[time-nuts] TBolt settings (was GPSDO Design)

[WarrenS]

> Lots of fun little details to worry about managing....
    ws) TOO True.
A temporary attenuator bypass switch or syncing at a higher freq are two of 
many ways to help the initial lock up delay caused
when a high value Dac attenuator is used to lower the effect of EFC noise 
and / or to increase the Loop TC without needing large Capacitors.

**
This reminds me of a couple of unconventional Tbolt setting that have been 
useful for me.
(Mostly applies ONLY to advanced Tbolt users that like to tweak things)

#1) Along with setting the "initial Dac voltage" to the nominal Dac out, 
ALSO
Set the "Min & Max Dac Voltages" to 0.100 mv or so below and above the 
nominal Dac output voltage.
(or more than  +- 100mv if your Osc is still drifting lots)
Reason:
Although these setting have NO effect under normal operation, every once in 
a while,
I'll set something wrong (like Dac gain to the wrong polarity) and the 
control loop will Drive the DAC voltage way off.
What I have seen is IF the EFC voltage is given a large temporary voltage 
change,
it can (will) cause a small Freq offset to occur even after the Dac is 
returned to its previous voltage.
This offset can then take a long time to recover, similar to cycling the 
power.
Clamping the Dac level helps that from happening by limiting the max EFC 
voltage change.

#2) Set the Tbolt to the "single Sat mode and the Sat number to "00"  (two 
zeros)
(Note, ONLY applies to units with a poor antenna signal)
Reason:
IF you have a poor antenna view caused by things like an indoor antenna or a 
partial blocking of signal in one or more directions,
That will cause excess Osc noise as the low level satellites switch in and 
out.
In the "single sat / double zero"  mode, the Tbolt will only use the highest 
Sat available.
This causes less satellite switching noise, but then there is only one 
Satellite at a time to average.
Always that trade off.
What I've seen is for poor antenna signals the single sat mode can help Osc 
and phase noise about two to one,
For the average and good antenna signals the single sat mode hurts by two to 
one or more.

ws

**************
From: "Bob Camp" <lists at cq.nu>
Subject: Re: [time-nuts] GPSDO Design
>

> Hi
>
> If you fire up a conventional 1 pps loop with 10 ppb tune range, it's 
> going
> to take it a while to track out any initial error. The most it can move by
> is +/-5 ns per second. You probably don't want to wait a year for it to
> catch up with a 158 ms initial error.
>
> Lots of fun little details to worry about managing....
>
> Bob
>
*****************************
> Subject: [time-nuts] GPSDO Design
>
> John ask
>>Translating nV/sqrt(Hz) to something
>>practical is basically the assistance I'm looking for here.
>>I would appreciate anyone being able to teach me a bit more about this.
>
> If that is ALL you want to know, That's easy and quick.
> For this application sounds like you already know ALL you need to know 
> about
> that,  nothing.
> Putting a 1 sec or so RC filter at the EFC input, takes care of all that 
> AND
> if you want it even better,
> and to get the long Control loop time constants needed, JUST reduce the
> (loop) gain, don't need no BIG caps.
> That is attenuate the output of the control amp by typically a hundred to 
> a
> thousand instead of multiplying by 1.6 and add a fixed, adjustable, 
> stable,
> offset source. (electrical or mechanical)
> The Buffer amp is not going to be your problem.
>
> ws
>
> ************
>
> [time-nuts] GPSDO Design
> John Foege john.foege at gmail.com
>
> Hi All,
>
> Quick question for the more experienced members here with GPSDO
> design/operation. Let's assume I'm using a 4096 phase comparator chip
> followed by some kind of long time constant lowpass loop filter,
> whether it be analog or digital, is not of concern for the following
> question.
>
> Obviously using a 74HCT4096 would mean that my EFC voltage range would
> be approx. 0-5V. If I wanted to use an OCXO with say a 0-8V EFC
> voltage range, then I would be inclined to simply use an op-amp
> amplifier with a gain of 1.6 to scale the EFC voltage accordingly.
>
> But not just any op-amp would do I take it? High-speed would of course
> be of no concern. Also low-offset would be of little concern, as the
> PLL would work to correct this, and it therefore seems to be
> negligible. However, the part that's got me thinking is noise.
> Obviously any noise at the ouput of the amp would adversely affect the
> frequency stability of the OCXO.
>
> I thought the best way to control this would be to use an extremely
> low noise op-amp employing a rather large compensation cap to give me
> a rather small bandwidth, perhaps only a few hundred hertz.
>
> Anyone have experience with this? Assuming I have an OXCO with a max.
> pulling range of 1ppm or 1e-6 over a 10V range, then I effectively can
> pull 1e-7 per volt. This translates to 1e-10 per millivolt and 1e-13
> per microvolt. Assuming that is a logical conclusion, then for a good
> OCXO, in which I can at best hope for 5e-12 stability for tau=1s (e.g.
> HP10811A), I would strive to to keep the noise at such a level that it
> is an order of magnitude better than the best short term stability
> figure. Accordingly, then I should shoot to keep any noise under 1
> microvolt?
>
> I don't have much experience with noise calculations. I know it is
> specified in nV/sqrt(Hz) generally. Translating this to something
> practical is basically the assistance I'm looking for here.
>
> I would appreciate anyone being able to teach me a bit more about this.
>
> Thank you in all in advance.
>
> Sincerely,
>
> John Foege
> **************