Super-heterodyne FM Receiver Design and Simulation
The design of a standard super-heterodyne receiver was performed with minor adjustments to remove interference to other ...
Super-heterodyne FM Receiver Design and Simulation Bhavya 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.
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. COMMERCIAL PARTS FOR RECEIVER Manufacturer RF Bandpass Filter RF Amplifier
Voltage Controlled Oscillator IF Bandpass Filter IF Amplifier
Part or Model Number 3303FM-20
Microwave Filter Company, Inc http://www.mwfilter.com/wideband _single_channel.htm Analog Devices http://www.analog.com/en/amplifier s-and-comparators/rfifamplifiers/adl5531/products/product .html Triquint Semiconductor http://www.triquint.com/prodserv/m ore_info/default.aspx?prod_id=WJZ 3020 Micronetics http://www.micronetics.com/produc ts/vcoseries.html?sort=freqrange&s ortdir=asc#null Network Sciences http://www.networksciences.com/21 4mhz.htm Richardson Electronics http://www.broadcastrichardson.com/ amplifiers.asp Analog Devices http://www.analog.com/en/rfifcomponents/modulatorsdemodulator s/ad8348/products/product.html
Impedance: 50 Ohm Insertion Loss: 1.5 dB (max) Passband: 88-108 MHz Frequency Range: 20-500 MHz Gain: 20 dB Isolation: -23 dB Noise Figure: 2.5 dB Output IP3: 41 dBm RF, LO, 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
Center Frequency: 21.4 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
Table 1 : Components of Receiver System
IV. PERFORMANCE ANALYSIS The performance is analyzed in terms of noise figure, gain and linearity. When a small signal is received by the receiver, sufficient gain must be present in the receiver in order for the FM radio to play the music data. The noise figure is important because it displays the difference in signal to noise ratio from the input to the output of the receiver. The degraded signal to noise ratio may affect the FM station’s music quality. If there is a high noise figure, the output signal to noise ratio is much less than the input signal to noise ratio. This causes the noise to be a huge interferer in the music or data being received. The noise figure should be low, for a good system. The linearity of the system is measured in terms of the third order intercept point. The farther the third order intercept point (output and input points) is away from the noise floor, the better the linearity of the system. The system performance was evaluated using SysCalc6. The system analyzed is shown in the figure below. The
input was chosen to be at 100 MHz frequency and input power of -60 dBm.
Figure 1: Receiver System in SysCalc
The noise figure of this system is 7.14 dB, which is reasonable for this system. The FM demodulator has a variable gain amplifier of 45 dB, therefore the total gain of the system is 104.40 dB. This seems like a high gain, but if the input signal is greatly attenuated, then the gain will be needed. The IIP3 is quite large, indicating that the system designed is highly linear. The minimum detectable signal (MDS) was found using the SysCalc6 software. The standard analysis revealed that
the MDS is about -122.9 dBm. 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, and the low MDS value indicates the capability of the system in detecting small signals. These performance parameters indicate the system is functioning well.
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. The VCO was simulated separately in Agilent to verify the functionality as well.
V. VERIFICATION OF FUNCTIONALITY The performance of the system was simulated, but the FM receiver should function like an FM receiver. The verification of this functionality was performed using the Agilent ADS software system. The first step was to FM modulate a 5 kHz tone onto an input frequency. The schematic for this system is shown below.
Figure 2: FM Modulation Circuit
Figure 3: VCO test circuit
The output of the VCO confirms that according to the voltage input, the frequency is varied on the output. The VCO is replaced with a frequency tone at the value required to achieve an IF frequency of 21.4 MHz. The entire FM system with the fm modulation, receiver and demodulation is shown in Figure. The input signal when modulated and sent through the receiver, the input tone was accurately receiver. The outputs of each stage were verified, but only the main figures are provided to show that the receiver works as intended.
Figure 4: Receiver System in Agilent ADS
Figure 5: FM modulated signal, center frequency 100 MHz
The modulated signal progresses through the RF filter because it is within the passband of FM radio. The signal is amplified then mixed down to 21.4 MHz IF frequency. The down converted signal is analyzed to make sure the modulated signal exists at the IF frequency. The signal is plotted with a center frequency of 21.4 MHz. dBm(fs(Downconverted,,,,,"Kaiser"))
The input to the receiver is a FM modulated signal with a 5 kHz tone. The input to the receiver is plotted with the center frequency being at 100 MHz.
50 0 -50 -100 -150 -200 -30
Figure 6: FM modulated signal, center frequency 21.4 MHz
The output of the receiver is indeed a 5 kHz tone, as seen in the following figure.
The voltage controlled oscillator creates a signal at 78.6 MHz. This signal directly leaks back into the RF input of the receiver, therefore a spurious emission results. The isolation and loss encountered by the oscillator signal leaking back attenuates it such that it doesn’t cause interference.
VII. FCC COMPATIBILITY
The FCC standards for intentional radiators state certain rules for FM broadcast. If the design of a transmitter was completed, these rules would have to be considered. The part 15 of the FCC rules places a broad requirement that the device doesn’t cause harmful interference. Since FM falls under the unlicensed operation category, the device mustn’t cause “harmful” interference and must accept any interference that may even cause undesired operation. Based on the emission spectrum analysis, it is noticeable that the other signals, besides the input signal, are not very strong. Most of the signals lie far below the sensitivity of most systems. The only signal that might cause some interference is at 78.6 MHz. The power of that signal is -101 dBm which is slightly above the sensitivity level of -102 dBm. This signal will not affect other FM radio receivers because it lies outside of the FM passband, therefore the RF filter will greatly attenuate the signal. For other systems the strength of the signal isn’t large, therefore harmful interference doesn’t seem to occur.
1.00 0.96 0.92 0.88 0.84 0.80 0.76 0.72 0.68 0.64 0.60 0.56 0.52 0.48 0.44 0.40 0.36 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00
Figure 7: FM Receiver Output
VI. EMISSION SPECTRUM ANALYSIS The spurious emissions at the antenna connector are simulated, envelope simulation, to check if the signals cause any interference. The isolation of the mixer prevents the leakage of signals affecting the antenna. When the modulation occurs at 100 MHz, the emission spectrum is as shown below. 200 0
-600 -800 -1000 -1200 -1400 0
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. Some of the spectrum values are shown in Table 2. The rest are around the -400 dBm mark. Frequency 200 MHz 178.6 MHz 157.2 MHz 135.8 MHz 121.4 MHz 100 MHz 78.6 MHz 57.2 MHz 42.8 MHz
Emission Spectrum Value -397.396 dBm -390.107 dBm -377.175 dBm -377.048 dBm -364.073 dBm 26.704 dBm -101.691 dBm -383.492 dBm -396.26 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. Although standard receiver architectures are utilized, the components, IF frequency, and gain distribution need to be greatly considered for a good receiver design. The concept that an FM receiver can cause interference was understood by completing this project. The tools for RF system simulation, SysCalc6 and Agilent ADS, were learned and proved to be very useful for this project. The receiver systems designed was chosen to obtain an understanding of the standard heterodyne receiver. REFERENCES  S. J. Erst, Receiving Systems Design. Dedham, MA: Artech House, 1984.  T. Vito and K. McClaning, Radio Receiver Design. Atlanta, GA: Noble Publishing Corp, 2000.  B. Razavi, RF Microelectronics, Upper Saddle River, NJ: Prentice Hall PTR, 1998.  D. Pozar, Microwave and RF Design of Wireless Systems, John Wiley & Sons Inc, 2001.