[time-nuts] GPSDO PLL Loop Filter Values

[Bob Camp]

Hi

Simple answer - adding gain will mess things up noise wise.

More detailed answer - The noise out of the GPS should be higher than the noise out of a reasonable TCXO out into the several seconds range. On a good TCXO it will be higher out into the 10's of seconds. On a very good one, you may get to the 100's of seconds. A lot will also depend on just how well the TCXO is shielded from drafts. Long time constants = low gain. 

Bob


On Mar 15, 2010, at 6:06 PM, Joe McElvenney wrote:

> Hi,
> 
> I'm cobbling together a very basic PLL GPSDO with components I have to
> hand which means I'm pushing my luck as far as the loop filter values
> are concerned.
> 
> These are -
> 
> N = 1000 (10kHz ref from a Jupiter-T),
> 
> Kv = 27Hz/V (10MHz Motorola VC-TCXO),
> 
> Kp = 0.4V/rad from a 74HCT9046 type-II PD with charge-pump.
> 
>> From these I've worked out some values graphically and with a couple of
> apps I've found. They are an old DOS program by KD9JQ and a web one at -
> http://www.aubraux.com/design/pll-design-tool.php
> 
> The first tells me I need some gain between the loop-filter and the VCO,
> the second (the third order active choice) seems (I think) to have the
> capacitor values the wrong way round.
> 
> My own efforts suggest I can get away with the values I've calculated
> but with a lower loop bandwidth and longer lock-up time than I'd like.
> The op-amp I'll be using for the filter, or extra DC gain for the VCO,
> will be an OPA-703.
> 
> How badly would sticking a gain block between the filter and the VCO
> screw things up noise-wise (if at all)? Any thoughts on getting out of
> the 'low resource' hole I've dug would also be much appreciated. BTW,
> I'm not pushing any envelopes and my previous projects, with different
> components, have worked out reasonably well
> 
> 
> Cheers - Joe G3LLV
> 
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