Use 16-bits if you have it, especially during cold/warm start. But if you only have 8-bits then I can think of two solutions. One is to extend the h/w 8-bit timer with one or two bytes of s/w overflow count. The other is to reduce the sample rate by 2^N (you can do this on a PIC with a prescaler; not sure about AVR) so the numbers are smaller by a factor of 2^N and still fit in 16- or 8-bits. It's one of the rare cases where a trade-off between range and resolution is in your favor.
If your CPU doesn't have a prescaler, or maybe even if it does, just use 16-bits. Or implement the overflow code -- but realize that it's really tricky to make it work at the boundary conditions. /tvb ----- Original Message ----- From: "KD0GLS" <[email protected]> To: "Tom Van Baak" <[email protected]>; "Discussion of precise time and frequency measurement" <[email protected]> Sent: Thursday, April 10, 2014 11:48 AM Subject: Re: [time-nuts] First success with very simple, very low cost GPSDO I should have said warm start, not cold. I was referring to the code, not the oscillator. So tell me, the OCXO is warm, there's no previous EFC information to draw upon, and the oscillator is off-frequency by more than can be measured with, let's say eight timer bits. What do those early measurements tell me, and which direction from midway should the EFC be adjusted? > On Apr 10, 2014, at 13:28, "Tom Van Baak" <[email protected]> wrote: > > I agree with Charles. Further, you don't even have to wait a predetermined > amount of time (this would be oscillator or environment dependent). Instead > simply monitor the rate of frequency change. When the drift rate drops to the > level where your PID is known to be able to track, then start the PID. > > Realize that just 2 seconds after power-up you have your first frequency > measurement. By 3 seconds you have your first drift measurement. Just wait > until it falls to however few ppm/second or ppb/second you need for your loop > to smoothly track. This avoids special case PID startup or wind-up code. > Although you can argue it merely replaces it with special case drift rate > code. > > I'm suspicious of fast/slow tracking loops. If you want to vary the tracking, > perhaps it is best to continuously, transparently, smoothly vary loop > parameters according to drift rate rather than use a hardcoded fast/slow > algorithm binary switch. I'm sure there's deep theory on this, which I have > not read yet. > > /tvb > > ----- Original Message ----- > From: "Charles Steinmetz" <[email protected]> > To: "Discussion of precise time and frequency measurement" > <[email protected]> > Sent: Thursday, April 10, 2014 10:55 AM > Subject: Re: [time-nuts] First success with very simple, very low cost GPSDO > > >> >>> In my case, the cold-start frequency of my OCXO with EFC at midpoint >>> was off sufficiently far that I needed a minimum number of remainder >>> bits to know which way to initially steer it. Don't recall the exact >>> number I needed, but it was more than eight at 10 MHz. Like this >>> design, I had 16 bits to work with, which gave me a usable range. >> >> Why even try to discipline an OCXO before it's warm? Just leave the >> control loop off for a predetermined time at startup. You can light >> up a bright red "unlocked" LED, and even inhibit the 10 MHz output >> until lock is achieved if you want. >> >> Alternatively, you could figure out the EFC voltage needed to zero >> the cold oscillator and load the corresponding DAC code at >> startup. However, if the control loop is slow enough for good GPSDO >> performance at tau out to 100 seconds or more, it would probably be >> too slow to track the oscillator as it warms up -- so you would >> likely need switched fast and slow loop filters. (Switched "acquire" >> and "maintain" time constants are often very useful for a number of >> reasons, and a GPSDO can benefit from several different "maintain" >> time constants for best performance in noisy conditions and >> recovering from holdover.) >> >> Best regards, >> >> Charles > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
