Filters

Posted on Feb 6, 2014

This option has the advantage that one does not need to change anything in the receiver itself. However, since any filter will give some attenuation of the signal, this will degrade the noise figure of the receiver, and thus limit the weakest signal which could be detected. This is not a big disadvantage as it might seem, since at 20 MHz noise fro

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m the Milky Way gives a background of about 50 000 K, while the internal noise of the receiver is about 1200 K. If the filter`s attenuation is e. g. 10 dB, the galactic background is at 5000 K, still much above the receiver`s noise. Thus it remains the limiting factor for detection of signals. Another disadvantage may be the large space occupied by the filter: In order to provide a large rejection to unwanted signals, the resonators must be of very low electrical losses, which unfortunately can only be provided by sufficiently large air coils. The best way to make low loss resonating circuits is to use helical resonators, i. e. the coils are in resonance with the capacitance of their shielding boxes. The filter shown above has two coils which are done from ordinary copper telephone or hook-up wire, about 0. 5 mm diametre, with red plastic insulation, wound on 2. 5 cm diametre plastic tubes. The hot end of each coil is terminated in a small trimmer capacitor. Since the trimmers I have are a bit jumpy when adjusting, I use two trimmers in parallel, each has 12 pF maximum capacitance. The two resonators are capacitively coupled via the silvered wire which leads from the top of the left hand coil through a hole in the partition and ends as a nearly vertical wire placed close to the top of the right hand coil. The input and output is done by a tap at 1/2 to 3/4 of a turn from the earthy end. The wires seen to bridge the front...

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