FM25B • 1
Ramsey Electronics Model No. FM25B
Own and operate your own FM Stereo broadcast station. The
FM25B has an exceptional synthesized transmission range and
improved audio quality that puts your favorite radio station to
shame.
• Synthesized 88 to 108 MHz for no frequency drift!
• No annoying Hum with even better stereo separation than the
original!
• New design features ‘Line In’ and ‘Loop Out’ 1/8” Stereo jacks!
• ‘F’ style RF output connector for easy connection to an external
antenna
• Fully adjustable RF output level for custom coverage capabilities!
• Kit includes case, AC adapter, 1/8” Stereo to RCA patch cable, and
whip antenna
• Great for schools, health clubs, realtors… or your back yard!
• The ideal campus or school radio station
• Clear, concise instructions guide you step-by-step to a finished
product that works FIRST time.
SYNTHESIZED FM
STEREO TRANSMITTER
FM25B • 2
PARTIAL LIST OF AVAILABLE KITS
RAMSEY TRANSMITTER KITS
• FM25B FM Stereo Transmitter
• AM1, AM25 AM Transmitters
• 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
• DDF1 Doppler Direction Finder
• 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. FM25B Rev 1.2a
First printing: November 2001
COPYRIGHT 2001 by Ramsey Electronics, Inc. 590 Fishers Station Drive, 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.
FM25B • 3
SYNTHESIZED FM
STEREO TRANSMITTER
KIT
Ramsey Publication No. MFM25B
Price $5.00
TABLE OF CONTENTS
Introduction ...................................... 4
Circuit Description ............................ 5
Parts Layout Diagram ...................... 9
FM25B Parts List ............................. 10
FM25B Assembly ............................ 12
Custom Case Assembly ................... 20
Choosing an Operating Frequency .. 20
Adjusting .......................................... 21
Home Use ........................................ 23
Projects ............................................ 23
Antenna Ideas .................................. 24
Troubleshooting ............................... 25
FCC Rules and Information ............. 26
Understanding Field Strength .......... 29
Summary .......................................... 30
Schematic Diagram .......................... 35
Warranty .......................................... 35
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
RAMSEY ELECTRONICS, INC.
590 Fishers Station Drive
Victor, New York 14564
Phone (585) 924-4560
Fax (585) 924-4555
www.ramseykits.com
FM25B • 4
INTRODUCTION
The Ramsey FM25B 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 FM25B’s low-power broadcasting capability and other practical uses can be fun and interesting for people
of all ages, but the FM25B 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 FM25B's capabilities in accordance with the law.
Typical uses for the FM25B 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 FM25B 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 21st century global community, we realized that our FM25B low-power Synthesized FM
Stereo Transmitter Kit was certain to attract worldwide interest among hobbyists, students and "pioneers." While the use of the FM25B 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 FM25B's just like yours are faithfully relaying news and information to listeners in remote areas around the world. The FM25B is most definitely not a toy!
FM25B • 5
CIRCUIT DESCRIPTION
We will begin by talking about the power supply of your new FM25B. While a
DC power source is provided with the kit, its DC output isn’t ‘clean’ enough to
provide us with the low-noise, stable supply that we would like for good audio
quality. We wouldn’t want our FM transmitter ‘Humming’ with the music now
would we! I mean… it should know the words!
Special care has been taken to filter the input DC signal to maximize our working voltage while still offering a clean, stable supply. Take a look at the schematic as we cover the kit’s circuit description. Right off the bat our input DC
voltage is channeled through an RF filter network composed of bypass caps
C35 & C36 and RF chokes L3 & L5. This filter isolates the plus and minus supply feeds coming from the wall transformer and helps to remove any unwanted
RF that might be coupled into your transmitter. The large electrolytic capacitor
(C32) that follows the RF filter stores energy so instantaneous peaks in demand for power do not cause dips in the supply voltage.
The final parts of the FM25B’s power supply section are composed of a 12 Volt
ripple filter and a 5 Volt regulator to obtain a clean well-filtered power source.
The capacitive multiplier formed by Q1, R4, and C4 together make a handy low
loss ripple filter by in effect multiplying C4’s capacitive value by the DC current
gain of Q1. This increases C4’s ripple filtering capability to roughly that of a
10,000uF capacitor without the typical 2 to 3 volt overhead loss you would get
with a 12 Volt regulator! Now that we have a clean power source, let’s dive into
the rest of the kit!
The custom FM stereo IC (U3) is the heart of the FM25B. U3 is a microprocessor controlled FM stereo generator with lots of built-in performance features.
The surrounding support circuitry configures U3 for proper operation under a
variety of conditions.
Potentiometers R27 and R28 allow for adjustment of the input audio levels to
match a wide sampling of audio sources for the best possible sound.
Capacitors C24 and C27 set the pre-emphasis characteristics for that of the region you intend to operate in (75 µs for USA, 50 µs for Europe).
Capacitors C22 and C25 are part of the 15kHz audio low-pass filter (LPF) that
is internal to the chip. The Bessel filter has flat delay characteristics and removes the higher frequency elements that would interfere with the stereo signal.
Capacitor C29 acts as a ripple filter for U3’s internal audio reference voltage.
This reduces any chance of audio distortion due to internal demands on its
power buss.
FM25B • 6
Crystal X1 along with C9 and C12 form the timing reference of the transmitter.
The signal from the crystal network is divided down and used to generate the
stereo components of the transmitted signal as well as by the chip’s Phase
Lock Loop (PLL) circuitry to provide ‘rock solid’ frequency stability. More on the
PLL side of U3 in a moment.
Inductor L1, C13, and D8 form the Voltage Controlled Oscillator (VCO) stage.
The RF oscillator is a tuned circuit formed by these elements that sets the base
frequency of our RF transmitter. The DC voltage applied to varactor D8 causes
its capacitive value to change up or down as needed. This allows us to vary the
frequency output of the circuit up or down simply by changing the DC voltage
feed to D8.
Transistors Q2 and Q3 work in conjunction with C6, C8, and R17 to filter the
PLL correction pulses coming from U3 pin 7. The inverting LPF they form pass
a DC voltage component, as a function of the correction pulses, to the VCO to
vary the final operating frequency of the RF oscillator.
Varactor D7 is used to ‘wiggle’ the VCO voltage in accordance with the applied
audio signal. The resistor network around D7 serves a multitude of functions. In
a nutshell, D7’s capacitive value changes slightly with a given applied AC signal
(the processed composite audio signal) causing the VCO to deviate from its
base frequency. This is how we get our FM (Frequency Modulation) signal. By
using a dual varactor diode modulation scheme, we can achieve a very tight
deviation tolerance across the entire FM band and improve the overall performance of the system greatly!
Resistor R25 varies the combined modulated RF signal from U3 before its final
amplification stage. R25 gives the user full control to vary the final output level
to match their custom applications and coverage.
Amplifier U4 (the Gal-5 is phenomenally rated from DC to 4GHz operation!)
boosts the output level of U3 in one super clean stage without introducing harmonics or other spurious signals even before the signal goes through a low
pass filter!
The low pass RF filter consisting of C34, L2, C37, L4, and C38 allows the fundamental (operating) frequency to pass through while rejecting any unwanted
harmonics. Harmonics are multiples of the desired fundamental frequency and
in this case, they can cause unwanted emissions in critical areas of the RF
spectrum. A cleaner RF output means happier neighbors and the ‘piece of
mind’ that you are not causing unwanted interference.
U1 is an LM358 opamp. This little work-horse can be found in use for all types
of different applications. Our application for this circuit gives the user feedback
about the operating status of the unit. The combined stages used with their
support components act as a dual purpose pulse detector. The opamp (U1) cir-
FM25B • 7
cuit tells the user when their transmitter has locked on frequency and when an
applied audio signal is being transmitted. U1:A takes the PLL correction
pulses and amplifies them. The feedback resistor network formed by R6 and
R8 set the gain of the amplifier (remember that classic formula ‘G=1+Rf/Ri’ =
1+1 MegΩ / 10KΩ). The output of U1:A is then rectified by D2 and the peak
voltage is stored by C2. R2 is used as a discharge ‘Bleeder’ resistor so that
the sampled peak-hold voltage on C2 will vary up and down fast enough to
give the reliable dual indication features we need. U1:B monitors the peak
voltage stored on C2 and turns on or off the D1 LED Frequency Lock / Audio
Modulation indicator. When a large number of correction pulses are present
on the PLL output, due to the user changing frequency or the unit become
unlocked, the resulting voltage on C2 will be high enough on the inverting input of U1:B to swing the output Low. When the output of U1:B is low, the LED
(D1) is turned off indicating that the unit is unlocked. The similar scenario applies when audio is being transmitted. The audio being transmitted varies the
frequency up and down (FM – Frequency Modulation) in accordance with the
music. The PLL tries to correct for these deviations from the center transmission frequency by sending out short pulses. The positive feedback provided by
R9 adds a bit of hysteresis to U1:B’s response by changing the crossover trigger point switching the output back and forth from High to Low. The resulting
effect is to smooth the response of the LED (D1) indicator making it more
pleasant to the eye. The detection circuit formed by U1 will indicate when audio is being transmitted by flashing D1 along with the music. Not bad…two for
the price of one!
U2 acts as the brains of the whole circuit. This microcontroller looks at the settings of each of the dip switches S1 through S3 one at a time and from these it
calculates the desired frequency. The switches allow you add up the closed
(down) positions 1, 2, 4, and 8 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.
Closing 1 and 4 on S2 (1 MHz switch) is equal to 5 MHz. Closing 2 and 1 on
S1 (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 U3 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 U3. The frequency information
takes 10 bits of data along with an additional 6 bits sent for the internal control
and transmission mode (stereo / mono) selection. You may think that all this
would take a long time but in fact the whole process of sending the data takes
less than 1/100th of a second!
U3’s internal phase locked loop (PLL) synthesizer requires a 7.6 MHz refer-
FM25B • 8
ence crystal (X1) as we discussed before. All the internal operations of U3 are
truly amazing! The reference signal of X1 is divided by 4 and then again by 19
to obtain a stable reference frequency of 100KHz that will be used to keep our
transmitter on frequency. U3 then internally samples the RF output and divides it by a number (N). N is the frequency data that was sent by U2 and is
always equal to the desired frequency in Megahertz times 10. Using the previous example, a frequency of 95.3 MHz gives an N of 953. This means the
sampled RF output signal will be divided by 953 by U3 and then compared
with the reference frequency of 100 KHz. If the desired RF output frequency
happens to be too low (lower than the calculated reference frequency), U1
sends a series of controlled pulses close to the chip’s 0 Volt rail from a 1/2
VCC midpoint on pin 7. The inverting LPF (Q2, Q3 in conjunction with C6, C8,
and R17) in turn raises the DC control voltage on D8. 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
o
= 1/[2π(LC)½]), bring-
ing the FM25B’s output frequency back on frequency to match that of the reference. If the desired frequency is higher than the reference, U1 does just the
reverse and sends a series of controlled pulses close to the chip’s 5 Volt rail
from a 1/2 VCC midpoint on pin 7. If the frequency is just right then pin 7 floats
at a constant DC level, keeping the VCO voltage constant on D8. 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 (assuming of course that
no audio is being applied). If the frequency starts to drift for any reason (such
as a temperature change) then U3 instantly corrects the tuning voltage to
bring it back to the proper frequency.
FM25B • 9
FM25B PARTS LAYOUT DIAGRAM
FM25B • 10
PARTS SUPPLIED WITH FM25B: Note the extra chip capacitors included.
Capacitors
7 .001 µF disc capacitors (marked .001, 102 or 1nF)
[C13,14,18,31,33,35,36]
3 .01 µF disc capacitors (marked .01 or 103 or 10 nF) [C2,5,26]
3 .1 µF disc capacitors (marked .1 or 104 or 100 nF) [C7,21,23]
1 .047 uF ceramic capacitor (marked .047 or 473) [C6]
2 10 pF disc capacitors (marked 10 or 10K) [C11,15]
2 33 pF disc capacitors (marked 33 or 33K) [C9,12]
2 47 pF disc capacitors (marked 47) [C34,38]
1 75 pF disc capacitor (marked 75 or 75K) [C37]
2 100 pF disc capacitors (marked 100 or 101) [C39,40]
2 150 pF disc capacitors (marked 150 or 151) [C22,25]
2 2200 pF disc capacitors (marked 222 or .0022) [C24*27*] see text
2 3300 pF disc capacitors (marked 332 or 0.0033) [C24*,27*] see text
6 10 µF electrolytic capacitors [C1,10,20,28,29,30]
1 47 uF electrolytic capacitor [C8]
2 100 µF electrolytic capacitor [C3,4]
1 1000 µF electrolytic capacitor [C32]
4 .1 uF SMT capacitors [C16,17] Preinstalled!
Resistors
1 10 ohm (brown-black-black) [R1]
1 100 ohms (brown-black-brown) [R17]
1 120 ohms, larger, 1 watt (brown-red-brown) [R26]
1 270 ohms (red-violet-brown) [R15]
2 470 ohms (yellow-violet-brown) [R3,4]
2 4.7k ohms (yellow-violet-red) [R9,16]
8 10k ohms (brown-black-orange) [R7,8,10,11,12,14,19,22]
2 22k ohms (red-red-orange) [R5,18]
1 47k ohms (yellow-violet-orange) [R24]
3 100k ohms (brown-black-yellow) [R2,13,20]
1 220k ohms (red-red-yellow) [R23]
1 470k ohms (yellow-violet-yellow) [R21]
1 1M ohms (brown-black-green) [R6]
1 1k ohm trimmer potentiometer [R25]
2 10k ohm trimmer potentiometers [R27,28]
Semiconductors
3 2N3904 NPN transistors [Q1,2,3]
13 1N4148 diodes (small glass diodes) [D2, D3 - 6, D9 - 16]
1 1N4000 series diode (can be any number from 1N4002 to 1N4007
(black with white band) [D17]
2 Varactor diodes (transistor shape, two leads, marked MV2105)[D7,8]
1 LED [D1]
FM25B • 11
Inductors
1 Shielded can inductor, 0.18 uH [L1]
2 Pre-wound spring style inductors, 44 nH [L2,4]
2 1 µH axial inductors (brown-black-gold) [L3,5]
Hardware, Misc.
1 PIC 16C505 microcontroller IC (marked with white sticker) [U2]
1 BH1415F Stereo generator IC [U3] Preinstalled!
NOTE: These surface mount parts may be pre-installed on your circuit board.
Check the solder side of the PC board for these parts.
1 7.6 MHz crystal (thin shiny rectangle marked 7.600000) [X1]
1 78L05 +5 volt voltage regulator [VR1]
1 14-pin socket for U2
1 LM358 Low Power Dual Operational Amplifier IC [U1]
1 GAL5 SMT Amplifier IC (surface mount chip with 4 leads, 3 on one
side) [U4] Preinstalled!
1 Two pin jumper and jumper block [J1]
1 ‘F’ type board mount connector [J3]
2 3.5mm stereo jacks [J4,5]
1 2.1mm DC power jack [J2]
1 DPDT push-button switch [S4]
3 DIP switches (8 pin dip with 4 sliding tabs) [S1,2,3]
1 Whip antenna [ANT1]
1 FM25B printed circuit board
1 1/8” stereo to RCA cable
1 AC125 12 volt DC power transformer
Required, not supplied
Line level audio source (such as a tape deck or CD player)
Case and Knob Parts
Top cover with drilled antenna hole
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
Multimeter for voltage adjustment
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