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Saturday, July 1, 2023

Building a 10MHz Master Clock

Even though I have a GPSDO, and in the procees to upgrade it, I still would like to build a 10MHz reference clock.

I have a spare Oscilloquartz 8663-XS OCXO that I wanted to use for my GPSDO, but the lack of room on the PCB stopped me in my tracks.

This OCXO is too good to not use, so I embarked on a little side project to put it to good use.

For a very long time, I already collected information from another user that built a frequency reference with that same OCXO, so I borrowed some of his ideas.

Here is the link to that design :

It's a Dutch forum but I take it you can either follow it, or use Google translate.

There are three areas where I'm going to deviate from that original design. First of all, I'm not going to create a separate power supply for it. The unit will be on 24x7 and I did not want a normal transformer to power the unit. I have a nice 12VDC 1A wall-wart that uses a transformer, so I'm going to use that. The stability and precision of the 12V is not that important for this design.

The other area I'm deviating is by using different output transformers. There was a large effort to create very special hand-made transformers, but I did not want to go that far. I'm going to use the commercially available transformers that I already use for several other projects.

Lastly, I'm not going to use the Oscilloquartz reference voltage to power the frequency setting circuit. It seems the logical thing to do, but this so called reference voltage is anything but a reference voltage. They are typically created by using a zener (reference) diode like the TL431, but that is simply not good enough as you will see.

During my work with the Reciprocal Counter, described in another post, I found to my dismay how poor that reference voltage really is. Have a look at that post and search for the following update: "Testing the counter (11-05-2023)" towards the end.

Here is a display of the vref output that shows what I mean:

This OCXO has been powered for many weeks 24x7 and is in a plastic container isolating it from drafts, but not the room temperature of course. It's easy to see that using the Vref of the OCXO will fall short when you really want a very stable 10MHz, because any variation in the Vref output will show-up as a change in the output frequency.

The Oscilloquartz is currently running while using the following circuit, put together after my "discovery". Here it is using a 10V reference I had in my stash already. This prototype is built on protoboard.

My plan is to use the above frequency setting circuit, but with a REF5050, which is a 5V reference.

Here is the schematic I decided on.

I only need two isolated 10MHz sine wave outputs.

Most of the resistor values are taken from the original design but could possibly change once I start to  test it on the PCB.

My intent is to put the circuit in a metal enclosure, but put all the electronics in an isolated foam box inside the enclosure so the OCXO will warm-up the inside far enough above the maximum room temperature, hopefully reducing the temperature dependencies of the components.

This is a side-project so may not get a lot of attention while I'm working on the GPSDO version 4 and the Reciprocal Counter.

[update July 2023]

I did manage to create a PCB layout.

The two long multi-turn trimmers top left are from my old part stash, even through they date back to the 70's, they are genuine and of good quality. The adjustments of these course and fine trimmers will protrude through the front panel, the same as the two SMA output connectors. The length is 125mm and the width of the board is 99.5mm so it will slide in the typical enclosures I use.

Because PCBWay is sponsoring my PCB's, I'll wait until I have another PCB ready to order, to save on the shipping costs. Maybe that will be the front panel for this instrument.

Stay tuned for more...


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