PTS Frequency Synthesizers
Amongst the dinosaurs in our labe are "Programmed Test Sources" (PTS) frequency synthesizers from the late 1970's - early 1980's. Earlier this week I observed that for certain frequency settings (26 612 123.818 000 000 Hz +- a few mHz, for example), their phase noise becomes pretty big. So I dug into the (excellent!) documentation to find out how this can be. Did we miss this behaviour entirely for the last 4 years?
No. As it turns out, the problem was intermittent and went away after two days... But I did learn how they operate:
A 10 MHz reference signal (either internal or external source) is transformed (analog) 14 MHz, 16 MHz, 18 MHz, 20 MHz, 22 MHz, 112 MHz, and 113 MHz. These are then used to form a 14 MHz carrier signal that carries all the fine digits up to the 100kHz dial using several identical sections. Each section takes the 14.X MHz carrier of the previous section (first one takes pristine 14 MHz), and then adds a combination of the 14 MHz, ... 113 MHz signals such that the total signal is between 140.X MHz and 149.X MHz. The specific combination depends on the dial setting for the particular digit, so that you end up with 143.X MHz if the current dial was set to 3. Finally, the section divides the carrier by 10 so that the output is between 14 MHz and 15 MHz again (in this case, 14.3X MHz). They daisy-chain 7 of these modules, with an option to connect an external source to the 14MHz input of the first section, so that you can daisy-chain two or more synthesizers and thereby get arbitrary precision.
As of 2014, the company still exists and sells frequency synthesizers with impressive specs (just look at that phase noise!), but by now they mostly switched to direct digital synthesis for the lower digits (although their FAQ mentions the analog mixing as an option).
As a side note: The other, popular way to synthesize arbitrary frequencies is to use a phase locked loop (PLL): You run a voltage controlled oscillator (VCO), divide it's output by a user-set number so that the divider-output coincides with the frequency of the reference oscillator. You compare the phase of the ouput with the reference and servo the VCO to keep the phase difference constant.