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


Filters
Click here to download the full size of the above Circuit.

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...




Leave Comment

characters left:

Related Circuits

  • New Circuits

    .

     


    Popular Circuits

    Reversing motor drive dc control
    home automation Installation
    CD4040 Generating Long Time Delays Circuit
    How to properly snub transient spikes from a transformer and prevent switch mode power supply burn out
    bluetooth curtain system
    Basic Controls of a CRO
    TPA2011D1 3.2W Mono Filter-Free Class-D Audio Power Amplifier With Auto-Recovering Short-Circuit Protection
    Speaker Microphone Circuit
    DIY ECG Machine On The Cheap
    Op-Amp Microphone Preamp
    High Fidelity Valve audio Amplifier 40W



    Top