[time-nuts] Re: Realtime comparing PPS of 3 GPS

[glen english LIST]

Be aware not to confuse the antenna ground plane  (the patch will always 
have its own plane because the top metalization must be fed against a 
plane or counterpoise -Â  and a ground plane behind the antenna.

I can see the usefulness of the larger ground plane for any purchased 
patch antenna to reduce the likelihood of interference underneath (if 
the feed coax has a good RF contact with the plane), and if the plane is 
coupled well, it may improve the low angle response .

The supplementary ground plane doesnt have to have a galvanic connection 
if the gap between the underside of the patch is low- IE use purely a 
capacitive coupling to tie the patch antenna ground to the large ground 
sheet-

If we consider the patch area to be 10x10mm  = 100uM^2, and the gap 
being air (for simplicity sake) of 0.25mm, the capacitance is 
Epsilon-nought times area, all divided by the distance between the two 
plates

For the above example this is about 3.5pF or (-)j30. Really needs to be 
< j5 .

That means reducing the gap to about 0.05mm  OR increasing the area- 
probably means using a bigger patch.

You might be able to sweat solder the patch antenna (bottom)Â  to a sheet 
of FR4- that would be my approach.

-glen



On 31/05/2022 1:24 am, Carsten Andrich via time-nuts wrote:
> Hi Erik,
>
> have you tried running all receivers off the same antenna via a power 
> splitter (make sure to dc block all but one receiver)? That should 
> remove the uncertainty due to antenna differences (location, RF 
> characteristics, etc.).
>
> Also, are you using ground planes for your puck antennas? These types 
> of antennas typically require a ground plane for optimal performance [1].
>
> Best regards,
> Carsten
>
> [1] 
> https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16
>
> On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote:
>> Further evaluation did shown the time differences between the 3 GPS 
>> modules was due to difference in the trigger level setting of the 
>> timer/counter and difference in length of GPS antenna cables.
>> After removal of the phase drift due to Rb frequency offset the 
>> attached image shows the phase differences of the 3 modules versus a 
>> Rb reference.
>> The two ATGM modules are very consistent over a 2.8 hours period. The 
>> NEO-7M varies wildly  with phase errors above 100 ns. Possibly due to 
>> a somewhat less optimal antenna position.
>> It seems phase variations over time in the order of 10-20 ns are 
>> indeed unavoidable, even with a good antenna.
>> Erik.
>>
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-- 
Glen English
RF Communications and Electronics Engineer

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