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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
Halve and then multiply
7 MHz from a 14 MHz VXO
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)
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Books by VK3YE
Ham Radio Get Started (USA)Australian Ham Radio Handbook (Aust) More Hand-carried QRP Antennas 99 things you can do with Amateur Radio Getting back into Amateur Radio Illustrated International Ham Radio Dictionary Make your Passion Pay (ebook writing)
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