SYNTHESIZED FM
STEREO TRANSMITTER
Ramsey Electronics Model No. FM25A
Own and operate your own FM Stereo broadcast station.
Definitely not a toy, the FM-25A has an exceptional synthesized
transmission range and improved audio quality that puts your
favorite radio station to shame.
• Great for transmitting your tape deck or CD player throughout the
house, yard or even your car.
• Powerful enough for college or neighborhood radio stations - in use
all over the world.
• Fantastic improved audio quality, sounds better than most stations
on the dial. And we’ll tell you why!
• Easily connects to the line-level outputs on any tape deck, stereo
system or CD player.
• Some users hook up one channel to the scanner and the other to
their two way radio. Now you can hear what’s going on around town
or a couple blocks away from your house with a simple stereo
receiver... and adjust the volume of each individually with your
balance control!
• Add a mike-mixer and tape or CD deck for a “PRO” sounding radio
station.
• Operates on 8 to 14 volts DC, 12 volt DC power supply provided.
• Easily tunable anywhere in the 88-108 MHz FM band.
• Clear, concise instructions guide you step-by-step to a finished
product that works FIRST time.
FM25A • 1
PARTIAL LIST OF AVAILABLE KITS
RAMSEY TRANSMITTER KITS
• FM25, MP3FM FM Stereo Transmitters
• AM1 AM Transmitter
• TV6 Television Transmitter
• FM100 Professional FM Stereo
Transmitter
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• SR2 Shortwave Receiver
• AA7 Active Antenna
• SC1 Shortwave Converter
RAMSEY HOBBY KITS
• SG7 Personal Speed Radar
• SS70A Speech Scrambler
• MX5, MX10 Mixers
• MD3 Microwave Motion Detector
• PH10 Peak hold Meter
• STC1 Stereo Transmitter Companion
RAMSEY AMATEUR RADIO KITS
• FX 146 VHF Transceiver
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• CW7 CW Keyer
• CPO3 Code Practice Oscillator
• QRP Power Amplifiers
RAMSEY MINI-KITS
Many other kits are available for hobby, school, scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.
SYNTHESIZED FM STEREO TRANSMITTER KIT INSTRUCTION MANUAL
Ramsey Electronics publication No. MFM25A Rev 1.4
First printing: August 1996
COPYRIGHT 1996 by Ramsey Electronics, Inc. 793 Canning Parkway, Victor, New York
14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.
FM25A • 2
Ramsey Publication No. MFM25A
Price $5.00
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
SYNTHESIZED FM
STEREO TRANSMITTER
KIT
TABLE OF CONTENTS
Introduction ...................................... 4
Circuit Description ............................ 5
Parts Layout Diagram ...................... 7
FM25A Parts List .............................. 8
Assembly Procedure ........................ 10
Schematic Diagram .......................... 16
Case and Knob Assembly ................ 18
Choosing an Operating Frequency .. 19
Adjusting and Alignment .................. 19
At Home Use .................................... 21
Projects and Experiments ................ 21
Antenna Ideas .................................. 22
Troubleshooting ............................... 23
FCC Rules and Information .............. 24
Understanding Field Strength .......... 26
Summary ......................................... . 28
Warranty ........................................... 31
RAMSEY ELECTRONICS, INC.
793 Canning Parkway
Victor, New York 14564
Phone (716) 924-4560
Fax (716) 924-4555
www.ramseykits.com
FM25A • 3
INTRODUCTION
The Ramsey FM25A is a true SYNTHESIZED STEREO FM broadcast transmitter, which any person may build and use in accordance with the rules of your
nation’s telecommunications authority. For U.S. residents, that authority is the
Federal Communications Commission (FCC). The FM25A’s low-power broadcasting capability and other practical uses can be fun and interesting for people
of all ages, but the FM25A is not a toy. We will refer to the FCC regulations frequently in this manual and provide you with some information necessary to enjoy the FM25A's capabilities in accordance with the law.
Typical uses for the FM25A include the following:
• Extension of home stereo system - without wires.
• Listening aid for auditoriums, churches.
• Student-operated school radio station.
• College dorm favorite music broadcast service.
• Short-range, two-channel experiments and demonstrations.
We think you will be very pleased with the transmitting range, audio quality, frequency stability and stereo channel separation of this build-it-yourself synthesized FM stereo transmitter. If you follow our assembly directions carefully and
use your FM25A in accordance with applicable FCC rules, a whole new world
of sharing music, news and views with friends and neighbors awaits you.
Since the sharing of music and information is vital to the culture of our late20th-century global community, we realized that our FM25A low-power Synthesized FM Stereo Transmitter Kit was certain to attract worldwide interest among
hobbyists, students and "pioneers." While the use of the FM25A may need to
be limited to "wireless stereo extensions" in some USA households (to comply
with FCC Rules, Part 15), we have seen it serve very well as a serious, though
simple, broadcast station for remote villages throughout the world where low
cost AM-FM receivers are available to people of all economic levels. After
you're done building your kit, sitting back and listening to your handiwork, consider this: many other FM25A's just like yours are faithfully relaying news and
information to listeners in remote parts around the world. The FM25A is most
definitely not a toy!
FM25A • 4
CIRCUIT DESCRIPTION
We will begin by talking about the power supply of your new FM25A. While a
DC power source is provided with the FM25A, the DC isn’t ‘proper’ enough to
provide us with the low-noise, stable supply that we would like for good audio
quality. Voltage regulator VR3 provides us with a means to take the raw DC
output from the wall transformer and ‘smooth’ it out, keeping it at a constant 12
volts. Filter capacitor C5 reduces ripple noise from the DC wall transformer.
VR1 provides us with a good clean +5 volts for the CMOS circuits of U1 and
U2, while VR2 gives us about 2.6-2.8 volts which is what the custom IC U3
likes to see.
The custom FM stereo IC (U3) is the heart of the FM25A. The control of U3 is
determined by its surrounding circuitry. Potentiometers R11 and R16 allow for
adjustment of the audio levels. Resistors R9 and R15 set the pre-emphasis
characteristics (75 µs for USA, 50 µs for Europe). R6 permits adjustment of stereo balance and L1 and D21 form an adjustable resonant circuit to set the carrier operating frequency. Y2, C32, and C30 provide the 38 KHz subcarrier for
stereo transmission. C33, R22, R21 and C34 set the proper multiplexed audio
carrier levels for the modulator. The combined modulated RF signal is seen on
pin 7 of U3, and is amplified by Q3 and surrounding biasing components.
Q3 amplifies the output of U3. This signal is fed to U1 through C26 and R12
which is the transmitted frequency feedback for the PLL to compare the crystal
frequency to. It is also fed to Q2 through C29 and R19. Q2 provides further amplification before the signal goes to the low pass filter. The low pass RF filter
consisting of C35, L2, C36, L3, and C37 allows us to pass the fundamental
(operating) frequency while rejecting the harmonics. Harmonics are multiples of
the fundamental frequency, and in this case are undesirable in transmission
since they can transmit in critical areas of the RF spectrum.
U2 is the brains of the whole circuit. This micro-controller looks at the settings
of each of the dip switches S3 through S5 one at a time and from these it calculates the desired frequency. On these switches you add up the closed positions
1, 2, 4, and 8 to make to make any number between 0 and 9. For example closing position 1 and 8 on S3 (10 MHz switch) is equal to 90 MHz, plus closing 1
and 4 on S4 (1 MHz switch) is equal to 5 MHz, while closing 2 and 1 on S5 (0.1
MHz switch) is equal to 0.3 MHz. This makes the final frequency equal to 95.3
MHz. These switches may be set to any frequency between 88 and 108 MHz.
To set the frequency above 100 MHz, the S3 positions must add up to ten. Any
switch setting greater than 9, with the exception of S3, is invalid and will be
read as 0.
Once this frequency is determined, the information needed to control U1 is sent
serially from U2. This information is a string of binary data, (1's and 0's). In this
way data is sent one bit at a time to U1. The frequency information takes 16
bits, and there are an additional 32 bits sent for the internal control of U1. You
FM25A • 5
may think that all this would take a long time but in fact the whole process from
the time you press the program switch until the data is completely sent is less
than 1/100th of a second!
U1 is a phase locked loop (PLL) synthesizer IC. It takes the 6 MHz crystal frequency and divides it by 60 to obtain a stable reference frequency of 100KHz.
How does it know to divide the crystal frequency by 60? That is part of the data
that is sent from U2 each time the program button is pressed or the loop is out
of lock. U1 also takes the output frequency of U3, and divides it by a number,
N. N is the frequency data that was sent by U2. N is always equal to the desired frequency in megahertz times 10.
Using the example from above, a frequency of 95.3 MHz gives an N of 953, so
the frequency from U3 is divided by 953 and then compared with the reference
frequency of 100 KHz. If the desired frequency is less than the reference frequency U1 increases the output voltage on pin 13. This increases the voltage
across diode D21, a varactor diode. As the voltage across the varactor increases, it causes a decrease in capacitance (Increasing reverse bias essentially increases the distance between the capacitor’s plates by increasing the
depletion region in the diode (C = kA/d). The decrease in capacitance causes
an increase in U3’s RF oscillator (f
= 1/[2π (LC)½]), bringing the FM25A’s out-
o
put frequency back on frequency. If the desired frequency is higher than the reference, pin 13 is driven low. If the frequency is just right then pin 13 becomes a
high impedance, basically disconnecting it from the circuit so it will cause no
change in the voltage on D21. The voltage changes on pin 13 are filtered by
R8, C16, R10, and C19 to provide a steady, noise free tuning voltage for D21.
In this way the output frequency of U3 is "locked" to that desired by U2. When
the frequency is locked, U1 will cause led D1 to be brightly lit. If D1 is dim or off
there is a problem and the frequency is not locked. If the frequency starts to
drift for any reason (such as a temperature change) then U1 instantly corrects
the tuning voltage to bring it back to the proper frequency.
Notes on Operation
Any time the unit is turned "on" or the "program" switch is pressed the microcontroller reads the frequency switches and sends the data to U1.
To change the frequency simply set the switches to the desired frequency and
press "program". The frequency is limited to within 88 to 108 MHz. Any setting
outside this range will be programmed as 88.1 MHz. If you set your frequency
and press program but don’t hear the signal on the frequency you expected try
listening on 88.1 MHz. If the signal is here there is an error in the switch settings or maybe a solder bridge somewhere making the frequency setting invalid.
A switch must be closed to be active. Add up the positions 1, 2, 4, and 8 to
make any number between 0 and 9. 10 -15 are invalid and will be read as 0.
FM25A • 6
FM25A PARTS LAYOUT DIAGRAM
FM25A • 7
PARTS SUPPLIED WITH FM25A TRANSMITTER KIT
Capacitors
8 .001 µF disc capacitors (marked .001, 102 or 1nF) [C10,C12,C13,C26,
C27,C28,C29,C31]
7 .01 µF disc capacitors (marked .01 or 103 or 10 nF) [C7,C8,C15,C23,
C24,C42,C43]
2 .1 µF disc capacitor (marked .1 or 104 or 100 nF) [C9,C16]
2 .022 uF ceramic capacitors (marked .022 or 223) [C14,C22]
2 5 pF disc capacitors (marked 5 or 5K) [C18,C20]
2 10 pF disc capacitors (marked 10 or 10K) [C30,C32]
2 27 pF disc capacitors (marked 27 or 27K) [C1,C2]
2 47 pF disc capacitors (marked 47) [C35,C37]
1 75 pF disc capacitor (marked 75 or 75K) [C36]
1 220 pF disc capacitor (marked 220 or 221) [C33]
8 10 µF electrolytic capacitors [C3,C6,C11,C17,C19,C21,C25,C34]
1 100 µF electrolytic capacitor [C4]
1 1000 µF electrolytic capacitor [C5]
Resistors
1 100 ohms (brown-black-brown) [R12]
2 220 ohms (red-red-brown) [R14,17]
1 270 ohms (red-violet-brown) [R13]
2 470 ohms (yellow-violet-brown) [R2,R10]
1 1K ohms (brown-black-red) [R31]
3 2.2K ohms (red-red-red) [R9*,R19,R15*]
2 3.3K ohms (orange-orange-red) [R9*,R15*]
3 4.7K ohms (yellow-violet-red) [R21,R28,R30]
9 10K ohms (brown-black-orange) [R1,R4,R5,R18,R20,R23,R24,R25,R26]
3 47K ohms (yellow-violet-orange) [R8,R27,R29]
1 100K ohms (brown-black-yellow) [R7]
1 150K ohms (brown-green-yellow) [R22]
1 10M ohms (brown-black-blue) [R3]
2 1K ohm potentiometer [R11,R16]
1 100K ohm potentiometer [R6]
Semiconductors
1 2N3904 NPN transistor [Q1]
2 2SC2498 or 2570 NPN transistors [Q2,Q3]
12 1N4148 diodes (small glass diodes) [D9 - D20]
1 Varactor diode (transistor shape with two leads, marked MV2105)[D21]
1 LED [D1]
Inductors
1 Shielded can inductor [L1]
2 Pre-wound spring style inductors [L2,L3]
FM25A • 8
Hardware, Misc.
1 6 MHz crystal (thin shiny rectangle marked 6.00) [Y1]
1 38 KHz crystal (small silver cylinder with two small leads), taped to
paper. [Y2]
1 78L05 +5 volt voltage regulator [VR1]
1 78L02 +2.6 volt voltage regulator [VR2]
1 7812 +12 volt voltage regulator [VR3]
1 68HC705K1 Microcontroller IC (marked with white sticker) [U2]
1 16-pin socket for U2
1 18-pin socket for U3
1 145170 Serial PLL IC [U1]
1 BA1404 Stereo modulator IC [U3]
1 FM25A printed circuit board
3 RCA-style jacks [J2,J3,J4]
1 DC power jack [J1]
1 Horizontal push-button switch [S1]
1 Small black mini push-button switch [S2]
3 DIP switches (8 pin dip with 4 sliding tabs) [S3,S4,S5]
1 Whip antenna [ANT1]
1 12 volt DC power transformer
1 Two pin jumper and jumper block [JMP1]
Required, not supplied
Shielded stereo audio cables
Line level audio source (such as a tape deck or CD player)
Case and Knob Parts
Top Cover
Bottom Base Tray
4 - Short Phillips Head Screws
2 - Long Phillips Screws
Front and Rear Plastic Panels
Front and Rear Labels
4 - Rubber Feet
Appropriate Knobs for Kit
Required Tools
Small Phillips Head Screwdriver
Pen or Pencil
Sharp hobby knife or hand held paper punch
Ruler at least 6 inches long
Optional Tools
5/16” Drill
FM25A • 9
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