Make your own crystal oscillator.


Introduction

Sometimes there might be a requirement for a non-standard crystal oscillator. While most crystal manufacturers will grind a custom crystal for you, it would obviously be much cheaper and quicker for you to make your own. Bear in mind this project is suitable only for those people who are well acquainted with electronics and soldering. You will need a frequency counter, or at least an oscilloscope. In this document the word "crystal" refers to a simple quartz crystal, usually packaged in a metal can with two leads. The word "oscillator" or "crystal oscillator" refers to an electronic device which is crystal controlled and generates a clock signal suitable for digital circuits. It is also packaged in a metal or plastic case with 4 leads.

This is a design, using very cheap, common parts which will allow you to build a drop-in equivalent for the popular metal can 14-pin DIL layout crystal oscillator modules. One problem, however, is that most crystals available to the home hobbyist are of frequencies less than around 36 MHz. There is a way to obtain higher frequencies by causing the crystal to oscillate in a different "mode". For example, a crystal labelled as 14.31818 MHz can be induced to oscillate at 42.9 MHz in third overtone mode. Likewise in 2nd overtone mode, you will get 28.636 MHz. A 25MHz crystal will give you either 25MHz, 50MHz or 75MHz. The one crystal gives you a choice of at least 3 frequencies.

To select which frequency you want is simple. You first need to work out what crystal you need. Divide your desired frequency by either 1, 2 or 3, aim for a resulting frequency that is available. For example, you need a 64MHz oscillator. Dividing by 2 gives 32MHz, which is a crystal you can buy from most electronics retailers. To make your crystal oscillate at the higher frequency, you will need to vary the value of C1 shown on the schematic. A smaller value capacitor will force the crystal to oscillate at it's higer frequency. Generally, the capacitor would be somewhere in the range of 1pF to 15 pF. The actual value required would be best found by experiment as there are quite a few variables involved such as wire lead lengths (try to keep them short), the IC used and most importantly, the capacitance inherent in the crystal used.

As an example, to generate a 55.6 MHz clock used in overclocking the A2386 Bridgeboard, I used an 18.5 MHz crystal operating on it's 3rd overtone. The value of C1 required was 3pF. Be aware, that if you make a similar oscillator, you may need a slightly different value for C1. Another example is a requirement for 32 Mhz. In this case I had an actual 32 Mhz crystal, so to make the crystal oscillate on it's fundamental mode, I used a value of 10 pF for C1.

Parts Required

1x 74F04 hex inverter IC.

2x 1.2 Kilohm resistors.

1x crystal

1x ceramic disc capacitor, preferably NPO type.

Frequency counter and appropriate tools...

Construction

Make sure you have a 74F04 as the more common 74LS04 may not work reliably at high frequencies. Start by cutting off all pins except for pins 1,7,8 and 14 (the pins nearest to each corner) Leave a stub for pins 9,10,11,12 and 13 as you will need to solder to them. Referring to the layout diagram, solder on the resistors and crystal as shown, making sure leads are short. The resistors can lie flat on the IC, with the crystal soldered in vertically. Short together pins 9 & 10 with a solder blob. Try to avoid overheating the IC. Temporarily attach the capacitor to the appropriate points. Try starting with around 5 pF.

Using an experimenter's breadboard or an IC socket, connect +5V DC to pin 14 and GND to pin 7. Connect your frequency counter to pin 8. Power up. You should get a reading on your counter. If it is too high, increase the value of the capacitor. If too low, reduce the value of the capacitor. Make sure the frequency does not vary if you heat or cool the oscillator - or if you touch the capacitor with your finger. If the frequency varies, it is not oscillating under crystal control - keep changing the capacitor value until you get the desired frequency that is stable under all conditions. It might be best to find the "mid point" by finding the minimum and maximum capacitor value that results in the desired frequency and then using a capacitor value midway between the two points. You can trim down the value of a ceramic capacitor that is a little too high by using a file to remove part of the capacitor.

If you are using an oscilloscope, check for a stable waveform that does not drift when you vary the temperature or touch the capacitor.

You might find that the home made oscillator is a little loose in the socket when you plug it in to your computer. You may need to arrange something to hold it down in it's socket if your computer is subject to vibration.


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Introduced 5th July 1998, updated 30th July 1998. Version 1.1