rf-design wrote on Mar 9^th, 2010, 2:34pm:



Sounds simple!

Now - go do something very quantitative:

1. Find the frequency accuracy needed to hit a tuned circuit (the crystal) with enough accuracy to realize less than 3dB-6dB of passband loss. The Q of your circuit is 0.5 x 10E6. For the sake of argument, lets call it a 10MHz crystal.

I expect you will find an answer that is under 50Hz, and if my memory serves me correctly the answer is around  20Hz.

2. Go model that RC oscillator, put in all the PVT corner variables and determine the frequency variance.

I expect you will find a +/- 40% variance (give or take)

3. Determine how much resolution you are going to need to get an adjust circuit that can be "tuned" within 1/2 of the tuned accuracy stated above. (best guess is 10Hz) - so with an RC oscillator that varies from 14MHz to 6MHz (delta = 8 MHz) your adjust system needs 800,000 adjustment points.

Thats a lot of tuning and adjustment to hit a very narrow frequency window.

Now, after you get that circuit tuned up to hit the right frequency, change something, pretty much anything, but lets say change the temperature, or the power supply voltage.

You just lost your tuning point, and you are outside the frequency window.

Your numbers may vary, but the above is the concept. Problems are seen in simulation, and I have seen this idea in the lab and seen that it has problems there too.

there is a much simpler solution and everyone seems to keep ignoring what I said originally -

put a lot of gain after the crystal oscillator so the signal is usable at a small amplitude and does not need to need to go thru the long period of start up amplitude growing.

that works. its simple and works reliably.