[time-nuts] FE-5680A GPS discipline board on sale now

[Attila Kinali]

On Fri, 20 May 2016 07:03:10 -0700
Nick Sayer via time-nuts <time-nuts at febo.com> wrote:

> > The discharge resistor of the integrator (R13) is with 10M way too high.
> > The PCB resistance is usually in the same order of magnitude. Ie with
> > a resistance that high, the actual resistance highly depends on how
> > clean the board is (finger prints, dust) and the amount of humidity
> > it absorbed (both the epoxy and the solder resist are quite hygroscopic).
> > A resistance of 1M usally is the maximum recommended value. For systems
> > that should run reliably for a long time, i would even try to keep it lower.
> > Of course, you have to increase the capacitor accordingly, which also makes
> > the capacitor less dependent on parasitics. 
> 
> The discharge time constant is not critical to the design.
> It needs to be long enough to not have a significant impact
> own the cap over the course of the first few microseconds,
> but it needs to clear out the cap before the next second.
> A larger cap requires a smaller resistor, which increases 
> the current from the PLL output pin. I joked at the time that
> I could just clip a fingernail and glue that onto the pads
> instead of using an 0805, but the standard deviation would be quite high.
>

Ah.. ok... so all you needed was something that defined the minimum
discharge resistance?

> As with my TCXO/OCXO based designs, there are a lot of choices that
> just wind up being “good enough,” given that the target ADEV is easily
> an order of magnitude higher (10^-11 ish for the OH300) than many Time
> Nuts(tm) aspire to.

Jupp... I just worte a long text block detailing the temperature
dependence of your phase measurement circuit, only to realize that
its error is likely to be burried in the temperature dependence and
noise of the rest of the circuit. Hence i deleted it again...

(In case you wonder: the phase measurement has probably a temp dependence
of about 0.1%/°C, in both variants. Plus the dependence of the ADC)

 
				Attila Kinali

-- 
Reading can seriously damage your ignorance.
		-- unknown