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Experiments with variable crystal oscillators (VXOs) Crystals are normally thought of as fixed frequency devices. Put one in a transistor oscillator circuit and you get a signal on the frequency marked on the can. That's useful for many applications. For example just about every piece of consumer electronics with an IC in it has a crystal oscillator to function as a 'clock' to govern the timing of internal processes. Any computer or modem will have at least two or three quartz crystals or oscillator modules. Crystals are also extensively used in receivers and transceivers. Several can be connected to form a narrow filter to provide selectivity on reception or SSB generation on transmit. And they are required for detector and mixer stages to work in receivers. And the simplest amateur homebrew transmitters have a crystal right on the operating frequency. A ceramic resonator is basically a less stable but still useful type of crystal. As I mention on my QRP page, frequency agility is king. You will get many more contacts if you can vary your transmit frequency. That's because agility gives you flexibility to dodge interference or to reply to a station calling on another frequency. If you don't want to go the full way with a free running or DDS VFO you can shift the frequency of a crystal oscillator slightly if you use a variable crystal oscillator (VXO) circuit. This is just an ordinary crystal oscillator with a variable capacitor and (usually) also an inductor in series with the crystal. It's quite an art to get maximum frequency shift while retaining the frequency stability that crystal oscillator circuits are known for. Read and watch the videos below for tips on how you can achieve this in the VXOs you build.   Super VXO shifts 80 kHz on 80m   Cheap 10.111 MHz crystals - how far do they swing in a VXO circuit?   10.111 MHz super VXO with 10 crystals   Four crystal 7.023 MHz super VXO pulls 5 kHz higher (ideal for US Technician/General hams)   Can a 16 MHz crystal multiply to generate a signal inside the 144 MHz band?   Changing a quartz crystal's frequency with sandpaper and a pencil   Experiments with 1.8 MHz ceramic resonators   Filing a 3.58 MHz ceramic resonator to change its frequency   Filed up 4.92 MHz ceramic resonator shifts over 100 kHz   Frequency shifting a crystal oscillator module by varying its voltage   Three features every homebrew QRP CW rig must have (describes tx/rx offset VXO circuit)   Disclosure: I receive a small commission from items purchased through links on this site. Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.

Books by VK3YE Ham Radio Get Started (USA) Australian Ham Radio Handbook (Aust) Hand-carried QRP Antennas More Hand-carried QRP Antennas 99 things you can do with Amateur Radio Getting back into Amateur Radio Minimum QRP Illustrated International Ham Radio Dictionary Make your Passion Pay (ebook writing)   All material on this site (c) Peter Parker VK3YE 1997 - 2024. Material may not be reproduced without permission. Read privacy policy.