EE3333 001

Project 2 Group 4

FM Remote Control System

Peng Zhao

Texas Tech University

December 06, 2006

Instructor: Dr. Dickens

Advisor: Dr. Karp

Team Members: Rachel Moore, Ikenna Okonkwo, Kevin Hooper

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Abstract

This paper describes a technical overview of a FM remote control system for West Texas

Best robotics competition. The current FM remote control system provided by West Texas Best

is somewhat expensive. The exploration of a new inexpensive system will be carried out through

this project. The FM communication system has to comply with FCC requirements and operate

in a frequency range between 75.410 MHz to 75.990 MHz. This project uses channel 90 at

frequency 75.990 MHz with a bandwidth of no more than 8 KHz. The system consists of various

components such as voltage controlled oscillator, mixers, filters, power amplifier, and phase

locked loop to work together. These individual components will be discussed in further detail

including their requirements, analysis, and capabilities. Finally, the integrated of the transmitter

and receiver circuits will be reviewed.

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Table of Contents

1 Introduction............................................................................................................................. 3

1.1 Transmitter...................................................................................................................... 3

1.2 Receiver .......................................................................................................................... 4

2 Voltage Controlled Oscillator ................................................................................................. 5

3 Local Oscillator..................................................................................................................... 10

4 Mixer..................................................................................................................................... 11

5 Bandpass Filter...................................................................................................................... 13

6 Power Amplifier.................................................................................................................... 16

7 Phase Locked Loop............................................................................................................... 19

8 Overall System...................................................................................................................... 21

8.1 Transmitter Integration ................................................................................................. 21

8.2 Receiver Integration...................................................................................................... 23

8.3 FM Transmission .......................................................................................................... 26

9 Conclusion ............................................................................................................................ 28

References..................................................................................................................................... 29

Appendix A List of Figures ....................................................................................................... 30

Appendix B Data Tables and Pictures ....................................................................................... 31

Appendix C Gantt Chart and Budget ......................................................................................... 33

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1 Introduction

Modern robotics systems widely use FM modulation to control various components. The

basic principle is to modulate a control signal by varying the frequency accordingly which is

called frequency modulation, or FM. This signal is then prepared by filters and amplifiers to

transmit a signal through air to the receiver. The receiver does the exact opposite by

demodulating the signal to obtain its original control signal.

Project labs at Texas Tech University have close ties with West Texas Best robotics. The

remote control system used in these competitions consists of a Conquest T4NBF transmitter and

a Futaba R127DF receiver and they are becoming more expensive. The objective of this project

is to design, build, and test a single stage transmitter and a dual stage receiver operating at

channel 90 with a center frequency of 75.990 MHz with a bandwidth of no more than 8 KHz.

Each team member is assigned with an individual component and the whole idea of the project is

to complete separate pieces and then integrate the overall system at the end.

1.1 Transmitter

The transmitter uses a single stage design for mixing and filtering, which is part of the

design requirement. In frequency modulation, the modulated signal varies in frequency

depending on the input voltage. The typical control signal of remote control units is a square

wave of 1s and 0s with a certain baud rate. The FM modulator has corresponding frequencies for

these states. The modulated signal has a bandwidth of no more than 8 KHz. This is also the

modulation deviation, which is the amount of variation in frequency. A voltage controlled

oscillator, or VCO, is commonly used to achieve this modulation. An overview of the transmitter

is shown in figure 1.1.

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Figure 1.1: Single Stage Transmitter Block Diagram

First, the VCO modulates an input signal at 10.7 MHz, and then a mixer will mix this signal with

a local oscillator operating at 65.290 MHz to step up the frequency to 75.990 MHz. This signal

has many harmonics, so a bandpass filter is connected to remove the unwanted signals. Due to

the losses in the system and to achieve long transmission distance, the signal has to be amplified

using a power amplifier.

1.2 Receiver

The receiver is a dual stage design which mixes and filters the signal twice before feeding

the signal to the demodulator. Upon receiving the transmitted signal, the first priority is to clean

the signal with a 75 MHz bandpass filter. Then a mixer mixes that signal with a local oscillator at

65.290 MHz as indicated in figure 1.2 to step down the frequency to 10.7 MHz. At this point, the

signal needs to be amplified again using an amplifier. The 10.7 MHz bandpass filter takes the

lower frequency from the mixer. At the second stage, the signal is mixed again with a local

oscillator at 10.245 MHz to produce a frequency difference at 455 KHz.

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Figure 1.2: Dual Stage Receiver Block Diagram

A 455 KHz bandpass filter further filters out the noise and neighboring channels, which are 20

KHz away from the desired frequency. At this point, the demodulator, in this case, is a phase

locked loop demodulates the signal and ultimately produces a signal that corresponds to the input

signal from the transmitter side.

2 Voltage Controlled Oscillator

The voltage controlled oscillator, or VCO, is essentially a dynamic oscillator that

translates the voltage difference from an input signal to a frequency difference, which achieves

FM modulation. The VCO can be simplified as a normal oscillator with a given frequency, then

combining it with a dynamic network to changes this frequency according to the voltage.

An oscillator cannot be modeled ideally that only occurs in the real world by using

unstable elements to cause it to oscillate at some fixed frequency.

Figure 2.1: Colpitts Oscillator [6] Figure 2.2: Varactor Capacitance Curve

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A Colpitts oscillator design (Figure 2.1) is perfect this purpose and as it connects to a common

base amplifier as a feedback, it creates an overall gain of unity and phase shift of 360 , which are the requirements for an oscillation to start. Further, this type of oscillator is advantageous as

it only has one inductor, because the project requirement states that the system must be

constructed using discreet components and hand-wound inductors, which is difficult to construct

with precision. There is also another condition to be considered; initially the amplifier should

have an open loop gain higher than unity and the Colpitts ratio of two capacitors to initialize the

oscillation. The equation below describes the initial condition:

1

2

CCRG Lm > (2.1) [6]

The dynamic network is formed by setting up a varactor diode in shunt with the oscillator.

This diode is the key to modulating the signal. When the varactor diode is reverse biased by a

DC voltage, the electrons in the diode gets pulled away to form a gap that looks like a capacitor.

A capacitance curve of the diode in figure 2.2 shows when the voltage increases, the capacitance

decreases exponentially. According to equation 2.2, the oscillation frequency is inversely

proportional to total inductance and capacitance of the system.

LCf 2

1= (2.2) [6]

When the input signal act as the voltage source for the varactor diode, the capacitance change

corresponds to the signal, and ultimately changes the oscillation frequency, which creates the

modulation signal.

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Figure 2.3: 10.7 MHz VCO with Buffer Amplifier

In the single stage transmitter, the VCO is required to modulate at 10.7 MHz with a

deviation of no more than 8 KHz and enough power to compensate for power loss in the

upcoming stage. The design (Figure 2.3) uses two 2N3904 transistors that have a minimum gain

bandwidth product of 300 MHz [8], which sets the maximum gain of the amplifier to be 30 when

operating at 10.7 MHz. One for the oscillator and the other is for the buffer stage in the output

side which will be discussed later in this section. To the left side is the normal oscillator with

Colpitts configuration formed by L1, C1, and C2. The modulation frequency is determined by the

equation below.

)||(21

211 CCLf = (4.3)

The varactor network consists of the varactor diode Cv, offset capacitor Co, diode bias resistor R5,

and input voltage source Vin. With this network the overall frequency equation changes to:

)||||(21

211 vo CCCCLf += (2.4)

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The varactor diode is a NTE612 diode at 10 to 13pF when supplied with 4V [7]. The capacitance

slope, the capacitance at 30V supply divided by capacitance at 2V, is 2.9 for this diode [7]. There

is an offset capacitor Co is in series with the varactor diode, so mathematically, the overall

capacitance is obtained by calculating the values in parallel. Therefore, this capacitor sets the

influence of the varactor diode on the system and ultimately sets the modulation deviation.

Figure 2.4: VCO Circuit

The actual circuit (Figure 2.4) is tightly packed together on the protoboard to reduce stray

inductance or capacitance from interfering with modulation frequency. The capacitor values have

decreased compared to initial calculations due to the increased inductance at this frequency.

Most of the components have a tolerance of 10% which is appropriate for this project.

To analyze the capability of this system, a modulation curve (Figure 2.5) is constructed

by collecting frequency data while varying the input voltage from 0 to 5V in steps of 0.5V. The

offset capacitor is found to be 3pF to set the modulation deviation at 7 KHz from 10.708 MHz to

10.715MHz indicated in the results below. It is somewhat unstable due to increased size of the

Colpitts capacitors as it has a standard deviation of 3 KHz. The frequency range is obtained by

forming a moving average of the data points.

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VCO Voltage Vs. Frequency All Data

10.70310.70410.70510.70610.70710.70810.70910.71010.71110.71210.71310.71410.71510.71610.71710.71810.719

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Input Voltage (V)

Out

put F

requ

ency

(MH

z)

Data Points

MovingAverage

Figure 2.5: Modulation Curve

The output power is checked by using a spectrum analyzer which sweeps the power in

dBm in a frequency range. When the circuit is fed with 12V supply, the output power as shown

in figure 8 is 17.11dBm, which is higher than the 10dBm of what the function generators can

output.

Figure 2.6: Output Power

Since the modulation frequency depends on capacitance and inductance according to

equation 2.2, any output capacitors and inductors can change the modulation frequency. Even

when testing the VCO, if the output is connected to a probe that has enough capacitance, the

frequency indicated will be off. Connecting a buffer to the output (Figure 2.6) allows the VCO to

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be isolated from the other devices and reduces its output impedance. A buffer amplifier has high

input impedance, low output impedance, and a gain of unity. An emitter follower amplifier is

used between the VCO and the mixer as a buffer amplifier. The biasing on the emitter follower is

similar to the common emitter configuration used in the VCO except that since output is on the

emitter side, the emitter voltage has to be great enough to allow the voltage swing seen from the

VCO.

Figure 2.7: Buffer Stage Output

The buffer amplifier response (Figure 2.7) obtained from the oscilloscope is the yellow

curve and the original output from the VCO is the green curve. The buffer output has lower

amplitude with a gain of 98% and about 7.83V peak-to-peak voltage (inconsistent with 17dBm

output power because this is tested using 9V supply).

3 Local Oscillator

Mixers use local oscillators as reference to step up or down the frequency. The overall signal

quality depends on the quality of local oscillators and VCO. These local oscillators are

essentially just the Colpitts oscillator part (Figure 2.3) of the VCO. There are two 65.290 MHz

oscillators and one 10.245 MHz oscillator. These should have fairly strong power output to make

the mixer output signals strong.

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Figure 3.1: (a) Local Oscillator Layout (b) Local Oscillator Response

The oscillator layout (Figure 3.1a) is made so the circuit can be milled multiple times. This

layout also has a buffer stage for convenience. A spectrum analyzer is used because of its

accuracy to help adjusting the center frequency. As shown in (Figure 3.2b), the oscillators each

can generate 16dBm of power using a 12V supply. There are many spikes along the side of the

main peak, and it moves left and right, which shows the oscillators to be unstable. The size of the

offset capacitors may be too small to stabilize the oscillator operating frequency.

4 Mixer

Mixers are widely used to step up or step down the frequency in radio communication

systems. This is done based on a trigonometric identity as the following.

( ) ( )( ) ++= coscos21coscos (4.1) [1]

This equation indicates that if two sinusoidal waves with different frequencies are to be

multiplied, the result has half the magnitude with two components. One is the sum and the other

is the difference of the two frequencies. A mixer is used in each stage of the transmitter and the

receiver. A transmitter mixer operates at the frequency of the sum to step up the overall

frequency. A receiver mixers operating frequency is the difference of the originals to step down

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the overall frequency for demodulation. All three mixers in the system use the same design

(Figure 4.1) have different operating frequencies (Table I).

Figure 4.1: General Mixer Design [1]

This design uses a bipolar transistor in common emitter configuration and also uses 2N3904

because of its gain bandwidth product. The modulated signal, or RF, that goes into the base of

the transistor and the local oscillator, or LO, which goes into the emitter leg. At the collector side,

after multiplying the two signals together, two frequencies are produces along with some amount

of harmonics. A filter is usually connected after the mixer to take out the unnecessary

frequencies such as harmonics. The operating frequency of the mixer is set by L1 and C6 in ratio

(Table I) that satisfies equation 2.2.

Table I: Mixer Component Values [1]

A series of tests are conducted to determine the performance of the mixers. The most

important characteristic is the gain. This is tested by connecting them to a network analyzer to

check the gain at many frequencies. The input from the analyzer is at 0dBm.

Transmitter Receiver 1 Receiver 2 fIF = 75.990 MHz fIF = 10.7 MHz fIF = 455 KHz

L1 = 400 nH L1 = 2.903 H L1 = 3.6 H C6 = 11 pF C6 = 76.21 pF C6 = 0.033 F

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Figure 4.2: (a) 455 KHz Mixer Loss (3.6 H) [1] (b) 455 KHz Mixer Loss (33 H) [1]

The 455 KHz mixers initial loss (Figure 4.2a) is -21dB using a 3.6 H inductor. The analyzer results show the operating frequency with the best gain is at 4.5 MHz, so increasing the inductor

(Figure 4.2b) to 33 H makes the loss at 455 KHz to be -7dB. The other mixers are also checked for matching operating frequencies. The 10.7 MHz mixer has a loss of -3dB at 0dBm input

power, and 75 MHz mixer has a loss of -2dB. All three mixers are adjusted to their best

performance by shifting center frequency, and best response by switching RF and LO inputs.

5 Bandpass Filter

A bandpass filter is commonly used to filter out the unwanted signal such as harmonics.

Only the frequencies around a center frequency can be seen after the filter. The lower

frequencies and higher frequencies have attenuation below -20 dB which is neglected. There are

four filters in this project, which consist of two 75 MHz filters, one 10.7 MHz filter, and one 455

KHz filter. All of these have strict requirements to make the whole system work. The filter has

several properties to determine its quality and usefulness. Bandwidth is the amount of

information to be passed and is usually determined by the difference between the two 3dB points

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below the center frequency. Quality factor, Q, is the slope of the two sides of the frequency

response curve. This is important, because it determines the rate of the attenuation rate away

from the center frequency. The attenuation for the center frequency is also important. If the

center frequency has a lot of loss then it means the filter is unacceptable.

The two 75 MHz bandpass filters and one 10.7 MHz bandpass filter use the third order

Butterworth PI section design shown below.

Figure 5.1: 3rd Order Butterworth PI Filter [4]

The three stages have to match the same center frequency conditions according to equation 2.2

for choosing inductors and capacitors. The center stage has more influence on the Q of the filter.

The two side stages set how the side slopes drop off. The Ls value for the 455 KHz filter is

around 500 to 700 H which is too big to wind and induces a lot of attenuation. The values for the first two filters are chosen as shown in the figure below.

Figure 5.2: Butterworth Filter Values

The response is obtained using the network analyzer which sweeps the gain for a wide frequency

range. The 75 MHz filter response shows (Figure 5.3a) a loss of -5dB with -20dB frequencies at

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65 MHz and 85 MHz, which means the filter will reject signals below 65 MHz and 85 MHz

significantly. Its Q is 19, which makes the bandwidth to be 4 MHz.

Figure 5.3: (a) 75 MHz Filter Response (b) 10.7 MHz Filter Response [2]

The 10.7 MHz filter has a loss of -5dB (Figure 5.3b) with a Q of 26, and rejects signals below 9

MHz and above 12 MHz.

The 455 KHz bandpass filter uses a different design (Figure 5.4) that has only a capacitor

to be on the top stage as shown in figure 19. This design is slightly more difficult to adjust the

center frequency because the two side stages have equal influence on the center frequency. The

inductor and capacitor values are obtained from equation 5.1.

Figure 5.4: 455 KHz Bandpass Filter Design

RffffCC

RffffC

ffRffLL

)(2

4

2)(

122

132

12

121

12

1221 ==

+=== (5.1) [4]

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Since this filters output side connects to the phase locked loop, which has an input impedance of

approximately 2 K , the values will be different.

Figure 5.5: 455 KHz Bandpass Filter Response [2]

The result from analyzer (Figure 5.5) shows that it has a gain of almost 0dB with a Q of 9 and a

bandwidth of 45 KHz. The filter is not sufficient for this projects requirement, since the

neighboring channels are at 20 KHz away and has a bandwidth of 5 KHz; the bandwidth of this

filter should be no more than 30 KHz. But due to time constraints, this is the best the team can

offer.

6 Power Amplifier

The signal must be amplified before transmitting to obtain better transmission distance.

The power amplifier design (Figure 6.1) consists of a preamplifier and a class C amplifier

configuration. The connection between these stages and the overall output has impedance

transformers to match the impedances and ultimately perform maximum power transfer. The

transistors bandwidth product or Ft plays an important role here since it determines the gain

limit at a certain frequency. 2N5109 is the bipolar transistor used and it has a Ft of 1200 MHz.

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The preamplifiers job is to bring a low level signal up to a level that the power amplifier can

take.

Figure 6.1: Power Amplifier Design [3]

The second stage is the main part of this amplifier and it uses a diode with fast reverse

recovery time and low forward voltage drop to reduce the DC build up in the capacitors that

hinder the overall performance.

Figure 6.2: Diode Clipping Effect [3]

The diode turns on (Figure 6.2) when the base reaches its forward voltage and clips the negative

side, which mirrors the transistor switching effect and creates a wave with 50% duty cycle. The

diode FD700 has a reverse recovery time of 900ps and is used for the second stage.

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21

21.5

22

22.5

23

23.5

24

24.5

25

-10 -5 0 5 10

Power In (dBm)

Pow

er O

ut (d

Bm

)

P_out 1P_out 2P out 3

Figure 6.3: Overall Gain [3]

The overall curve (Figure 6.3) is obtained by recording the output power while varying

the input power. The power output reaches saturation when input power becomes 8.5dBm as

seen from the three trials. This class C stage is tested to be promising with another teams

preamplifier and produced an overall 24dBm output from 5dBm input power. The power

amplifiers operating frequency is checked by using a network analyzer using a 10dB attenuator.

Figure 6.4: Center Frequency Adjustment [3]

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The response (Figure 6.4) shows that it is centered at 76 MHz, but since the peak flats out at that

frequency, adjusting to 75 MHz makes no difference. This also confirms that gain to be 24 dBm,

which translates to 250 mW of power.

7 Phase Locked Loop

The phase locked loop, or PLL, is used as the demodulator of the system at the very end

of the receiver which is the counter part of the modulator. It has three main components, the

phase comparator, the oscillator, and the feedback loop. The chip used is TI CD74HC4046A,

which has a PLL and a VCO. First, the VCO is set up to be 455 KHz as the reference signal for

the PLL. The phase comparator compares the phase of the input and the reference signal. The

output voltage changes linearly with the difference between the input signal and the reference

signal, and therefore it is demodulating a radio signal. After the phase comparator, a feedback

loop consist of a low pass filter connects back to the input of the VCO. The cutoff frequency of

the low pass filter controls the PLLs modulation rate. This is the capacity of data the

demodulator can handle.

Figure 7.1: (a) PLL Schematic [2] (b) PLL Layout [2]

The overall schematic of the PLL demodulator is shown in figure 7.1a. The top half is the

PLL, and the bottom half is the VCO. Once the feedback loop closes, the PLL starts locking to

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the frequency and continues the loop. The output voltage after the filter should be similar to the

control signal modulated at the transmitter side.

Figure 7.2: (a) Milled Board Response [2] (b) Bread Board Response [2]

The finalized circuit is milled from the layout in figure 7.1b, and a comparison test was

conducted between the milled board and bread board. In figure 7.2a, the milled board PLL has

higher amplitude compared to the bread board from figure 7.2b. This is due to more stray

capacitance seen in the milled board since the ground plane is surrounding the traces, but at a 30

mil distance.

Figure 7.3: Demodulation Curve (Okonkwo)

The demodulation response (Figure 7.3) is a frequency vs. voltage graph. Here, the voltage

depends on the frequency linearly, which should be opposite of modulation curve (Figure 2.5),

which the frequency depends on the voltage. This demodulator is able to lock to frequency range

from 430 KHz to almost 480 KHz with voltage variation of 1V to 6V [2]. Since the input is a

sinusoidal wave, the output will also be a sinusoidal wave. The output can further be fed into a

Amplitude vs. Span Frequency

0

0.5

1

1.5

2

0 5 10 15 20 25

Span Frequency

Am

plitu

de

Serie1

Amplitude vs. Span Frequency

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25

Span Frequency

Am

plitu

de

Serie1

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comparator that spits out 1s and 0s to change the final signal to a square wave that is identical to

the input signal.

8 Overall System

When all the individual components are completed, it is time to evaluate the overall

capability of the transmitter and receiver. Local oscillators all have stability issues, which mean

the signal will not be very clean. The local oscillators have deviations of 80 KHz, so function

generators with 30 KHz deviation will be used instead. Each system is tested using a step by step

add-on method. First, the function generator signals are checked to ensure they are working

properly or the coaxial cables are not broken. Second, each stages are checked to ensure that

problems occurred at the end of each stage are fixed before moving on to the next. The final

signal is reviewed using a spectrum analyzer and the demodulated signal will be reviewed by

using an oscilloscope.

8.1 Transmitter Integration

Figure 8.1: Transmitter Test Setup

The transmitter testing station is exactly setup as the block diagram in figure 8.1. The

only issue with the transmitter side is that the local oscillator at 65.290 MHz (Figure 8.2a) has

very strong signal that it bleeds through the filter. This can be fixed by strengthening the 75 MHz

signal and weakening the local oscillator signal. The transmitter mixers new gain after

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increasing the inductor size strengthened the 75 MHz signal and by changing local oscillator

signal to the base side of the transistor of the mixer reduces the 65 MHz shown in figure 8.2a,

which is taken right after the 75 MHz filter. The VCO signal is increased after switching to the

emitter side of the transistor. The results are shown in figure 8.2b that it is significantly improved.

Figure 8.2: (a) Strong Signal at 65 MHz (b) Reduced 65 MHz Signal

Then with the power amplifier connected, since the loss before the amplifier is -1.7dBm (Figure

32), the power amplifier puts the final signal to 23dBm (Figure 8.3). There are many noises seen

at the antenna right out the transmitter, and this is due to interferences from close by frequencies.

This can be improved by improving the Q of the filter used and the system can be shielded from

the interferences.

Figure 8.3: Transmission in the Antenna

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A transmission distance test is conducted by moving a spectrum analyzer that has an

antenna on the port to distances away from the transmitter. This is to see how far the transmitter

can reach.

Transmission Powery = -1.2843x - 9.8952

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40

Distance (ft)

Pow

er (d

B)

Figure 8.4: Transmission Power vs. Distance [3]

The transmission power versus distance graph (Figure 8.4) shows that the power decreases

linearly with distance. With the closest distance tested at 9ft, the power decreases to -16dBm.,

and when reached to 37ft, the power decreases to a low level at -56dBm, which is a good level to

call the transmission distance to be 37ft.

8.2 Receiver Integration

Figure 8.5: Receiver Test Setup

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In order to test the receiver, three function generators are required and they are all set at

13dBm. One is used for simulating the modulated signal and the other two are in place as local

oscillators. After stepping down the frequency in the first stage, the 10.7 MHz mixed signal

(Figure 8.6a) has a loss of -3dBm, but when arriving at the end of the second stage, the signal

(Figure 8.6b) loses to a low level of loss at -27dBm. This is quite low to feed into the

demodulator.

Figure 8.6: (a) Stage 1 Mixer Output (b) Stage 2 Mixer Output

An amplifier is built using the preamplifier design [9] from the power amplifier (Figure 6.1) used

in the transmitter to amplify the 10.7 MHz signal in between the first stage and the second stage.

After implementing such amplifier, the results are shown in figure 8.7b, and by comparing the

one before amplification to adding this amplifier, the 10.7 MHz signal is improved by 12dBm

from -4dBm (Figure 8.7a) to 8dBm (Figure 8.7b).

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Figure 8.7: (a) 10.7 MHz Signal without Amplification (b) with Amplification

Since the input signal to the second stage is increased, the second stage should also improve. In

Figure 8.8b has a gain improvement from -26dBm to -18dBm, which increased by 8dBm. The

455 KHz at this level should be sufficient to feed the demodulator.

Figure 8.8: (a) 455 KHz Signal without Amplification (b) with Amplification

The function generator that outputs a modulated signal has two different modulation

deviations, or rate, and they are 400 Hz and 1 KHz. So if the receiver is working correctly, the

demodulator should lock on to the 455 KHz signal and output a wave that corresponds to this

modulation rate.

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Figure 8.9: (a) Demodulation at 400 Hz Rate (b) Demodulation at 1 KHz Rate

In figure 8.9a, the modulation rate is set to 400 Hz in the function generator. The output is then

checked by a high speed oscilloscope by direct coaxial cable connection. The receiver is working,

since the output shows an 890mV peak to peak 409 Hz signal. In figure 8.9b, the modulation rate

is changed to 1 KHz, and the output gives 1 KHz sinusoidal signal with 894mV peak to peak

voltage. Such result also corresponds to the PLL response in figure 7.2a.

8.3 FM Transmission

Since the two systems are ready to transmit and receive, they are setup about 6ft away

from each other using four function generators and three power supplies. One function generator

is used to simulate the input square signal that is fed to the VCOs varactor network, and the

other three are used in place of local oscillators. Antennas with 1m length are used since they are

designed for 75 MHz transmission. Since the transmitters output contains a lot of generated

noise and interference, this affects the receiver as well.

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Figure 8.10: Demodulated Signal at 100 Hz

Figure 8.11: Demodulated Signal at 1 KHz

In figure 8.10, the input signal frequency is set to 100 Hz by the function generator, and

the demodulated signal shows 100 Hz with 924mV peak to peak voltage. There seems to be a lot

of noise picked up along the way in this demodulated wave. Figure 8.11 shows the demodulated

wave when the input signals frequency is changed to 1 KHz and it corresponds to 1 KHz with a

peak to peak voltage of 1V.

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9 Conclusion

The project is considered complete at this point since the receiver is able to demodulate a

signal from the transmitter. There are many shortcomings to the final product because of

problems in individual components that could not be addressed within the given time frame. First,

the VCO and local oscillators are not stable (Figure 3.1b) enough, and this could be fixed by

decreasing Colpitts capacitor sizes, increasing the inductor, and adding offset capacitors with

sufficient sizes. The transmitted signal generated by using either unstable local oscillators or 30

KHz deviated signals from function generators have a undesired bandwidth of at least 30 KHz,

which means the signal is crossing over to the neighboring channels. Second, the power

amplifiers gain can be improved through further adjustment to improve the transmission

distance. Third, the bandpass filters are very important here, and some of them do not provide

adequate filtering capability because of their low Q (Figure 5.5), therefore, unwanted signals can

easily affect the overall quality. If crystal filters are implemented then the results could be

significant. All the mixers have sufficient gain and are the most successful components built for

the project. Overall, filtering and stability are the main shortcomings. The transmitter can send

signals to about 37ft distance (Figure 8.4) but the noise along the way causes problems for the

receiver.

This remote control system is built using discreet components and hand-wound inductors

and is able to transmit a radio signal at 75.990 MHz with a bandwidth of 30 KHz at a distance of

37ft. It did not satisfy one of the project requirements since the bandwidth is required to be 8

KHz maximum. This main issue can be addressed by improving a few components, and then the

remote control system will be sufficient for West Texas Best robotics competition.

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References

[1] K. Hooper, FM Transmitter & Receiver, Presented at Project Lab 3 Final. [PowerPoint] November 2006. Available: http://www.ee.ttu.edu/lab/Weekly/EE3333/EE3333001P24.ppt.

[2] I. Okonkwo, FM Transmitter & Receiver, Presented at Project Lab 3 Final.

[PowerPoint] November 2006. Available: http://www.ee.ttu.edu/lab/Weekly/EE3333/EE3333001P24.ppt.

[3] R. Moore, FM Transmitter & Receiver, Presented at Project Lab 3 Final. [PowerPoint]

November 2006. Available: http://www.ee.ttu.edu/lab/Weekly/EE3333/EE3333001P24.ppt.

[4] The National Association of Amateur Radio, Bandpass Filters. [Online] October 2006.

Available: http://www.arrl.org. [5] TI, CD74HC4046A Datasheet. [Online] September 2006. Available:

http://www.ti.com/lit/gpn/cd74hc4046a. [6] R. E. Ziemer, W. H. Tranter, D. R. Fannin, Principles of Communication: Systems,

Modulation and Noise, Fifth Edition, Prentice Hall, 2002. [7] ChipDocs, NTE612 Datasheet. [Online] October 2006. Available:

http://www.chipdocs.com/pnsearch/download.html?okwd=NTE612&partid=448923&ReR=GG.

[8] Datasheet Catalogs, 2N3904 Datasheet. [Online] September 2006. Available:

http://www.ortodoxism.ro/datasheets2/a/0s18la5f3csj4dzug8wfyow5zqfy.pdf. [9] G. Ford (Private Communication), 2006.

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Appendix A List of Figures

Figure 1.1: Single Stage Transmitter Block Diagram..................................................................... 4 Figure 1.2: Dual Stage Receiver Block Diagram............................................................................ 5 Figure 2.1: Colpitts Oscillator......................................................................................................... 5 Figure 2.3: 10.7 MHz VCO with Buffer Amplifier ........................................................................ 7 Figure 2.4: VCO Circuit ................................................................................................................. 8 Figure 2.5: Modulation Curve......................................................................................................... 9 Figure 2.6: Output Power................................................................................................................ 9 Figure 2.7: Buffer Stage Output.................................................................................................... 10 Figure 3.1: (a) Local Oscillator Layout (b) Local Oscillator Response .................................... 11 Figure 4.1: General Mixer Design [1]........................................................................................... 12 Figure 4.2: (a) 455 KHz Mixer Gain (3.6 H) [1] (b) 455 KHz Mixer Gain (33 H) [1] ...... 13 Figure 5.1: 3rd Order Butterworth PI Filter [4] ............................................................................. 14 Figure 5.2: Butterworth Filter Values........................................................................................... 14 Figure 5.3: (a) 75 MHz Filter Response (b) 10.7 MHz Filter Response [2]............................. 15 Figure 5.4: 455 KHz Bandpass Filter Design ............................................................................... 15 Figure 5.5: 455 KHz Bandpass Filter Response [2] ..................................................................... 16 Figure 6.1: Power Amplifier Design [3] ....................................................................................... 17 Figure 6.2: Diode Clipping Effect [3]........................................................................................... 17 Figure 6.3: Overall Gain [3].......................................................................................................... 18 Figure 6.4: Center Frequency Adjustment [3] .............................................................................. 18 Figure 7.1: (a) PLL Schematic [2] (b) PLL Layout [2] ............................................................ 19 Figure 7.2: (a) Milled Board Response [2] (b) Bread Board Response [2] ............................. 20 Figure 7.3: Demodulation Curve (Okonkwo)............................................................................... 20 Figure 8.1: Transmitter Test Setup ............................................................................................... 21 Figure 8.2: (a) Strong Signal at 65 MHz (b) Reduced 65 MHz Signal ................................... 22 Figure 8.3: Transmission in the Antenna...................................................................................... 22 Figure 8.4: Transmission Power vs. Distance [3] ......................................................................... 23 Figure 8.5: Receiver Test Setup.................................................................................................... 23 Figure 8.6: (a) Stage 1 Mixer Output (b) Stage 2 Mixer Output ............................................. 24 Figure 8.7: (a) 10.7 MHz Signal without Amplification (b) with Amplification ...................... 25 Figure 8.8: (a) 455 KHz Signal without Amplification (b) with Amplification........................ 25 Figure 8.9: (a) Demodulation at 400 Hz Rate (b) Demodulation at 1 KHz Rate ..................... 26 Figure 8.10: Demodulated Signal at 100 Hz................................................................................. 27 Figure 8.11: Demodulated Signal at 1 KHz.................................................................................. 27 Figure B1: Bandpass Filter Calculation Table.............................................................................. 31 Figure B2: VCO Capacitance Picker ............................................................................................ 31 Figure B3: Transmitter Circuit...................................................................................................... 32 Figure B4: Receiver Circuit .......................................................................................................... 32 Figure C1: Gantt Chart Weeks 5 to 8............................................................................................ 33 Figure C2: Gantt Chart Weeks 9 to 13.......................................................................................... 33 Figure C3: Gantt Chart Weeks 14 to 15........................................................................................ 34 Figure C4: Budget......................................................................................................................... 34

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Appendix B Data Tables and Pictures

Figure B1: Bandpass Filter Calculation Table

Figure B2: VCO Capacitance Picker

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Figure B3: Transmitter Circuit

Figure B4: Receiver Circuit

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Appendix C Gantt Chart and Budget

Figure C1: Gantt Chart Weeks 5 to 8

Figure C2: Gantt Chart Weeks 9 to 13

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Figure C3: Gantt Chart Weeks 14 to 15

Figure C4: Budget