Super-heterodyne FM Receiver Design and SimulationBhavya Daya University of Florida, Gainesville, FL, 32608, USA Abstract – The design of a standard super-heterodyne receiver was performed with minor adjustments to remove interference to other FM radios. Spurious emission emitted from the receive antenna can directly affect other nearby FM radio receivers. The local oscillator is at the heart of the problem. The design was adjusted to have a higher IF frequency to avoid interference emissions to occur in the FM radio passband. Even though spurious emissions resulted, most were too low to be detected by other systems. The components of the system were carefully chosen to ensure that the receiver doesn’t affect other systems and good receiver performance resulted. I. INTRODUCTION The FM radio receiver can cause interference to nearby FM radios. It may seem unlikely because the radio receiver only receives signals and doesn’t transmit signals causing interference. This interference phenomenon was observed in a demo performed in class. Two nearby portable FM radios, when stationed at 102.3 MHz and 91.6 MHz, cause a jamming effect on the 102.3 MHz radio station. The 91.6 MHz station is not broadcasted on, but the static on this channel interferes with the other station. The source of this interference is the local oscillator (LO) used in the receiver. The local oscillator usually creates an intermediate frequency (IF) frequency at 10.7 MHz for a FM receiver. This means the local oscillator is tuned such that the IF frequency is always equal to 10.7 MHz. As can be observed from the experiment, the interference is caused by a channel that is 10.7 MHz away from it. This generated 10.7 MHz signal mixes with the signal of a channel and creates interference at about 10.7 MHz up and down from the channel. The objective of this project was to design a FM radio receiver that will not cause interference to other FM radios. The most common receiver architecture is the superheterodyne receiver and this architecture was chosen for the design. II. OVERVIEW OF RECEIVER DESIGN When designing using the superheterodyne architecture, certain considerations must be addressed. The first decision was to use a down conversion or up conversion receiver. Down conversion means the input signal frequency converts to an IF frequency that is lower than the input frequency. Up conversion is when the conversion to a higher IF frequency occurs. Since the input FM signal ranges from 88 to 108 MHz, down conversion is easier to accomplish due to availability of filters and number of conversions required. When choosing an IF frequency, the availability of the channel select filter is a large determining factor. The two most readily available channel select filters are at 10.7 MHz and 71 MHz. More than one conversion, mixer and local oscillator, is needed for up conversion which unnecessarily complicates the FM radio receiver. Sometimes down conversion requires multiple conversion but since the FM radio frequencies aren’t too high, only one conversion stage is utilized. The second decision was the IF frequency. The problem of interference to other FM radios stemmed from the IF frequency and local oscillator. In order for the interference to not occur, the IF frequency was increased to be fixed at 21.4 MHz. A higher IF frequency decreases the need for an image rejection filter because the image is greatly attenuated through the band pass filter. In this case, the image occurs at 21.4 MHz below the local oscillator frequency and the desired band occurs at 21.4 MHz above the local oscillator frequency. The higher band is used as the desired band, thus the local oscillator can be lower than FM radio band. The oscillation frequencies were chosen to range from 66.6 MHz to 86.6 MHz, resulting in a tuning ratio of 1.3. Minimizing the LO frequency facilitates the design of the oscillator, making it highly desirable. The image frequency, two times the IF frequency away from the input RF frequency, was found to range from 45.2 MHz to 65.2 MHz along the FM radio band. Therefore, the image frequency will be attenuated by the filtering performed by the RF Bandpass Filter. The gain distribution of the cascaded receiver architecture was contemplated to yield high linearity. Most of the gain was placed after the IF filter for stability and linearity. Enough gain had to be supplied so that the system can process its minimum discernable signal (MDS). The excess gain is the amount of gain between the antenna and any given point in the receiver. At low levels of excess, individual components contribute too much noise to the cascade and at high levels individual components add distortion. The effects of excess gain were considered when performing gain distribution. III.broadcastrichardson.com/21 4mhz. The FM demodulator has a variable gain amplifier of 45 dB.com/wideband _single_channel. but if the input signal is greatly attenuated.14 dB.52 Table 1 : Components of Receiver System IV.micronetics.mwfilter.5 dB (max) Passband: 88-108 MHz Frequency Range: 20-500 MHz Gain: 20 dB Isolation: -23 dB Noise Figure: 2. This seems like a high gain. the better the linearity of the system. PERFORMANCE ANALYSIS The performance is analyzed in terms of noise figure.aspx?prod_id=WJZ 3020 Micronetics http://www. The farther the third order intercept point (output and input points) is away from the noise floor. The degraded signal to noise ratio may affect the FM station’s music quality.networksciences.com/produc ts/vcoseries. therefore the total gain of the system is 104.5 dB Output IP3: 41 dBm RF. indicating that the system designed is highly linear. The standard analysis revealed that . gain and linearity. The IIP3 is quite large. then the gain will be needed. The system performance was evaluated using SysCalc6.analog.html Triquint Semiconductor http://www. for a good system.htm Richardson Electronics http://www.htm Analog Devices http://www. The noise figure should be low.4 MHz Bandwidth: 200 kHz Insertion Loss: 6 dB Freq: 1-500 MHz Gain: 40 dB Noise Figure: 6 dB RF Input Range: 50-1000 MHz Demod Bandwidth: 75 MHz Noise Figure: 11 dB Input IP3: 28 dBm RPAMO1500M10 AD8348 $1. The noise figure is important because it displays the difference in signal to noise ratio from the input to the output of the receiver. This causes the noise to be a huge interferer in the music or data being received. The linearity of the system is measured in terms of the third order intercept point. LO. COMMERCIAL PARTS FOR RECEIVER Manufacturer RF Bandpass Filter RF Amplifier Mixer Voltage Controlled Oscillator IF Bandpass Filter IF Amplifier FM Demodulator Part or Model Number 3303FM-20 Microwave Filter Company.87 N/A N/A N/A N/A $5.com/en/amplifier s-and-comparators/rfifamplifiers/adl5531/products/product .html ADL5531 WJZ3020-PCB MW500-1412 20024 Specifications Cost Impedance: 50 Ohm Insertion Loss: 1.com/en/rfifcomponents/modulatorsdemodulator s/ad8348/products/product.40 dB. The minimum detectable signal (MDS) was found using the SysCalc6 software.com/prodserv/m ore_info/default. When a small signal is received by the receiver. The system analyzed is shown in the figure below. Figure 1: Receiver System in SysCalc The noise figure of this system is 7. Inc http://www. which is reasonable for this system.com/ amplifiers. If there is a high noise figure. sufficient gain must be present in the receiver in order for the FM radio to play the music data.asp Analog Devices http://www. The input was chosen to be at 100 MHz frequency and input power of -60 dBm.analog. IF: 10-250 MHz Conversion loss: 7 dB LR Isolation: 64 dB (54 min) LI Isolation: 46 dB(36 min) RI Isolation: 38 dB Frequency Range: 50-100 MHz Tuning Voltage: 1 V to 18 V N/A Center Frequency: 21.triquint. the output signal to noise ratio is much less than the input signal to noise ratio.html?sort=freqrange&s ortdir=asc#null Network Sciences http://www. KHz Figure 6: FM modulated signal.4 MHz. but the FM receiver should function like an FM receiver. dBm(fs(Downconverted[1]. The signal is amplified then mixed down to 21. Figure 4: Receiver System in Agilent ADS 50 0 -50 -100 -150 -30 -20 -10 0 10 20 30 freq. center frequency 100 MHz The modulated signal progresses through the RF filter because it is within the passband of FM radio.4 MHz.4 MHz IF frequency. the frequency is varied on the output."Kaiser")) The input to the receiver is a FM modulated signal with a 5 kHz tone. the input tone was accurately receiver. The outputs of each stage were verified. This FM modulation schematic is connected to the receiver designed earlier to determine if the receiver obtains a 5 kHz tone after the processing steps. V. The entire FM system with the fm modulation."Kaiser")) dBm(fs(FMmodulated[5]. The input signal when modulated and sent through the receiver. These performance parameters indicate the system is functioning well. center frequency 21... The sensitivity of the system is equal to the MDS because a signal to noise ratio requirement wasn’t specified. The sensitivity is better when it is low.. VERIFICATION OF FUNCTIONALITY The performance of the system was simulated..the MDS is about -122.9 dBm. Figure 2: FM Modulation Circuit Figure 3: VCO test circuit The output of the VCO confirms that according to the voltage input.. 50 0 -50 -100 -150 -200 -30 -20 -10 0 10 20 freq. The signal is plotted with a center frequency of 21... The input to the receiver is plotted with the center frequency being at 100 MHz. receiver and demodulation is shown in Figure.4 MHz 30 . The VCO is replaced with a frequency tone at the value required to achieve an IF frequency of 21. The first step was to FM modulate a 5 kHz tone onto an input frequency. but only the main figures are provided to show that the receiver works as intended. The schematic for this system is shown below.. and the low MDS value indicates the capability of the system in detecting small signals. The VCO was simulated separately in Agilent to verify the functionality as well. The down converted signal is analyzed to make sure the modulated signal exists at the IF frequency. The verification of this functionality was performed using the Agilent ADS software system. KHz Figure 5: FM modulated signal. 492 dBm -396. MA: Artech House. MHz Figure 8: Spurious Emissions at Antenna Connector The emission spectrum shows that spurious emissions are present at the antenna connector other than the input signal at 100 MHz. The concept that an FM receiver can cause interference was understood by completing this project. Receiving Systems Design.691 dBm -383. were learned and proved to be very useful for this project. J.4 -0.6 MHz. Based on the emission spectrum analysis. therefore a spurious emission results. McClaning. Pozar.20 0.6 MHz 157. IF frequency. -0.72 0. Dedham. GA: Noble Publishing Corp.8 MHz 121. . 200 0 dBm(FMmodulated) -200 -400 VIII. besides the input signal. EMISSION SPECTRUM ANALYSIS The spurious emissions at the antenna connector are simulated. 1984. Since FM falls under the unlicensed operation category. real(FMdemodulated[0]) 0.16 0.60 0.2 MHz 135. Frequency 200 MHz 178.88 0. RF Microelectronics. The rest are around the -400 dBm mark. 2001. the components. envelope simulation. The isolation and loss encountered by the oscillator signal leaking back attenuates it such that it doesn’t cause interference.107 dBm -377.396 dBm -390.048 dBm -364. CONCLUSION -600 -800 -1000 -1200 -1400 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 freq.2 The FCC standards for intentional radiators state certain rules for FM broadcast.073 dBm 26.4 MHz 100 MHz 78.175 dBm -377.6 1.56 0.0 VII. Some of the spectrum values are shown in Table 2.6 0. [3] B. msec Figure 7: FM Receiver Output VI. The only signal that might cause some interference is at 78. 1998. as seen in the following figure.2 0. Vito and K. Razavi. Radio Receiver Design. these rules would have to be considered.2 MHz 42. to check if the signals cause any interference. and gain distribution need to be greatly considered for a good receiver design.64 0.12 0.96 0. the device mustn’t cause “harmful” interference and must accept any interference that may even cause undesired operation. This signal will not affect other FM radio receivers because it lies outside of the FM passband.24 0. [4] D.704 dBm -101. This signal directly leaks back into the RF input of the receiver. The power of that signal is -101 dBm which is slightly above the sensitivity level of -102 dBm. Upper Saddle River.8 MHz Emission Spectrum Value -397. NJ: Prentice Hall PTR.26 dBm Table 2: Spurious Emission Spectrum Values The design and simulation of a FM radio receiver greatly increased my understanding of the function of the receiver.48 0. SysCalc6 and Agilent ADS. For other systems the strength of the signal isn’t large. the emission spectrum is as shown below. it is noticeable that the other signals.76 0.68 0.28 0. Erst. [2] T. 2000. REFERENCES [1] S.08 0. The part 15 of the FCC rules places a broad requirement that the device doesn’t cause harmful interference.The output of the receiver is indeed a 5 kHz tone. If the design of a transmitter was completed.04 0. Atlanta.84 0.00 time.32 0.92 0.6 MHz 57. are not very strong. Microwave and RF Design of Wireless Systems.80 0.44 0. When the modulation occurs at 100 MHz.52 0.36 0. The receiver systems designed was chosen to obtain an understanding of the standard heterodyne receiver. 0. The tools for RF system simulation. therefore harmful interference doesn’t seem to occur.40 0. The isolation of the mixer prevents the leakage of signals affecting the antenna. Although standard receiver architectures are utilized.00 0.4 The voltage controlled oscillator creates a signal at 78.6 MHz. FCC COMPATIBILITY -0. Most of the signals lie far below the sensitivity of most systems. John Wiley & Sons Inc. therefore the RF filter will greatly attenuate the signal.
Report "Super-heterodyne FM Receiver Design and Simulation"