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.
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. 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, 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
ADL5531
WJZ3020-PCB
MW500-1412
20024
Specifications
Cost
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
N/A
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
RPAMO1500M10
AD8348
$1.87
N/A
N/A
N/A
N/A
$5.52
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
50
0
-50
-100
-150
-30
-20
-10
0
10
20
30
freq, KHz
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[1],,,,,"Kaiser"))
dBm(fs(FMmodulated[5],,,,,"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
-20
-10
0
10
20
freq, KHz
Figure 6: FM modulated signal, center frequency 21.4 MHz
30
The output of the receiver is indeed a 5 kHz tone, as seen
in the following figure.
real(FMdemodulated[0])
0.6
0.4
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.
0.2
0.0
VII. FCC COMPATIBILITY
-0.2
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.
-0.4
-0.6
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
time, msec
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
dBm(FMmodulated)
-200
-400
VIII.
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, 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. 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
[1] S. J. Erst, Receiving Systems Design. Dedham, MA:
Artech House, 1984.
[2] T. Vito and K. McClaning, Radio Receiver Design.
Atlanta, GA: Noble Publishing Corp, 2000.
[3] B. Razavi, RF Microelectronics, Upper Saddle River,
NJ: Prentice Hall PTR, 1998.
[4] D. Pozar, Microwave and RF Design of Wireless
Systems, John Wiley & Sons Inc, 2001.