of them in an AM receiver. The colours are white, yellow and black (the coil of the local oscillator is also being placed in such housing, but is being painted in red, to distinguish it from the IFT).x

Figure 10: Details and pin description of IF Filter

Detector Stage The detector stage is implemented with the easiest method that is with envelop detection. No description is necessary, only the circuit is given below. Please not that this method is known asynchronous detection.

Audio Amplifier Stage In order to get good and loud voice from the speaker it is essential to have an audio frequency (AF) amplifier or simply audio amplifier. For this purpose well-known audio amplifier IC LM386 is used. It is low priced and good quality IC. We can get 20 to 200 times amplification from it. Pin 5 gives the output, which in turn is connected with the loudspeaker. The speaker should be round about 10Ω rated to 1W. If speaker is not available just omit the LM386 and place a headphone just after the detector.

Figure 11: Audio Amplifier and LM386 pin description

FM Receiver As I described earlier that FM receiver is not much different with AM superheterodyne except the detector stage. A number of FM detection schemes have evolved over the years. The principal discrete ones were:

(a) F.M. Discriminator (figure 12)

Figure 12

This discriminator simply works on the principal that with no modulation applied to the carrier there is no output at the detector. Briefly T1 converts the f.m. signal to a.m. and when rectified the output is still zero because they would be equal but opposite in polarity, if modulation is applied then there is a shift in the phase of the input component with a corresponding difference in the signals out of the diodes. The difference between these outputs is the audio. As an aside, this is somewhat similar to some Automatic Fine Tuning (A.F.T.) schemes in some a.m. receivers, notably early T.V. receivers. With no frequency variation there is no output, with frequency drift there will be an output difference (in either direction) which is amplified and applied to front end tuning diodes for correction.

(b) Ratio Detector The schematic looks a little similar to figure 6 but has a third (tertiary) winding on the secondary of T1, diode D2 has its polarity reversed and the two divider resistors are replaced by capacitors. This scheme was quite popular in entertainment type receivers. You detect f.m. but NOT a.m. and it placed some relaxation on the severe limiting requirements.

(c) Crystal Discriminator Once favored by radio amateurs but superseded by later I.C. designs

(d) Phase Lock Loops Among the relatively newer designs and PLL's overcome many of the drawbacks and costs associated with building and aligning LC discriminators.

FM receiver with TDA7088T This IC is the successor of the famous TDA7000, i.e. it is an improved model of TDA7000, that allows to implement both monophonic and stereophonic FM receiver. The basic features of TDA7088T are given in the following table.

The electronic diagram of the HF part of the monophonic FM receiver made with TDA7088T IC is given on following figure. The IC contains all the parts of the classic superheterodyne receiver: the local oscillator, IF amplifier and FM detector, but also some other circuits that extend the possibilities and improve the features of this IC. As far as practical use is concerned, the most significant novelty is the auto-tuning circuitry. No variable capacitor is necessary for tuning, as it was in all the previous projects, the BB910 varicap diode is used instead.

Its capacitance is being changed by varying the DC voltage supplied to its anode over the 5k6 resistor. This is how the tuning is performed: When the user press and releases the pushbutton marked with “RUN”, the positive voltage impulse is released to the S(et) input of the SEARCH TUNING circuit. The 100 nF capacitor then starts chargingl and the voltage on the pin 16 increases. This voltage is then transferred, over the 5k6, to the anode of the BB910, causing its capacitance to decrease, which increases the frequency of the local oscillator (VCO). The VCO voltage is led into the mixer (MIXER) which also receives, over pin 11, the signals of all the other FM stations. The mixer outputs the FM signals whose frequencies are equal to the differences of the oscillator and the original station frequency. The only signal that can reach the demodulator (FM detector) is the one whose carrier frequency is equal to the inter-frequency, i.e. fm=73 kHz (selectivity is being accomplished by two active filters whose components are the

capacitors connected to pins 6, 7, 8, 9 and 10). Therefore, the oscillator frequency increases until it gets the condition fO-fS=73 kHz is accomplished. When this happens, the charging of the capacitor is halted by the command that is sent into the SEARCH TUNING circuit by two detectors (diode-blocks) located in the MUTE circuit. The AFC (Automatic Frequency Control) circuit now gets its role and prevents the voltage on pin 16 to be changed, until the RUN button is pushed again (this voltage can vary from 0 V til 1.8 V during the tuning). When the RESET button is pushed, the 100 nF capacitor is discharged, the voltage on pin 16 drops down to zero, and the receiver is set to the low end of the reception bandwidth, i.e. 88 MHz. Let us get back to the mixer. On its output, the 73 kHz FM signal is obtained, and it is modulated by the programme of the first station that is found after the RUN button is pushed. This signal then passes the active filters, gets amplified in the IF amplifier (IF LIMITER) and passed onto the input of the demodulator. By connecting the demodulator exit, over the LOOP FILTER, the adder (+) and resistor, to the VCO, the so-called FFL (Frequency Feedback Loop) circuit is accomplished, reducing the deviations of the signal being received from ±75 kHz to ±15 kHz. The LF (AF) signal is led from the demodulator, over the LOOP FILTER stage, the invertor (-1) and MUTE circuit onto the pin 2. The detectors (diode-blocks) control the operation of the MUTE circuit, preventing the LF (AF) signal to reach the output pin (2) until the tuning on the station that creates the signal in the antenna that is strong enough for quality reception is obtained.

Comparison between AM and FM 1. The first difference between two is their modulation technique. 2. Frequency modulation is superior to AM in the sense that there is very little or no effect of random noise on FM as compared to AM. The reason is that the noise in its nature can only change amplitude as in AM. The phase or frequency is usually not effected by the noise because of its nature, as in FM. 3. Commercial AM bands are between 550 kHz and 1600 kHz while FM is between 88 MHz and 108 MHz.xi 4. FM radio stations have 200 kHz of bandwith, with the carrier frequency in the middle. The range allows a broad range of audio frequencies to be represented allowing for better music. By contrast, AM radio stations are allocated only 10 kHz of bandwidth, with the carrier frequency again in the middle of the range. 5. Am has longer range because its wavelengths are so much larger. An AM wave is about 500 meters long; An FM wave is only around 10 meter long. 6. An AM wave seldom notices things as small as houses and buildings, as it can travel through them. An FM wave can be affected by anything at least a few feet wide as it bounces back after colliding with it. 7. The frequencies used in AM broadcasting are reflected by the ionosphere and by the earth so these frequencies can bounce back and forth. On the other hand, the frequencies used in FM broadcasting are not reflected by the ionosphere, so the reception of these signals is pretty much "line of sight", approximately.xii 8. There can be at most a maximum of (108-88)/0.2 = 100 stations on the FM dial. Whereas there can be at most (1605-535)/10 = 107 AM stations in an area. 9. In practice, the number of AM stations in an area is much lower for a number of other reasons. AM signals can be reflected from the ionospheric layer back to earth, so that the signals can reach unintended places that are thousands of miles away. Further, the ionospheric reflection is increased during the night time. Therefore, the AM signal from a powerful station in one city can be received in another city, which may be in another country. Consequently, AM signals are often subject to regulations such as the use of directional antennae or reduced power at nighttime or even going off the air at night.xiii 10. AM receivers are easy to built but difficult to tune or adjust for good performance. On the other hand, FM receivers are bit difficult to make but they can be optimized easily. 11. Additional circuitry may be required in AM design such as Automatic Gain Control (AGC) and Low Noise Filter (LNF) for better performance. 12. FM receiver can be easily manufactured in ICs and very easy to use. Most FM receivers today are made with the use of ICs such as TDA7000 or TDA7088A. 13. FM receiver requires small antennas as compared to AM receivers.

Here I will cite some survey data from the TGI Argentina study. This is a survey of 12,346 persons between 12 and 75 years old conducted by IBOPE Argentina during 1999-2000. These respondents were presented with a list of radio program types and ask their preferences for either AM or FM reception in listening to each program type. Each respondent may indicate an preference for AM or FM or indifference. The survey results are shown in the form of a scatter plot below.

(Source: TGI Argentina, IBOPE Argentina) The scatterplot contains a 45 degree diagonal line. Any radio program type that is below this diagonal line is preferred more in FM, and any program program type above this diagonal line is preferred more in AM. Clearly, the consumers prefer to listen to music in FM and to spoken words in AM. There are two interpretations of this phenomenon. First of all, this reflects the strengths of each transmission method --larger coverage with theoretically poorer quality for AM, and smaller coverage with theoretically better quality for FM. Alternately, this simply reflects the fact that AM stations carry mostly spoken words and FM stations carry most music, each playing to its own perceived strengths. Thus, the most and the best music programs are found on the FM spectrum, and the most and the best news programs are found on the AM spectrum.xiv

References

i

www.flw.com/define_m.htm www.shopzilla.com/3E--Portable_CD_Players_-_cat_id--11570000 iii http://www.mikroe.com/en/books/rrbook/ iv en.wikipedia.org/wiki/Frequency_modulation v William B. Kuhn “Design of Integrated, Low Power, Radio Receivers in BiCMOS Technologies”, 1995 vi W. M. Dalton The Story of Radio, Volumes 1-3, Adam Hilger, London, 1975. vii Image courtesy of Peter A. Stark copyright © 2002 viii http://en.wikipedia.org/wiki/Automatic_gain_control ix http://en.wikipedia.org/wiki/Automatic_gain_control#AM_radio x http://www.mikroe.com/en/books/rrbook/ xi http://staff.science.nus.edu.sg/~parwani/htw/c2/node72.html xii http://www.newton.dep.anl.gov/archive.htm xiii http://www.zonalatina.com/index.htm xiv http://www.zonalatina.com/Zldata72.htm ii