Monday, July 6, 2020

100 MHz third overtone crystal oscillator

Couple of years ago I purchased from a local store 100 MHz crystal resonator and tried several times to make a working schematic on breadboard using standard circuits I found on the internet. It never worked good enough, usually oscillating at 33.3 MHz instead of 100 MHz. Finally, I found that the crystal is third overtone type. Here some documents, that were useful for this project:

I used as a base the schematic from first document and here what I design:



The schematic is traditional Pierce crystal oscillator with the addition of an inductor in parallel with C2 which should block the fundamental frequency of the crystal and allow it to oscillate at 3-th overtone frequency (3 x fundamental frequency).

The actual schematic which I used is this:



It is very simple circuit and the idea was to solder a female pin headers and then to put capacitors, inductor and feedback resistor on them and try different values until the circuit work stable, and then remove the pin headers and solder the element permanently on the board. The schematic have 5V voltage regulator allowing it to work with supply voltages from 7V to 12V. In case we want to use 5V there can be put jumper between pin 4 and 5 of JP3 which effectively bypass the voltage regulator. The schematic use very fast variant of 7404 hex inverter - 74AHC04 which have propagation delay below 10ns. The oscillator is build with one of the inverters and the rest of them are connected as a buffer with 50 ohm output impedance.

In the first document there is a bunch of formulas to calculate the values of the elements, but in the end the circuit didn't work with calculated values. The values with which it worked are:
C1 = 20pF, C2 = 20pF, C3 = 1nF, L = 0.32uH, Rfb = 100k. 
C3 is for DC blocking and its value is not critical - can be anything between 1 nF and 100 nF.

The output frequency (according to my oscilloscope) is 100.017 MHz, which is OK.
Is it worth the hassle? No. On ebay or aliexpress you can buy 100 MHz active crystal oscillator for a dollar. In fact I ordered 5 pcs to compare them to my schematic. But I made it as a proof of concept and I am happy it finally worked.

Here some additional pictures:






The signal waveform is supposed to be square, but my oscilloscope is only 70 MHz, and anything above 50-60 MHz shows as sine wave.

The Gerber files for this project can be downloaded from here: Link

2 comments:

  1. I stumbled upon your post while searching for what the failure mode of a crystal is.. I have a 12mhz crystal on an arcade board which is broken, and putting the scope to it, I find it's oscilating at 36mhz. I find it more than coincidental that you were TRYING to get a crystal to oscillate at the 3x overtone, and I'm trying to avoid it, or figure out why it is doing it.. Just curious if you're aware of a reason or typical cause that it would happen when NOT desired? My circuit is here: https://1drv.ms/u/s!ApthK9KHvqixhNYW39kPegxw6WZFng?e=bytuhO I pulled pin 2 of the inverter out of circuit to isolate just the oscilation and I am getting 36 mhz on every inverter input and output on the circuit.. Thanks in advance for any advice you may have!

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    Replies
    1. You can try to put some sort of filter to prevent oscillating at 36 MHz. For example you can increase the C12 value.
      The schematic in this arcade isn't most common and you can try different schematic on small add-on board. Also there are very cheap active oscillators with all sorts of frequencies - you can use that too:
      https://www.aliexpress.com/item/32847840207.html

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