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During the autumn of 2007 one focus of the construction group is the SP-1 transmitter. We're starting out building low-pass filters for 40m, then making the transmitters and, last off for those wanting the additional power, a linear using four 2N2222s. Chris, M0JRQ, has got kits of parts together for the LPFs and has SP-1 kits from the G-QRP Club. He's taken the photos of the construction evenings, too. That's why he isn't in any of the pictures. The LPF is based on the 7-pole Chebyshev design that appears in the ARRL handbook. A useful Universal LPF PCB is available from kitsandparts.com, making the filter's construction about as straightforward as it can get. |
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Design The low pass filter design is unoriginal but effective - Although this LPF is credited to the ARRL Handbook versions appear in almost every handbook published over the last few decades. |
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The circuit consists two pairs of capacitors and three toroidal inductors. The important feature of these - and any LPF designed to go on the output side of an RF stage - is their cut-off before the first harmonic. | |||
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| This design does the job with output 30dB down by 10MHz as compared with 7MHz. | ||||
Construction Making the LPF is straightforward. The kitsandparts.com PCB is drilled to accept either polyester capacitors or disc types. We used polyester caps.
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| Construction began with laying out the tools and heating the soldering irons. The capacitors were soldered to the PCB and their leads trimmed. The next step was to wind the inductors. We all ensured we held the toroids in the same way and wound the wire onto them in the same direction: Clockwise windings. Since the toroids have one natural (black) face and one yellow, so long as the winding operation always uses the same sense, the inductors can be wired to the PCB with confidence that they're all the right way round! | ||||
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It is extremely important to ensure that the connection to the toroidal coils is good. The wire we used was epoxy insulated and NOT solder-through (Which most of us don't trust, anyway. But that's another story!) We used variations on a craft knife: scraping; a triangular needle file: abrasive; folded sandpaper: abrasive again and the beveled edge of a pair of side cutters. The important next step in every case was to check continuity with a test meter before tinning the wires with plenty of solder. |
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Next step: The SP-1...
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| Mike, G7TKM, winds a coil for his filter | ||||
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G3FYQ G1FYQ
