XTAL Osc. Varactors

A fun new project I'm working on involves receiving WWVB from NIST in Colorado on 60KHz and using it to synthesize my own time bases at home. The heart of my project is the Hewlett Packart 10811A ovenized crystal oscillator. To make this all work I need to develop a PLL system to synthesize a base 10MHz signal in order for a microcontroller to compare this to the HP oven-crystal over long periods of time. The comparison will generate long-term error signals which are used to "discipline" the HP crystal to the excellent long-term accuracy of the NIST clock.

This section deals with the synthesized 10MHz reference for the time base. To make a PLL one needs a good VCO (Voltage Controlled Oscillator). I'm limiting myself for this project to only parts I have on hand and so I've sifted through and found a nice old 10MHz clock oscillator from a computer board. The oscillator has a trimmer capacitor allowing tuning over a range of a few kilohertz. This oscillator is old enough to be before they started putting them into hermetically sealed cans; I was able to pry it apart and get to the guts as you can see below.

The circuitry inside is quite simple and involves an ordinary 10MHz crystal. In order to turn this thing into a VCO one needs some way of varying the capacitance in the tuned circuit based on some input voltage. A device intended for this task exists and is known as a "Varactor". This is a diode specially designed to exploit the varying capacitance of a reverse-biased diode junction. Higher reverse voltages cause the barrier in the device to widen and thus reduce the effective capacitance. This allows very simple control of capacitance via voltage. An interesting fact about this effect is that it follows a "square-law": that is, capacitance varies as the square of voltage. I'm not taking advantage of the square-law function here but in some cases this can be quite useful.

Seen here is a schematic showing what I've done to add the VCO functionality to my oscillator.The original circuit is only shown in part. The additional circuit is exceedingly simple. The varactor is the bit that looks like a diode stuck to a capacitor. This is coupled to the resonator using a capacitor so as to isolate the DC voltage applied to the varactor through the 470k resistor from Vc. The 500pF capacitor serves as a bypass to prevent RF coming back out through the Vc line. For further reduction in RF coupling to Vc one would use an RF choke in series with Vc.

The varactor I used is something I pulled from an old radio. I don't know the specifications of my varactor but I tried measuring capacitance on my DMM. It shows about 3pF in one direction and is below the meter range in the other. I would assume this varactor is made to work in the 15pF range with plenty of bias. My circuit happens to tune nicely to 10.000000MHz with 5V applied to Vc. This is an excellent center voltage as I will be using a 0-10V signal to control the oscillator around the desired frequency. The varactor input allows me to change the frequency over a few hundred hertz within my desired voltage range and so is more than enough to synthesize against WWVB by far!

After modifications I added an SMA connector to the original oscillator can and removed the sticker. I also added a brass plate to the bottom and attached it with a bead of solder. The original can had no bottom cover and I prefer to have complete shielding if possible. The original output pin ended up being perfect to use for Vc for the varactor. I lifted the output signal and led it to the SMA connector instead. Pictures tell all below! You'll see more on this as I construct the rest of the elements required to make my WWVB disciplined time base system.


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