FM TRANSMITTER LOWPASS FILTER Ramsey Electronics Model No. FMLP1 In today’s world, the importance of sticking to the “specs.” is more important then ever! The FMLP1 has been designed specifically to meet the Low Power FM broadcasters needs. No more worrying about out-of-band harmonics interfering with your neighbors! Y!! !! BL M CE E N S MA AS OR SY F A E ER TP A E GR (Actual Size) • NO Tuning Required...
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Ramsey Publication No. MFMLP1 KIT ASSEMBLY AND INSTRUCTION MANUAL FOR FM TRANSMITTER LOWPASS FILTER TABLE OF CONTENTS Introduction ......................................4 Circuit Description.............................5 SMT Soldering Instructions...............7 FMLP1 Parts List .............................9 Parts Layout Diagram ......................10 Schematic Diagram ..........................11 FMLP1 Assembly .............................12 Custom Case Assembly ..................
INTRODUCTION While this kit and manual were designed with the beginner in mind, it should also be stated that this kit qualifies as a “Skill Level 2” due to the nature of the assembly using surface mount components. While this isn’t intended to “scare you off” it is not recommended that this be your first electronics assembly project.
CIRCUIT DESCRIPTION The FMLP1 is a lowpass filter. This means that certain frequency elements can pass from the input to the output unimpeded while others are almost stopped in their tracks! The name of this type of filter gives away its response characteristics when a signal is applied to the input… lowpass filter… low frequency signals pass through. Conversely, high frequency signals are greatly reduced in amplitude virtually rendering them to insignificant levels.
If you think for a moment about the fundamental operation of a capacitor and an inductor when an RF (radio frequency) signal is applied, the reasoning for the layout seen in the schematic is obvious. Thinking in terms of extremes and going back to the basics of reactive elements, let’s examine each one separately. At very high frequencies a capacitor looks like a short circuit meaning signals pass through it like it was just a piece of wire.
The previous paragraphs have help to shed a little light on the basic workings of a lowpass filter. To this point the reason for the grouped elements of ‘L1 & C3’ and ‘L3 & C8’ were not specifically mentioned. These components still follow the guidelines set forth earlier but their configuration yields an almost magical result. Our main concern for using a lowpass filter is to limit the harmful presence of any harmonic frequencies, especially those in the 2nd harmonic (176 - 216 MHz) realm.
components to the printed circuit board before soldering. The procedure is to take a small amount of glue (usually with a pin or toothpick) and “dab” the circuit board in the place where the component will be affixed. Be careful not to apply too much glue as when the part is placed it may “squash” the glue underneath the soldering tabs of the component. Carefully place the part into position, and when the glue dries, solder the connection.
PARTS SUPPLIED WITH FMLP1 TRANSMITTER KIT Capacitors -(50% more SMT capacitors are supplied with each kit) 4 2 4 10 pF chip capacitors (should be marked A1) 12 pF chip capacitors (should be marked C1) 27 pF chip capacitors (should be marked L1) [C1,2,9,10] [C3,8] [C4,5,6,7] Inductors 1 15” of #20 AWG enameled wire [used to make L1,2,3] Hardware, Misc. 1 2 2 2 1 10” long RG-188A Teflon coaxial cable ‘F’ style panel mount connectors 0.
FMLP1 PARTS LAYOUT DIAGRAM FMLP1 • 10
FMLP1 • 11
RAMSEY 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 the PC board is to deal with the surface mount components first, then work upward from the board as the parts overlap.
7. Install C7, 27 pF chip capacitor (marked L1) 8. Install C8, 12 pF chip capacitor (marked C1) 9. Install C9, 10 pF chip capacitor (marked A1) 10. Install C10, 10 pF chip capacitor (marked A1) This completes the first phase of construction of your FMLP1. Take a break now to carefully examine all solder connections. Make certain there are no solder ’blobs’ or small shorts on or around the components you have just installed.
Now the inductor coils are prepared. Let’s see how they will mount to the PC board. Look at the parts layout diagram. You will see that coil L1 and L3 mount directly over the top of C3 and C8. The enamel on the wire will keep them from shorting but leaving a bit of space never hurts. About 1/32nd of an inch does nicely. 15. Install L1, one of the four turn inductors, as shown on the parts layout diagram. Leave the coil loops spread apart about the same as the bolt threads.
20. Cut the 10” piece of RG-188A coax into two 5” pieces. When we are finished preparing the cable in the next few steps, all four ends should look similar to this: Proper Coax Preparation 21. Very carefully strip off the outer insulation on one end of a piece of coax about 1/2” back from the end. Be careful not to remove any of the outside ground shield braid. This is a gentle procedure, so take your time.
applying solder. Use enough to coat the wires and form a strong connection making for easier installation later. 27. Repeat steps 21 through 26 until both ends of each piece of coax are prepared. Examine your work with the RG-188A coax to this point. Use an ohmmeter or continuity checker to verify that center conductor is not shorted to the ground shield. Visual inspection is normally enough to check this. Remove any gobs of solder with the hot soldering iron if any exist. 28.
33. Solder the ground braid of the coax to the solder lug of the F-connector. Poor connections can lead to poor performance. Check your work and make sure the connection does not look like this: Poor F-Connector Installation (This connection style acts as a loop antenna and allows signals to radiate across the filter!) 34. Repeat steps 30 through 33 to attach the other F-connector. Your FMLP1 kit is now assembled and almost ready for use.
The Finished Product!! CONCLUSION We sincerely hope that you have enjoyed the construction and use of this Ramsey Kit. As always, we have tried to compose our manual in the easiest, most “user friendly” format that is possible. As our customers, we value your opinions, comments, and additions that you would like to see in future publications. Please submit comments or ideas to: Ramsey Electronics Inc.
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.
FM LOWPASS FILTER KIT Quick Reference Page Guide Introduction ...................................... 4 Circuit Description ............................ 5 SMT Soldering Instructions .............. 7 FMLP1 Parts List ............................. 9 Parts Layout Diagram ...................... 10 Schematic Diagram .......................... 11 FMLP1 Assembly ............................ 12 Custom Case Assembly .................. 17 Hook-up and Applications ............... 17 Conclusion .........................
