[time-nuts] GPS Filter

Bob Camp lists at rtty.us
Mon Mar 7 13:01:59 UTC 2011


Hi

In the case of a GPS, you really can't increase the aperture (gain = directivity) since you want to cover the entire sky. 

The filter needed here is far from trivial. The spacing on the "good to bad" signals is pretty tight in this case. A filter can indeed be built, no doubt about that. It's going to be a bit more than I can shoot up in the basement or make on a simple pc board. 

Bob


On Mar 4, 2011, at 8:59 PM, Mike Feher wrote:

> Well, as you said John, for FWIW. In this case not much. As said low loss,
> so increase in noise temp would be minimal, and, if it makes a difference
> between an overloaded front end or a 0.5 dB loss in NF, it will be welcome.
> Heck, we use filters in front of most of our Satcom LNBs at 21 GHz with
> minimal effect. Been there and done that, as the saying goes. If necessary
> the small increase in noise temp can easily be overcome by a slightly larger
> aperture, especially since a view of the full sky is not really necessary
> and birds below certain elevation angles are typically ignored by software
> settings of one's own choosing. And again, I was talking about people who
> have done this and can do it again. It would not be an issue for me. Regards
> - Mike 
> 
> Mike B. Feher, EOZ Inc.
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960 office
> 908-902-3831 cell
> 
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of J. Forster
> Sent: Friday, March 04, 2011 8:47 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPS Filter
> 
> I'm not so sure.
> 
> A filter ahead of a preamp significantly increases the system Noise
> Temperature.
> 
> GPS signals are weak and link margins are small. The receiver preamps are
> already very low noise.
> 
> I'd think that a narrow filter might well drive up the systen NF to the
> point it'd be useless.
> 
> FWIW,
> 
> -John
> 
> =============
> 
> 
>> I used to make some interdigital filters and amplifiers in the early 80's
>> for MDS TV reception in the 2.3 GHz range. One can easily fabricate a low
>> loss narrow band filter at 1.5 GHz if need be, and as mentioned before,
>> antennas should be easy as well. If this really becomes an issue, I am
>> sure
>> there will be a lot of solutions offered and anyone with some RF
>> experience
>> will also be able to handle it themselves. Regards - Mike
>> 
>> Mike B. Feher, EOZ Inc.
>> 89 Arnold Blvd.
>> Howell, NJ, 07731
>> 732-886-5960 office
>> 908-902-3831 cell
>> 
>> 
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>> Behalf Of Ziggy
>> Sent: Friday, March 04, 2011 7:47 PM
>> To: time-nuts at febo.com
>> Subject: Re: [time-nuts] GPS Filter
>> 
>> The discussion got me to thinking about how we used to filter out nearby
>> interference on amateur TV - namely by using interdigital filters. This
>> led to a search for GPS interdigital filters which i did indeed find.
>> See Alison Microwave website at
>> <http://www.amlant.co.uk/DetailsAD430.htm> for one example of an
>> integrated antenna/filter/preamp. (I'm sure these aren't cheap, but I
>> haven't asked.) As for retrofitting, you could add a filter after the
>> antenna/amplifier assembly but I might be concerned that the amplified
>> GPS antenna is pretty wide and may have trouble with a Lightsquared
>> transmitter nearby. There are passive antennas though, and there are
>> in-line amps - you'd need to add the filter in between. We made these
>> ourselves for 439 and 1296 MHz - GPS L1 isn't much above that so with
>> some care it should be doable. The tuning can be finicky though :\
>> 
>> Ziggy
>> 
>> On 03/04/2011 03:31 PM, John Ackermann N8UR wrote:
>>> Here's a measurement we did a few years ago on the HP 58535A:
>>> 
>> 
> http://www.febo.com/pages/hp_gps_splitter/port_1_hp_58535a_two_port_amp.png
>>> 
>>> 
>>> John
>>> ----
>>> On 3/4/2011 1:31 PM, Bob Camp wrote:
>>>> Hi
>>>> 
>>>> Ok, now it's pretty obvious the RF world near your GPS will be
>>>> changing a *lot* in the near future. Lightsquared and a bunch of
>>>> similar outfits will be camping out right next door with very high
>>>> power gear. They will be running 1.5KW from somewhere in town. GPS is
>>>> running 30 watts from off planet.
>>>> 
>>>> Has anybody tossed the various HP / Symmetricom GPS splitters on a
>>>> network analyzer? If so, what do the filters in them look like?
>>>> 
>>>> I probably should corner the market on these things before asking a
>>>> question like that.
>>>> 
>>>> The new neighbors will be at 1525 to 1559 MHz.  GPS L1 is at 1575.42
>>>> MHz.  That's what we are using for timing. L2 is down at 1227.5,
>>>> right now it's mainly military use. Obviously these guys are a bigger
>>>> deal for civilians than the military.
>>>> 
>>>> So the question is - do the built in splitter filters have any real
>>>> rejection 15 to 50 MHz off of center?
>>>> 
>>>> Probably worth checking. It would be a pleasant surprise if they
>>>> turned out to be useful.
>>>> 
>>>> Bob
>>>> 
>>>> 
>>>> 
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