Early Morning Sparks New Idea

Here is a technique I have been thinking of for years and tried one morning at 4:00 a.m.

Many instruments have an external reference on the rear panel. I'm thinking of RF generators here. Electronics enthusiasts always are looking for better and better 10MHz reference oscillators. My best in house is a Rubidium, and I want to lock it to GPS soon. There is another neat use for the 10 MHz ref in jack. That is what this note is about.

Some of us know about the wonderful HP 3325A and HP 3325B. These are very useful function generators. The salient feature here is 12-digit frequency resolution via fractional N PLL technique.

My IFR 1200S and other RF generators sometimes don't have enough resolution for my taste. The IFR is 146.5200 MHz, so there's 100 Hz of resolution. The trick is to use the HP 3325A as the 10 MHz reference. Now its 12-digit resolution is transferred to the RF generator.

I set mine to 10.000000000 MHz 2 V RMS. I pipe its output to my IFR's 10 MHz external reference in. Now I can control the IFR RF frequency with greater accuracy -- 146.52000001, for instance. The HP 3325A does need an oven or better local reference. I also needed my HP 5370B Time Interval Counter to read that small of a change. This unit reads 12 digits in one second. Very fine indeed.

In effect, I am varying the reference oscillator but maintaining phase lock.

This technique allows narrow filter eval and GPIB control of very fine frequency corrections, and fine control when multiplying a carrier up to 20 GHz etc. In this case, very small frequency changes are multiplied by the same factor, so fine frequency control is needed.

Steve Lindberg has loved electronics since he received his first Weller soldering gun when he was 12. He has 35 years of test, debug, and design experience.

Tell us your experience in solving a knotty engineering problem. Send stories to Lauren Muskett for Sherlock Ohms.

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