Frequency Modulator and Tuning Circuit

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 its 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.

High Frequency Generator Circuit

This is a design circuit for high frequency waveform generator is very useful in electronic experiment and design. This circuit is generate sine wave oscillation, but actually we can modify the circuit to generate triangle or square wave function. The core of this waveform generator is MAX038. This integrated circuit chip gives complete function to build a waveform generator/function generator. This is the figure of the circuit;

The frequency can be controlled using current. If we disconnect the 20k RIN from REF (pin 1) and connect it to a DAC, then we can control the frequency using microcontroller or digital interface. We can even control the chip using a quartz crystal (PLL) by controlling the current using a phase comparator output that compares the sync output (pin 14 of MAX038) and a reference clock from quartz crystal oscillator. This waveform generator integrated circuit chip is very interesting since it can generate 0.1Hz to 20MHz, very wide operating frequency, as expected for every waveform generator instruments.