I have been looking at some small, simple transmitter circuits (Eg. Mighty Mite) for CW on the HF frequencies around 3-10 MHz using a crystal and LC circuit in parallel. As far as I understand the LC can be used on it's own to produce and oscillation which would work but be sensitive to noise, temperature change, etc, and the crystal provides feedback which helps stabilize the frequency.

I was wondering if the crystal can be used alone, without the LC circuit. Resonating only at the crystals fundemental frequency? Is the main advantage of using the LC circuit together with the crystal to provide a bit more range and the ability to tune or does it also help in other ways?

 

Hello,

Have a look at the following pages:
https://www.electronics-notes.com/articles/analogue_circuits/transistor-rf/crystal-oscillator.php
https://www.electronics-tutorials.ws/oscillator/crystal.html

Bertus

 

I was wondering if the crystal can be used alone, without the LC circuit.

Hi there. They do & you can Crystals are produced to resonate at many different specific frequencies for particular applications accuracy and stability are of vital importance as radio bands are usually crowded with many transmitters operating in a given radio band. Transmitters therefore must not allow their transmission frequency to wander and interfere with adjacent transmissions using discrete L C tank circuits frequency's will drift . Receivers must be able to tune to known reliable frequencies .. With ordinary inductors and capacitors, Q factors more than a few hundred are not possible, but by using quartz crystals Q factors well in excess of 10,000 can be achieved.that equals stabilization.

 

To add to what's already been said, the equivalent circuit for a crystal is an LC circuit with high Q :

 

Like this?

 

Thanks for all the helpful replies. One thing I'm not sure I understand is how the crystal and the LC are working together in the circuit. Is the LC being used to surpress or isolate some of the crystals oscillations, giving the ability to tune?

 

Mechanical analogy. The crystal is the energy frequency (rpm regulator) agent while the LC is the energy storage (flywheel) agent at that rpm. There is interaction between the two agents but the crystal is relativity insensitive to resistive loading changes in the circuit.

 

Is the LC being used to surpress or isolate some of the crystals oscillations, giving the ability to tune?

Your on the right track! Crystals are produced to resonate at many different specific frequencies for particular applications, but the range of available frequencies is made much greater by various techniques such as frequency division, where the frequency of a crystal oscillator is sequentially divided by 2 many times by digital dividers, to a much lower frequency. Because any slight errors are also reduced by the same division process, the final low frequency is much more accurate.
Crystals can also be made to resonate at higher multiples of their basic resonant frequency. One of these higher multiples, called overtones) can then be selected using a conventional LC circuit. By using these frequency division and overtone techniques a much wider range of crystal frequencies can be achieved.
However, it is also possible to “tune” a crystal oscillator to any even harmonic of the fundamental frequency, (2nd, 4th, 8th etc.) and these are known generally as Harmonic Oscillators while Overtone Oscillators vibrate at odd multiples of the fundamental frequency, 3rd, 5th, 11th etc). Generally, crystal oscillators that operate at overtone frequencies do so using their series resonant frequency.
So... When crystal oscillators operate at even harmonic of the fundamental frequency.they do so in there parallel resonant frequency.

 

Great! Thanks for all the help and info everyone.

 

Doug DeMaw W1FB wrote several notebooks.
QRP notebook was fun to read. The book answers the question: Does LC do more ? absolutely.

 

Hello,

Here is one of the notebooks @sparky 1 mentioned, found in the Internet Archive:
https://archive.org/details/fea_W1FB_QRP_Notebook
The EYE has also a design notebook of W1FB:
https://the-eye.eu/public/Books/Electronic Archive/W1FB Design Notebook.pdf

Bertus

 

I'll definitely have a look at those, thanks.