Toroid Coil
Posted by admin on Saturday, December 5, 2009 · Leave a Comment
how to convert a 40meter ham radio receiver into a 80 meter receiver?
how to convert a 40meter ham radio receiver into a 80 meter receiver,
I have a kit to make a 40 meter ham radio receiver, they 80 meter receiver has been discontinued, I hope there is someone who could give me a little bit help.
the kit is a 40 meter direct conversion receiver it uses a sa602 chip, two varactor diodes and two toroid ferrite coils that are hand wound.
Dear Jimjuta,
Direct conversion receiver is oldest type of receiver that designed before superheterodyne receiver system. In direct conversion, joined carrier signal and AF signal directly extracted/filted to get AF signal. While in superheterodyne system, joined carrier and AF signals, first developed to get IF (Intermediate Frequency) signal. This IF signal then amplified in order receiver to be more sensitive, then as final result: AF (Audio Frequency).
Superheterodyne developed because IF amplifier which frequency is much more low than RF, is easier to designed.
If you like to change frequency of your receiver, you should be able to modify oscillator/local oscillator of your receiver. Oscillator developed from many models: with LC tank, VCO, Crystal, etc..
Eg. oscillator with LC tank: main formula for resonance frequency: Fr=1/(2π√(LC)) . If values of L and C increased, resonance frequency would be lower. Of course if you modify oscillator, you would need oscilloscope which maximum frequency measurement is approximately 5MHz (because 80m band is between 3.5-3.9MHz).
In order to modify, you should understand internal circuit of SA602, so please download this link (just click): http://go2.wordpress.com/?id=725X1342&site=frrl.wordpress.com&url=http%3A%2F%2Ffrrl.files.wordpress.com%2F2008%2F11%2Fne602.pdf although this is datasheet for SA602A. On that datasheet, figure 7 shows various tuning circuits attached to pin 6 and 7, and figure 8 shows circuit if you are using VCO with synthesizer. I do not know what circuit you are using. SA602A could produce oscillation up to 200MHz, so modify it to 3.5-3.9MHz, I hope is alright.
I could recommend You good book to design oscillator: "Transistor Circuit Design With Experiment" by Delton T. Horn, and another good book to design coil/L/inductor: "ARRL Handbook".
Have a nice experiment.
Thank You very much Jimjuta.
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