> Then, I am not sure all GPS receivers actually use their internal > crystal oscillator in a PLL as the timebase. I have read that some > work by removing or adding pulses in a discrete fashion rather than > phase locking, and this offers significant jitter, making the 1 PPS > only useable to phase lock a reference oscillator with a very long > integration period to eliminate the jitter.
> Now, I can't find the article describing that technique, so it may > have been old news. It's a reasonably common trick in the digital world. Direct Digital Synthesis. Analog Devices makes chips that do it all. They have good app notes and data sheets. If you have a clock running at X and you want Y when Y is (much) smaller than X, just make a big adder and add a magic number each clock tick. Think of the decimal (binary) point at the left end of the adder. So if you start with 8 MHz and you want 1 MHz, just add 001 (binary) to a 3 bit register each cycle. The top bit is the output. If the ratio doesn't come out even, you can get arbitrarily close by adding more bits. You are adding low bits on the right of the adder. If you want a sine wave, take the top N bits and feed them to a table lookup ROM and feed that to a DAC. The output frequency is solid - maybe not exactly what you want but you can compute it from the constant you add, the main clock frequency, and the number of bits in the adder. The output clock ticks will be on the closest main clock edge. That leaves you with a 1/2 cycle of jitter. In many applications that's not a big deal. Somebody mentioned a sawtooth jitter pattern. That's what I would expect from this sort of clock if the target frequency is very close but off slightly. Say you want 1 HZ but your crystal is running a tiny bit fast. Every now and then it will insert an extra cycle to slow down the output. I'll bet that turns into a sawtooth of phase error if you compare it to a good reference clock. The phase error will build up to 1/2 cycle then it will slip to -1/2 cycle and start building up again. -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
