[time-nuts] GPS active antenna delay ?

[John Ackermann N8UR]

OK, so I had an HP 58535A two-port GPS splitter handy and put it on the 
VNA.  It clearly has a filter of some sort, as shown by the S21 
frequency response.  The delay at the center of the passband is about 
21ns, and it increases to about 26ns at the edges.

That delay consists of the physical length of the signal path in the 
splitter, plus the effects of a 6dB amplifier, SAW filter, and a hybrid 
2-way splitter.

The noise in the delay plot is because I had to avoid overdriving the 
splitter amp, so the input signal was 40dB lower than normal for the 
VNA.  Thus the analyzer receivers had pretty weak signals to work with 
(and I didn't do anything heroic to compensate, other than use a large 
averaging factor plus smoothing).

I don't know how applicable this would be to the circuit in an antenna. 
  I suspect the biggest difference might be due to the higher gain amp 
in the antenna vs. 6dB in the splitter.

On 02/08/2015 08:29 AM, John Ackermann N8UR wrote:
> I think that the surplus HP/Agilent GPS splitters may have an SAW
> filter.  If so, measuring the delay of one of those could yield at least
> an approximation.
>
> I may have that data laying around; I'll do some digging.
>
> John
> ----
>
> On 02/08/2015 05:11 AM, Attila Kinali wrote:
>> On Sat, 7 Feb 2015 10:07:44 -0800
>> Tom McDermott <tom.n5eg at gmail.com> wrote:
>>
>>> While compensating for cable delay is relatively straight forward by
>>> measuring the length and compensating for
>>> the velocity factor, a question is: how much amplifier / filter group
>>> delay
>>> is to be expected within the antenna itself?
>>
>> The usual way is to calibrate the whole setup, including antenna, LNA,
>> cable and receiver. Ie. you drive to the national lab, set up your whole
>> system, then measure the timing difference of your GPS receiver to the
>> one of the lab, drive back home, and apply the correction.
>>
>>> Looking through GPS SAW filter datasheets seems to show none with group
>>> delay specifications.
>>
>> Not surprising. Group delay is not considered of any importance in most
>> RF designs.
>>
>>> googling leads to some research papers with delays of about:
>>>
>>> L1 - 20 MHz wide SAW filter has about 15 nsec of group delay
>>> L1 - 2 MHz wide SAW filter has about 65 nsec of group delay
>>> L1 - LC filter - can't find anything, but suspect it's probably just
>>> a few
>>> nanoseconds.
>>
>> I would be very much interested in those papers. Could you list their
>> titles
>> and authors at least?
>>
>>
>>> I'm not sure a consumer grade antenna even has a SAW filter, it may
>>> simply
>>> be an LC filter.
>>
>> Unlikely. LC filters are not sharp enough and difficult to build reliably
>> at those frequencies. I would rather assume that there are no filters
>> at all (beside the antenna characteristics).
>>
>>
>>             Attila Kinali
>>
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