SYNTHESIZED FM STEREO TRANSMITTER 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.
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 Compani
Ramsey Publication No. MFM25B 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 ...................... 9 FM25B Parts List ............................. 10 FM25B Assembly ............................ 12 Custom Case Assembly................... 20 Choosing an Operating Frequency .. 20 Adjusting .................................
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.
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.
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.
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.
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.
FM25B PARTS LAYOUT DIAGRAM FM25B • 9
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 .
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.
RAMSEY "LEARN-AS-YOU-BUILD" ASSEMBLY STRATEGY As you can see in examining the circuit board and components, there is a bit more to this kit than just soldering a few parts. So that you don't spend extra time "troubleshooting" instead of getting on the air, we strongly recommend that you follow the assembly strategy and step-by-step procedures we provide. Our strategy in installing parts on our PC board is to install the larger and more obvious parts such as the ICs and connectors first.
• Nip or "trim" all excess wires extending beyond each solder connection, taking care that wire trimmings do not become lodged in PC board solder connections. Enough said. . . Let's get building! We’ll start with the surface mount parts on the “solder” side of the PC board. That will get the tiniest, most easily lost parts “out of the way” . . .
socket check to be sure that none of the pins are bent under or outside of the socket. When you’re sure the pins are where they belong, press it down so that it is seated flat within the socket. 4. Install U1, the LM358 dual op-amp. Again, orient the dotted or notched end as shown on the silkscreen and be sure to solder all 8 pins. 5. Install S4, the DPDT push-button switch. You’ll want this part to be seated correctly so that the power button moves freely when in the case.
16. Install J4, the other stereo jack. Now we’ll install some electrolytic caps. Electrolytic capacitors have a right and wrong way to be installed. Usually, capacitors have a wide black stripe which indicates their negative lead and the PC board or Parts Layout Diagram will show the positive side of the capacitor's installation hole. Be sure to place the ( + ) capacitor lead into the PC board ( + ) hole and the ( - ) lead into the ( - ) hole.
36. Install C23, .1 µF disc capacitor (marked .1 or 104 or 100 nF). 37. You’ll notice a silkscreen position for C19. This part is not used; don’t install any component at that position. 38. Install C20, 10 µF electrolytic capacitor. This part was changed from a disc capacitor to improve the low frequency response so you’ll have to fit the electrolytic in where the disc would have gone. The positive lead should be placed toward C13 and the negative or striped lead should be placed toward R25.
58. Install C6, .047 uF ceramic capacitor (marked .047 or 473). 59. Install R13, 100k ohms (brown-black-yellow). 60. Install Q3, the transistor marked 2N3904. The flat side must be placed as shown on the PC board. Mount it as close to the board as possible without forcing it. 61. Install Q2, another 2N3904 transistor. If you can’t remember how you just did it, reread the step above! 62. Install L1, the shielded can inductor.
flat to the board before soldering. 73. Ok, you know the drill by now. Install S1, the last switch. If you have a short memory, read the previous two steps. You didn’t think I was going to type all that again, did you? We’re getting there, aren’t we? Just a few more parts to install and you’ll be finished! Take a break if you need to. Stretch your fingers and arms, get the crick out of your neck and let’s keep going. 74. Install R8, 10k ohms (brown-black-orange). 75.
89. Install VR1, the 78L05 +5 volt regulator. This part looks like a transistor and like a transistor, it must be place correctly to work. Follow the parts layout diagram and PC board silkscreen for orientation. Solder all three pins. 90. Install C1, the last 10 µF electrolytic capacitor. Check polarity before soldering. 91. Install C4, the other 100 µF electrolytic capacitor. Do I have to remind you about polarity again? 92. Install R4, 470 ohms (yellow-violet-brown). 93.
stations but your neighbors may be receiving them, using a good receiver and outdoor antenna. Interfering with such reception is a direct violation of federal law. The most reliable way of finding a truly open frequency on the FM band is to check the band with a very good FM receiving system using an external antenna.
board. With an operating frequency of 107.9 MHz programmed into the DIP switches, there should be around 9.0 volts (+- 0.2 volts or so) at TP1. If there isn’t, adjust L1 until you read 9.0 volts. The LED lock indicator (D1) should now glow brightly. If not, you may need to SLIGHTLY tune the control voltage up or down from the expected standard in order to compensate for building tolerances. Now let’s verify the other end of the band at 88.1 MHz. Set the DIP switches for 88.
quite well. An external antenna can be used for a larger coverage area if desired. Use the RF adjustment control (R25) to fine tune the RF output level to just reach the area you intend to cover. The audio connection consists of using shielded audio cables to connect the line or speaker level output to the audio inputs of the FM25B. Consult the literature that came with your stereo equipment.
through a few feet of coaxial cable (either RG-58, RG-59 or miniature RG-174, available at Radio Shack and other sources). Correct dipole lengths for major sections of the 88-108 MHz band are: 88 MHz, each side: 2.7 feet; 5.4 feet total 98 MHz, each side: 2.4 feet; 4.8 feet total 108 MHz, each side: 2.2 feet; 4.4 feet total You can see that there’s not a great difference in antenna length from 88 to 107 MHz. Some antenna designers have the view that an "approximate" dipole such as 2.5 ft.
rior wiring of your home or building. Do not attempt such an installation unless you know exactly what to do and not to do. Also, because such an installation is beyond the original purpose of this kit and the safety standards intended for all Ramsey kits, and because we have not tested the FM25B in such an installation, we cannot provide further details.
APPENDIX A: FCC RULES AND INFORMATION The Rules of the FCC (Federal Communications Commission) and your kit built FM Stereo Transmitter. An interim explanation of applicable FCC regulations supplied as a personal assistance to FM25B builders, by Dan F. Onley (K4ZRA) It is the policy of Ramsey Electronics, Inc., that knowing and observing the lawful use of all kits is a first responsibility of our kit user/builders.
if you are in doubt) the rules and do not bother the understaffed and busy FCC employees with questions that are clearly answered in the rules. Here are the primary "dos and don'ts" picked from the current FCC Rules, as of May, 1990. This is only a brief look at the rules and should not be construed to be the absolute complete legal interpretation! It is up to you to operate within the proper FCC rules and Ramsey Electronics, Inc. cannot be held responsible for any violation thereof. 1.
• a. The "bandwidth" of your transmission is limited to 200 KHz, centered on the actual operating frequency. Since 200 KHz is enough spectrum space for several different FM stations, this is a "generous" limitation deDISTANCE FROM TRANSMITTER ANTENNA METERS FEET FIELD TOTAL AREA 3 10 250 314 FT 6 20 125 1256 FT 12 39 63 4800 FT 24 78 31 19113 FT 48 157 15 1.8 ACRES 96 315 7.5 7.2 ACRES 192 630 3.8 28.6 ACRES 384 1.9 11.4 ACRES 768 1260 2520 .
• d. The most specific FCC regulation of 88-108 MHz FM Broadcast band unlicensed operation is that the "field strength" of the signal must not exceed 250 microvolts/meter at a distance of 3 meters from the transmitter (FCC rule 15.239). If you have any concern about this emission limit, have your device checked by a technician with accurate measuring equipment. Remember that the "field strength" of a signal is determined as much by the antenna as by the RF output of the transmitter itself.
to "die out" at a specific distance from your antenna, no matter what kind of transmitter power or extra-gain antenna you are using. On the other hand, the FCC standards do make it legal and possible for you to broadcast on a school campus, campground or local neighborhood, as long as you do not cause interference to broadcast reception. “Why talk about acres"? There are three reasons to translate our look at "field strength" into "acres". 1.
SUMMARY The present edition of Part 15 of the FCC rules does not provide detailed guidance on ALL aspects of using a low-power transmitter such as the FM25B. The main point is that you may not cause any interference whatsoever to licensed broadcast services and that you must be willing to put up with any interference that you may experience.
10. Do not assume that the mere fact that you purchased this kit gives you any specific right to use it for any purpose beyond generating a low-level RF signal which is barely detectable beyond the perimeter of your personal dwelling space. Finally, the FCC Rules call for the posting of printed notices on devices intended for non-licensed operation under Part 15 Rules.
of private citizens and school-community groups as never before in radio communications history. Learn the rules...observe them...
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The Ramsey Kit Warranty Please read carefully BEFORE calling or writing in about your kit. Most problems can be solved without contacting the factory. Notice that this is not a "fine print" warranty. We want you to understand your rights and ours too! All Ramsey kits will work if assembled properly. The very fact that your kit includes this new manual is your assurance that a team of knowledgeable people have field-tested several "copies" of this kit straight from the Ramsey Inventory.
SYNTHESIZED STEREO TRANSMITTER KIT Quick Reference Page Guide Introduction ..................................... 4 Circuit Description ............................ 5 Parts Layout Diagram ...................... 9 FM25B Parts List ............................. 10 FM25B Assembly ............................. 12 Troubleshooting ............................... 25 FCC Rules and Information ............. 26 Understanding Field Strength .......... 29 Summary .........................................
