[time-nuts] yet another GPSDO design, or so
attila at kinali.ch
Tue Jun 29 04:39:29 EDT 2010
On Sun, 27 Jun 2010 02:24:31 -0700
Said Jackson <saidjack at aol.com> wrote:
> The 4002 expects a tight phase lock on the two inputs to properly stay
Why does the ADF4002 need that? Or do you mean by "locked" that the
locked output of the ADF4002 does reflect the actual locked state?
If you mean that, i dont think i will use that at oput. Maybe as
a debugging help, but not for the control loop itself.
As for the design, i wanted to rely on the ADF4002 haveing a
linear phase difference response within [-2Pi,2Pi] and integrate
over the phase error to average out the jitter introduced by GPS.
> and your adc/dac will likely introduce too much phase lag and
> cause oscillation. In fact when using the Analog Devices PLL simulator
> one has to closely follow the component values of the loop filter that
> the software suggests otherwise the system won't be stable, unless you do
> a lot of manual math. The 4002 works well with loop bandwidths of 30 Hz
> or more.
The exact control of the loop parameters are exactly the reason why
i want to use a uC implementing the control loop. Implementing such
low frequency control loops is a lot easier in software (or rather
digitally) than it is doing with analog electronics. And yes,
i'm quite aware that i'd have to tweak the loop parameter for stability.
But i dont expect this to be difficult as long as the analog part
is operated in its linear range. Unless of course, there is
something in the electronics i have not taken into account. Which
is why i'm asking here for advice :-)
> For a GPSDO you are looking for loop bandwidths of 0.001Hz or less, a
> totally different world. Even if you use the 10 Mhz output rather than
> the 1 pps. This is because your Isotemp Ocxo will be much more stable
> than the gps for short time intervals, say 0.1s to 500 s
Yes, i'm thinking about a 1/f in the range of 100s to several 1000s.
> What would be easier to try is to replace the gps internal Tcxo with
> an external ocxo, but you have to generate the frequency the gps is
> using, such as 26 MHZ and do some soldering on the gps itself.
Yes, that would be an idea. But it's not that easy. I dont know
how the control loop in the LEA-6 works and whether they are actively
changing the TCXO frequency. If they are, i would need to provide
a VCO that is stabilized, either in form of an seperate OCXO or by
locking/syncinc the VCO to my 10MHz OCXO.
I have one open LEA4-S infront of me. That has crystal mounted
marked with R4130 and that looks from the configuration (two
of the 4 pads on GND, two connected to other components) that
it is a simple quartz without any voltage control.
The Atmel uC (probably a SAM7 OEM variant) has only one small
crystal connected to it that looks like a 32kHz quartz. No
other frequency source is on the uC side of the module.
Although the LEA4 is two generations older than the LEA6,
i expect it to be at least similar by design.
> It would be very interesting to see just how good the uBlox lea-6t can
> work with an ultra stable frequency source rather than a $5 tcxo..
Yes. But then i'd have to build two devices, one that uses a stock
LEA6-T and one that uses a modified LEA6-T and measure both to have
an indication how much improvement this brings :-)
If you want to walk fast, walk alone.
If you want to walk far, walk together.
-- African proverb
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