kb8tq@n1k.org said: > The gotcha with a DDS in this case are the sawtooth spurs. To get them down > to the 5 ps level, you would need a DDS with a clock that is well into the > 100’s of GHz. I think that assumes you want to be able to generate an arbitrary frequency. Suppose I start with 10 MHz. It's easy to get a clean 1 MHz or 5 MHz, but not so easy to get 3 MHz. But if I start with 9 MHz, I can get a clean 3 MHz. How do I figure out how many crystals I need? How good are PLLs relative to 5 ps? Can I use them to make pseduo-crystals? What's the term for PLLs where I divide the output by M and the reference by N? As M and N get bigger I get more possible output frequencies but the output of the phase comparator gets lower in frequency and hence harder to filter. How low can I go before 5 ps gets interesting? (Is that a reasonable question?) If I have an array of crystals and PLLs, I can get some more flexibility if the crystals are VXOs. -- These are my opinions. I hate spam.