Frequency Modulator and Tuning Circuit

Posted on Feb 4, 2014

Here`s a design circuit for frequency modulator that is completed with tuning circuit. In this circuit is using pair of 1N4007s are used as varactor diodes. I picked 1N4007s not because they are particularly well suited, but simply that I had some on the bench. One of the varactors is directly coupled to the xtal circuit through 1 nF so most of it

Frequency Modulator and Tuning Circuit
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s capacitance change is seen, the other is coupled through a small trimmer to adjust its maximum effect. This gives two independent channels, one for relatively coarse frequency tuning, the other for FSK modulation. Here`s the figure of the circuit; The data looks characteristically parabolic as expected. I suspect the oddness of the fine is measurement error. The y coordinates are in Hz relative to 10. 138000 MHz (x is reverse voltage) so it doesn`t take much error in measurement to make the fine trace look lumpy.

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